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Accepted papers to appear in an upcoming issue

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Artificial neural network for the reduction of birefringence-induced errors in fiber shape sensors based on cladding waveguids gratings

Hanrong Zheng, Yi Jiang, Martin Angelmahr, Günter Flachenecker, Haiwen Cai, and Wolfgang Schade

DOI: 10.1364/OL.386218 Received 23 Dec 2019; Accepted 18 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: Cladding waveguide fiber Bragg gratings (CWG-FBG) provide a compact and simple solution for fiber shape sensing. The shape sensing accuracy is limited by birefringence, which is induced by bending and non-isotropic FBG structure (written by point-by-point techniques). An algorithms based on artificial neural network (ANN) for fiber shape sensing is demonstrated, which enables an increased accuracy, better robustness, and less time latency. This algorithm shows great potential in the application of high-accuracy real-time fiber shape measurements.

Highly Efficient 3.7-kW-Peak-Power Single-Frequency Combined Er/Er-Yb Fiber Amplifier

Maksim Khudyakov, Denis Lipatov, Aleksei Guryanov, Mikhail Bubnov, and Mikhail Likhachev

DOI: 10.1364/OL.386831 Received 24 Dec 2019; Accepted 18 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: In this paper, we propose and realize a novel concept for a high-peak-power highly efficient fiber amplifier in the 1.55 µm spectral range. The amplifier is based on the simultaneous utilization of Er-doped Yb-free and Er-Yb codoped large-mode-area fibers spliced together. Using this approach, we demonstrate the amplification of single-frequency 160 ns pulses at 1554 nm to a peak power of 3.7 kW with a pump-to-signal conversion efficiency of .6% relative to the launched multimode pump power at 976 nm.

Effect of thermal lensing and micrometric degraded regions on the catastrophic optical damage process of high-power laser diodes

Jose Luis Pura, Jorge Souto, and J Jimenez

DOI: 10.1364/OL.389385 Received 28 Jan 2020; Accepted 18 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: Catastrophic Optical damage (COD) is one of the processes limiting the lifetime of high-power laser diodes. The understanding of this degradation phenomenon is critical to improve the laser power and lifetime for practical applications. In this paper we analyze the defect propagation inside the cavity of quantum well (QW) high-power laser diodes presenting COD. For this, we studied the effect of highly localized thermal gradients and degraded regions on the laser field distribution. Finite element method (FEM) simulations are compared to experimental cathodoluminescence (CL) measurements. The presence of micrometric hot spots inside the QW induces the thermal lensing of the laser field. The laser self-focusing inside the cavity eventually generates a new hot spot, and, in a repetitive way, a sequence of hot spots would be created. This would account for the propagation of the dark line defects (DLDs) characteristic of this degradation mode.

Tunable self-similar Bessel-like beams of arbitrary order

Nikolaos Efremidis, Michael Goutsoulas, domenico bongiovanni, Denghui Li, and Zhigang Chen

DOI: 10.1364/OL.387115 Received 03 Jan 2020; Accepted 17 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: We predict that Bessel-like beams of arbitrary integer order can exhibit a tunable self-similar behavior (that take an invariant form under suitable stretching transformations). Specifically, by engineering the amplitude and the phase on the input plane in real space, we show that it is possible to generate higher-order vortex Bessel-like beams with fully controllable radius of the hollow core and maximum intensity during propagation. In addition, using a similar approach, we show that it is also possible to generate zeroth order Bessel-like beams with controllable beam width and maximum intensity. Our numerical results are in excellent agreement with our theoretical predictions.

Experimental observation of shaking soliton molecules in a dispersion-managed fiber laser

Yiyang Luo, RAN XIA, Ping Shum, Wenjun Ni, Yusong Liu, Huy Lam, Qizhen Sun, Xiahui Tang, and Luming Zhao

DOI: 10.1364/OL.388113 Received 14 Jan 2020; Accepted 17 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: Recent progress in real-time spectral interferometry enables the access to the internal dynamics of optical multi-soliton complexes. Here, to the best of our knowledge, we report on the first experimental observation of shaking soliton molecules by means of the dispersive Fourier transform technique. Beyond the simplex vibrating soliton pairs, multiple oscillatory motions can jointly involve in the internal dynamics, reminiscent of the shaking soliton pairs. Both quasi-periodically and chaotically evolving phase oscillations are approached in the sense of different oscillatory frequencies. In addition, the shaking soliton pair combined with sliding phase dynamics is also observed, interpreted as the superposition of two different internal motions. All these results shed a new light on the internal dynamics of soliton molecules with higher degrees of freedom, as well as enrich the framework towards multi-soliton complexes.

Information integrated glass module fabricated by integrated additive and subtractive manufacturing

Qi Zhang, Jincheng Lei, yizheng chen, Jianan Tang, Yongji Wu, Liwei Hua, and Hai Xiao

DOI: 10.1364/OL.389203 Received 27 Jan 2020; Accepted 17 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: In this letter, we report a novel integrated additive and subtractive manufacturing (IASM) method to fabricate information integrated glass module. After a certain number of glass layers are 3D printed and sintered by direct CO2 laser irradiation, a microchannel will be fabricated on top of the printed glass by integrated picosecond laser, for internal Fabry-Perot interferometer (IFPI) optical fiber sensor embedment. Then glass 3D printing process continues for the realization of bonding between optical fiber and printed glass. Temperature sensing up to 1000C was demonstrated using the fabricated information integrated module. Besides, the long-term stability of the glass module at 1000C was conducted. Enhanced sensor structure robustness and harsh temperature sensing capability makes this glass module attractive for harsh environment structural health monitoring.

Homogeneous enhancement of near-fields in all-dielectric metasurfaces with cluster-based unit cells

Anton Kupriianov, Kateryna Domina, Vyacheslav Khardikov, Andrey Evlyukhin, and Vladimir Tuz

DOI: 10.1364/OL.384983 Received 03 Dec 2019; Accepted 17 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: In order to construct a dielectric analog of spaser, we study theoretically and experimentally several configurations of cluster-based unit cells for an all-dielectric metasurface characterized by resonant conditions of the trapped mode excitation. Excitation of the trapped mode is realized by performing either specific displacement of particles in the cluster, or perturbation of the equidistantly spaced particles by off-centered holes. The latter approach is more advantageous for enhancement of the electric near-field with homogeneous distribution in-plane of the structure and strong field localization outside the high-refractive-index dielectric particles. This feature opens prospects for realization of subwavelength flat lasing structures based on strong near-field interaction with substances exhibiting pronounced nonlinear characteristics and properties of gain media.

Controlling light emission by engineering atomic geometries in silicon photonics

Arindam Nandi, Xiaodong Jiang, Dongmin Pak, Daniel Perry, Kyunghun Han, Edward Bielejec, Yi Xuan, and Mahdi Hosseini

DOI: 10.1364/OL.385865 Received 13 Dec 2019; Accepted 17 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: By engineering atomic geometries composed of nearly 1000 atomic segments embedded in micro-resonators we observe anomalous photon emission at the telecommunication wavelength. Erbium atoms geometrically arranged into a lattice inside a silicon nitride microring resonator give rise to a minor cavity effect enhancing light-atom coupling. We confirm dependency of light emission to the atomic positions and lattice spacing and also observe Fano interference between resonant modes in the system.

Omni-resonant space-time wave packets

Ayman Abouraddy, Abbas Shiri, Murat Yessenov, and Rohinraj Aravindakshan

DOI: 10.1364/OL.383650 Received 27 Nov 2019; Accepted 16 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: We describe theoretically and verify experimentally a novel class of diffraction-free pulsed optical beams that are `omni-resonant': they have the remarkable property of transmission through planar Fabry-Pérot resonators without spectral filtering even if their bandwidth far exceeds the cavity resonant linewidth. Ultrashort wave packets endowed with a specific spatio-temporal structure couple to a single resonant mode independently of its linewidth. We confirm that such 'space-time' omni-resonant wave packets retain their bandwidth (1.6 nm), spatio-temporal profile (1.3-ps pulse width, 4-μm beam width), and diffraction-free behavior upon transmission through cavities with resonant linewidths of 0.3-nm and 0.15-nm.

Multiple scattering of light by a very large agglomerate of small silica spheres

Timo Väisänen, Johannes Markkanen, Edith Hadamcik, Jean-Baptiste Renard, Jeremie Lasue, Anny-Chantal Levasseur-Regourd, Karri Muinonen, and Jürgen Blum

DOI: 10.1364/OL.382240 Received 06 Nov 2019; Accepted 16 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: We model the phase function and the degree of linear polarization of an agglomerate made of equisized silica spheres using the methodology of radiative transfer with reciprocal transactions (R²T²). In the experimental work, the agglomerate was produced in the laboratory by random ballistic deposition and characterized by scanning electron microscopy, and the light scattering properties were measured with the PROGRA² instrument. In this study, we model the entire agglomerate with the new R²T² by starting from the physical parameters derived in the experimental papers. The simulated normalized phase function is in agreement with the measured data, but the degree of linear polarization has some systematic discrepancies, which can be mitigated by adjusting the physical parameters. The study emphasizes the relevance of the degree of linear polarization and gives insights into the effects of particle aggregation on the scattering characteristics. To our best knowledge, this is the first time the degree of linear polarization has been modeled this well for the large agglomerate made of small particles using multiple scattering methods.

Broadband pumping enabled flat-amplitude multi-wavelength random Raman fiber laser

Jun Ye, Yang Zhang, Xu Jiangming, Jiaxin Song, Tianfu Yao, Hu Xiao, Jinyong Leng, and Pu Zhou

DOI: 10.1364/OL.389071 Received 23 Jan 2020; Accepted 16 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: Flat-amplitude multi-wavelength random Raman fiber laser with broad spectral coverage and high optical signal-to-noise ratio (OSNR) is challenging and of great interest. In this paper, we theoretically and experimentally proved that broadband pumping can help realize a broader, flat-amplitude multi-wavelength random Raman fiber laser. The influence of pump bandwidth, the tunability of spectral envelope and channel spacing are investigated. As a result, with 40 nm pump bandwidth, a spectral coverage of 1116–1125 nm with 19 laser lines and 31 dB OSNR is achieved, and the standard deviation in the peak intensities of the central 9 lines is ~1.1 dBm. This technique can also be applied to the multi-wavelength Raman (or random Raman) fiber lasers at other wavelengths, and provide a reference for the multi-wavelength applications in sensing, communication and optical component testing.

Generation of broadband chaotic laser by active optical feedback loop combined with a high nonlinear fiber

Mingjiang Zhang, Qiang Yang, Lijun Qiao, Jianzhong Zhang, Shaohua Gao, Tao Wang, Chai Mengmeng, and MEHJABIN MOHIUDDIN PROMI

DOI: 10.1364/OL.387963 Received 13 Jan 2020; Accepted 16 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: We propose and demonstrate a method to generate flat broadband chaotic laser by using an active optical feedback loop combined with a high nonlinear fiber. The feedback strength and nonlinear effect, especially the four-wave mixing effect of high nonlinear fiber, are studied to improve the bandwidth and flatness of chaos. When the feedback strength is 6.6 and injected fiber power is 1.0 W, a chaotic signal with a frequency range over 50 GHz, 80% bandwidth of 38.9 GHz, and flatness of 4.2 dB is experimentally achieved.

High-speed photography of shock waves with an adaptive illumination

Rok Petkovsek, Vid Agrez, and Tomaz Pozar

DOI: 10.1364/OL.388444 Received 24 Jan 2020; Accepted 15 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: An adaptable, laser-diode-based illumination system was developed to simultaneously visualize the dynamics of slow and fast phenomena in optically transparent media. It can be coupled with still or high-speed cameras and allows for the generation of arbitrary train of illumination pulses with variable pulse duration, pulse energy and intrapulse delay with temporal resolution of 12.5 ns. Its capabilities are presented with the selected illustrative visualizations of the dynamics of shock waves and cavitation entities generated after laser-induced breakdown in water.

Demonstration of the mode-selection rules obeyed in a single-helix helical long-period fiber grating

Peng Wang, hua zhao, Taishu Detani, Yuhta Tsuyuki, and Hongpu Li

DOI: 10.1364/OL.384665 Received 29 Nov 2019; Accepted 15 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: A simple method enabling to validate the mode-selection rules obeyed in a single-helix helical long-period fiber grating (SHLPG) has been demonstrated both theoretically and experimentally, which is realized just by investigating and analyzing the circular dichroism (CD) and the polarization-dependence-loss (PDL) spectra of the utilized SHLPG. It is confirmed in the first time that in terms of the SHLPG’s helicity, the core mode HE11 but with either the left circular polarization (LCP) or the right circular polarization (RCP) will be selectively coupled into the higher HE or the TE/TM mode, respectively. As a result, the SHLPG would exhibit an enhanced circular dichroism (CD) as well as an enhanced PDL near the resonant wavelengths of the HE and TE/TM modes, respectively. The results presented in this study will be helpful for us to well design and produce the SHLPG-based circular polarizers and optical-angular-momentum generators.

Nonlinear optical vortex coronagraph

Peter John Rodrigo and Einstom Engay

DOI: 10.1364/OL.383311 Received 20 Nov 2019; Accepted 14 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: A nonlinear optical vortex coronagraph (n-OVC) based on sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) crystal is presented. We demonstrate an n-OVC by mixing the image of an on-axis point source (λ_s = 1.6 µm) inside the PPLN crystal with a pump beam (λ_p = 1064 nm) imprinted with a helical phase profile from a vector vortex mask (topological charge l = 2). Due to quasi-phase matching and orbital angular momentum conservation, a coronagraphic image is produced at the SFG wavelength (λ_up ~ 630 nm). We validate that n-OVC is tunable to signal wavelength but only requires a vortex mask operating at the pump wavelength. The acceptance bandwidth of the SFG process provides the n-OVC a degree of achromaticity even with a monochromatic vortex mask. The n-OVC exhibits an inner working angle of ~λ_s/D and an experimental contrast of 10-⁴ at 3λ_s/D was achieved.

Quantum statistical signature of PT symmetry breaking

Stefano Longhi

DOI: 10.1364/OL.386232 Received 17 Dec 2019; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: In multiparticle quantum interference, bosons show rather generally the tendency to bunch together, while fermions can not. This behavior, which is rooted in the different statistics of the particles, results in a higher coincidence rate P for fermions than for bosons, i.e Pbos≤Pferm. However, in lossy systems such a general rule can be violated because bosons can avoid lossy regions. Here it is shown that, in a rather general optical system showing passive parity-time (PT) symmetry, at the PT symmetry breaking phase transition point the coincidence probabilities for bosons and fermions are equalized, while in the broken PT phase the reversal Pbos≥Pferm is observed. Such effect is exemplified by considering the passive PT symmetric optical directional coupler.

Polarization-maintaining and single-mode large mode area pixelated Bragg fiber

Olivier Vanvincq, Remi Habert, Andy Cassez, Karen Baudelle, Damien LABAT, sylvain delobel, Yves Quiquempois, Geraud Bouwmans, and Laurent Bigot

DOI: 10.1364/OL.388010 Received 14 Jan 2020; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: This letter reports on a large mode area pixelated Bragg fiber in which some high refractive index rods were replaced by boron-doped rods that allows polarization maintaining behavior while keeping single-mode behavior. The realized all-solid fiber has a core diameter of 35 µm and the fundamental mode is circular with a 25 µm mode field diameter. Its birefringence was measured at 8E-5 while the polarization extinction ratio reaches 30 dB. Finally, this fiber is flexible up to a 20 cm bending radius.

Partial quantum revivals of localized condensates in distorted lattices

Ivan Savenko, DOGYUN Ko, Meng Sun, Alexey Andreanov, and Yuri Rubo

DOI: 10.1364/OL.386848 Received 03 Jan 2020; Accepted 14 Feb 2020; Posted 18 Feb 2020  View: PDF

Abstract: We report on a peculiar propagation of bosons loaded by a short Laguerre-Gaussian pulse in a nearly flat band of a lattice potential. Taking a system of exciton-polaritons in a kagome lattice as an example, we show that an initially localized condensate propagates in a specific direction in space if anisotropy is taken into account. This propagation consists of quantum jumps, collapses, and revivals of the whole compact states, and it persists given any direction of anisotropy. This property reveals its signatures in the tight-binding model and, surprisingly, it is much more pronounced in a continuous model. Quantum revivals are robust to the repulsive interaction and finite lifetime of the particles. Since no magnetic field or spin-orbit interaction is required, this system provides a new kind of easily implementable optical logic.

Chip-scale Blue Light Phased Array

Min Chul Shin, Aseema Mohanty, Kyle Watson, Gaurang Bhatt, Christopher Phare, Steven Miller, Moshe Zadka, Brian Lee, Xingchen Ji, Ipshita Datta, and Michal Lipson

DOI: 10.1364/OL.385201 Received 09 Dec 2019; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Compact beam steering in the visible spectral range is required for a wide range of emerging applications such as augmented and virtual reality displays, optical traps for quantum information processing, biological sensing and stimulation. Optical phased arrays (OPAs) can shape and steer light to enable these applications with no moving parts on a compact chip. However, OPA demonstrations have been mainly limited to the near-infrared spectral range due to the fabrication and material challenges imposed by the shorter wavelengths. Here we demonstrate the first chip-scale phased array operating at blue wavelengths (488 nm) using a high confinement silicon nitride platform. We use a sparse aperiodic emitter layout to mitigate fabrication constraints at this short wavelength and achieve wide-angle beam steering over a 50° field of view with a full-width at half-maximum beam size of 0.17°. Large-scale integration of this platform paves the way for fully reconfigurable chip-scale three-dimensional (3D) volumetric light projection across the entire visible range.

Dynamic control of spontaneous emission rate using tunable hyperbolic metamaterials

SANDEEP CHAMOLI, Mohamed ElKabbash, Jihua Zhang, and Chunlei Guo

DOI: 10.1364/OL.385844 Received 12 Dec 2019; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We numerically investigate the dynamic control over the spontaneous emission rate of quantum emitters using tunable Hyperbolic Metamaterials (HMMs). The dispersion of a metal-dielectric thin film stack at a given frequency can undergo a topological transition from an elliptical to a hyperbolic dispersion by incorporating a tunable metal or dielectric film in the HMM. This transition modifies the local density of optical states of the emitter, hence, its emission rate. In the visible range, we use an HMM consisting of TiN and Sb2S3 and show considerable tunability in the Purcell enhancement and quantum efficiency as Sb2S3 phase changes from amorphous to crystalline. Similarly, we show tunable Purcell enhancement in the telecommunication wavelength range using a TiN/VO2- HMM. Finally, tunable spontaneous emission rate in the mid-IR range is obtained using a Graphene/MgF2 HMM by modifying the Graphene conductivity through changing its chemical potential. We show that using a metal nitride (for the visible and NIR HMMs) and a fluoride (for the mid-IR HMM) is important to get an appreciable change in the effective permittivity of the thin-film multilayer stack.

All-fiber Mamyshev oscillator with high average power and harmonic-mode-locking

Emmanuel Hugonnot, Etienne Poeydebat, Florent Scol, Olivier Vanvincq, and Geraud Bouwmans

DOI: 10.1364/OL.389522 Received 03 Feb 2020; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: In this Letter, we present the first experimental demonstration of high-order harmonic mode-locking of an all-fiber Mamyshev oscillator. The laser is entirely realized using standard step-index fiber. It delivers time-stable pulses trains with average powers reaching more than 100mW at the fundamental mode-locked repetition rate (7.7MHz) and 1.3W at the 14th harmonic (107.8MHz).

Bend-induced long period grating in helical core fiber

Kinga Zolnacz, Maciej Napiórkowski, Anna Kiczor, Mariusz Makara, Pawel Mergo, and Waclaw Urbanczyk

DOI: 10.1364/OL.388866 Received 22 Jan 2020; Accepted 14 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We report on a new type of long-period-grating generated in a helical core fiber by bending. The grating arises from bend-induced modulation of an equivalent refractive index in the helical core with a period equal to the helix pitch. We experimentally demonstrate that such grating induces multiple resonant couplings between the fundamental modes guided in the central core and the helical side-core. We have also shown that by varying a direction of bending, one can generate the phase shifted grating. The experimental results are supported by numerical simulations based on the coupled mode equations.

Fast reconstruction algorithm for structured illumination microscopy

Xiang Hao, Shijie Tu, Qiulan Liu, Xin Liu, Wenjie Liu, zhimin zhang, Taojin Luo, Cuifang Kuang, and Xu Liu

DOI: 10.1364/OL.387888 Received 10 Jan 2020; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Structured illumination microscopy (SIM) is a powerful technique for providing super-resolution imaging, but its reconstruction algorithm, i.e. linear reconstruction structured illumination microscopy (LRSIM) algorithm in the Fourier domain, limits the imaging speed due to its computational effort. Here, we present a novel reconstruction algorithm that can directly process SIM data in the spatial domain. Compared to LRSIM, this approach uses the same number of frames to achieve a comparable resolution but with much faster processing speed. Our algorithm was verified on both simulated and experimental data using sinusoidal-pattern illumination. Moreover, this algorithm is also applicable for the speckle-pattern illumination.

Optically computed optical coherence tomography for volumetric imaging

Yahui Wang, Qi Kang, Yuanwei Zhang, and Xuan Liu

DOI: 10.1364/OL.382045 Received 31 Oct 2019; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We describe a novel optically computed optical coherence tomography (OC-OCT) technology for 3D microscopy. The OC-OCT system performs depth resolved imaging by computing the Fourier transform of the interferometric spectra optically. The OC-OCT system modulates the interferometric spectra with Fourier basis projected to a spatial light modulator and detects the modulated signal without spectral discrimination. The novel optical computation strategy enables volumetric OCT imaging without performing mechanical scanning and without need for Fourier transform in a computer.

Endoscopic pyrometric temperature sensor

Sergio Vilches, Caglar Ataman, and Hans Zappe

DOI: 10.1364/OL.383337 Received 15 Nov 2019; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We demonstrate a pyrometric contact-less temperature sensor using a flexible fused silica fiber of 360 µm diameter as support for endoscopic/laparoscopic surgery, able to measure down to 30°C with a precision better than 1°C at 10 Hz. Fused silica fibers, as opposed to dedicated MIR fibers, are non-degrading, low-cost and bio-compatible. The large bandwidth (up to several kilohertz) and the broad temperature range (up to 5°C) of the sensor can be instrumental for time-resolved analysis and control of laser ablation and electrothermal surgery procedures.

Bionic-compound-eye structure for realizing compact integral imaging 3D display in cellphone with enhanced performance

Juan Liu, Zi-feng Zhao, Zhiqi Zhang, and Liangfa Xu

DOI: 10.1364/OL.384182 Received 27 Nov 2019; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: A bionic-compound-eye structure (BCES) is proposed to enhance the performance of integral imaging (II) 3D display, which is a substitute of micro-lens array and consists of a series of ocelli arranged on the curved substrate. Hexagonal ocelli without pupils and gaps are predesigned to obtain continuous image, uniform parallax and high resolution. Curved substrate is designed to enhance viewing angle. When the BCES is placed above a normal display device, continuous and full-parallax 3D images with 150 μm effective resolution, 130mm depth of field, 28° horizontal, 22° vertical and 35° diagonal viewing angle are achieved. The weight of BCES is 31g and the thickness of the whole system is 22mm, thus the BCES based II (BCES-II) is very compact. In addition, this structure can be easily integrated into a cellphone or iPad for compact quasi-2D and 3D adjustable display.

Adaptive optics for time-resolved Förster Resonance Energy Transfer (FRET) and fluorescence lifetime imaging microscopy (FLIM) in vivo

Simao Coelho, Simon Ameer-Beg, Simon Poland, and Viviane Devauges

DOI: 10.1364/OL.385950 Received 13 Dec 2019; Accepted 13 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Förster Resonance Energy Transfer (FRET) and Fluorescence lifetime imaging (FLIM) have been coupled with multiphoton microscopy to image in vivo dynamics. However, the increase in optical aberrations as a function of depth significantly reduces the fluorescent signal, reduces spatial resolution and fluorescence lifetime accuracy. We present the development of time-resolved FRET-FLIM imaging system with adaptive optics. We demonstrate the improvement of our AO-FRET-FLIM instrument over standard multiphoton FRET-FLIM imaging. We validate our approach using fixed cellular samples with FRET standards and in vivo with live imaging in a mouse kidney.

Experimental extraction of nonlocal weak values for demonstrating the failure of product rule

Xiao-Ye Xu, Wei-Wei Pan, Yaron Kedem, Wang Qin-Qin, Kai Sun, Jin-Shi Xu, Yong-Jian Han, Geng Chen, Chuanfeng Li, and Guang-can Guo

DOI: 10.1364/OL.375448 Received 14 Aug 2019; Accepted 13 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: The nonlocal weak values, defined as the weak values of nonlocal observables, can be adopted to study quantum paradoxes and quantum fundamentals. Unfortunately, owing to the absence of nonlocal observables in the physical Hamiltonian, performing weak measurements on such nonlocal observables for extracting the corresponding nonlocal weak values faces challenges. An approximate method for fulfilling this task has been presented taking into account the high order terms in the evolution operator of weak measurement. Here we report an experimental extraction of nonlocal weak values without performing the weak measurement on nonlocal observables. Our method is based on the concept of modular values. Additionally, comparing the weak values of local and nonlocal observables, we demonstrate the failure of the product rule for entangled systems. Our results significantly simplify the task of obtaining nonlocal weak values, thereby extending their applications.

Solitons supported by intensity-dependent dispersion

Ray-Kuang Lee, Chun-Yan Lin, Jen-Hsu Chang, and Gershon Kurizki

DOI: 10.1364/OL.387642 Received 10 Jan 2020; Accepted 13 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: Soliton solutions are studied for paraxial wave propagation with intensity-dependent dispersion. Although the corresponding Lagrangian density has a singularity, analytical solutions, derived by the pseudo-potential method and the corresponding phase diagram, exhibit one- and two-humped solitons with almost perfect agreement to numerical solutions. The results obtained in this work reveal a hitherto unexplored area of soliton physics associated with nonlinear corrections to wave dispersion.

Photonic serial implementation of flash analog-to-digital converter

He Hongxia, Shuna Yang, Tao Jin, and Hao Chi

DOI: 10.1364/OL.387164 Received 02 Jan 2020; Accepted 12 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: A novel photonic analog-to-digital converter (ADC) scheme based on multi-wavelength sampling and balanced detection is proposed and experimentally demonstrated. In the approach, a power-weighted multi-wavelength pulsed source is employed to implement sampling, a dispersion element is used to realize temporal walk-off of multi-wavelength pulses, and a thresholding module is applied to implement serial thresholding. The principles of quantization and encoding are similar to that of an electronic flash ADC, but the serial implementation in the given approach avoids the use of a large number of comparators. A proof-of-concept experiment with 3 wavelengths is successfully demonstrated. We also discuss the feasibility of the approach and the design of non-uniform quantization by properly setting the power ratio of the multi-wavelength pulses.

Verdet constant of potassium terbium fluoride crystal as a function of wavelength and temperature

David Vojna, Martin Duda, Ryo Yasuhara, Ondrej Slezak, Wolfgang Schlichting, Kevin Stevens, Hengjun Chen, Antonio Lucianetti, and Tomas Mocek

DOI: 10.1364/OL.387911 Received 14 Jan 2020; Accepted 12 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Potassium terbium fluoride KTb₃F₁₀ (KTF) crystal is a promising magneto-active material for creating multi-kW average-power Faraday isolators operating at the visible and near-infrared wavelengths. Nevertheless, the key material's parameter needed for the design of any Faraday isolator - the Verdet constant, has not been comprehensively investigated yet. In this letter, we report on measurement of the Verdet constant of the KTF crystal for the wavelengths between 600 and 1500 nm and for the temperatures ranging from 15 to 295 K. A suitable model for the Verdet constant as a function of wavelength and temperature has been developed and may be conveniently used for the optimal design of the KTF-based high-average-power Faraday isolators.

Laser micromachined zebra patterned graphene as a mode-locker with adjustable loss

Alphan Sennaroglu, Yagiz Morova, Fabian Rotermund, and Ji Bae

DOI: 10.1364/OL.389290 Received 27 Jan 2020; Accepted 12 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We describe a novel device based on micro-structured graphene, referred to as zebra patterned graphene saturable absorber (ZeGSA), which can be used as a saturable absorber with adjustable loss to initiate femtosecond pulse generation. Femtosecond laser micro machining was employed to ablate monolayer graphene on an infrasil substrate in the form of stripes with different duty cycle, resulting in the formation of regions with variable insertion loss in the range of 0.21-3.12%. The mode-locking performance of the device was successfully tested by using a Cr4+:forsterite laser operating near 1250 nm. In comparison with mode locking using non-ablated graphene, the ZeGSA device with regions of decreasing graphene enabled improved power performance, where the mode-locked output power increased from 68 mW to 114 mW and the corresponding pulse duration decreased from 62 to 48 fs at the same incident pump power of 6.3 W. These experiments show that ZeGSA has a great potential to be employed as a laser mode-locker with adjustable loss and should find applications in the development of femtosecond lasers over a broad spectral range.

Continuous-wave diode-pumped passively Q-switched eye-safe 1537 nm Er:Yb:Lu2Si2O7 pulse laser

Chen Yujin, Jian Huang, Yanfu Lin, Xinghong Gong, Zundu Luo, and Yidong Huang

DOI: 10.1364/OL.389873 Received 04 Feb 2020; Accepted 11 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: It is still a technical challenge to obtain a 1.55 μm passively Q-switched Er3+/Yb3+ pulse laser with high energy and high repetition frequency, especially when it is end-pumped by a continuous-wave 0.98 m diode laser. Benefited from its long fluorescence lifetime of the 4I13/2 upper laser level and high thermal conductivity, Er:Yb:Lu2Si2O7 crystal may be an excellent gain medium for realizing a high energy and high repetition frequency 1.55 μm pulse laser. At a continuous-wave absorbed pump power of 6.1 W, a 1537 nm pulse laser with energy of 45.5 μJ, repetition frequency of 1.32 kHz and duration of 25 ns was firstly realized in an Er:Yb:Lu2Si2O7 crystal.

Triplex radiometric, photoacoustic and ultrasonic imaging based on single-pulse excitation

yue zhao, Siqi Wang, john merrill, jesus arellano, Luis Trevisi, yizhou li, Liangzhong Xiang, Junle Qu, and Liwei Liu

DOI: 10.1364/OL.387501 Received 08 Jan 2020; Accepted 11 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: In this Letter, we proposed a novel triplex-parameter detection method to realize simultaneous radiometric, photoacoustic and ultrasonic imaging based on single-pulse excitation. The optical attenuation, optical absorption and acoustic impedance properties can be obtained simultaneously by analyzing the photoacoustic signals and the ultrasonic echo signals. To test the feasibility and accuracy of this method, agar phantoms with different absorption coefficients and elastic coefficients were measured. Then, this method was experimentally verified by imaging a leaf skeleton piece embedded in an agar cylinder. Furthermore, pilot experiments were performed by triplex imaging of a pig ear tissue ex vivo to characterize the cartilage and surrounding tissue. Experimental results demonstrated that this technique has future potentials for visualizing and providing the functional and structural information of biological tissues.

UV - NIR Femtosecond Laser hybrid Lithography for efficient printing of complex on-chip waveguides

Zhi-Shan Hou, Jia-ji cao, Feng Yu, Zhen-Nan Tian, Xiao Xiong, MU-TIAN LI, Xifeng Ren, Qi-Dai Chen, and Hong-Bo Sun

DOI: 10.1364/OL.386861 Received 26 Dec 2019; Accepted 11 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We propose UV - NIR Femtosecond Laser hybrid Lithography for efficient printing of complex on-chip waveguides, which offers good performance in terms of processing efficiency and accuracy. With this 3D printing technology, waveguides with complex cross-section shapes, such as owls and kittens can be easily fabricated with efficiency increased by 750 % (for 6x6 μm²). In addition, a circular cross-section waveguide with extremely low birefringence and complex 8 × 8 random walk networks were quickly customized. Which implies that in the design and preparation of large-scale optical chips, the proposed maskless method allows for the preparation of highly customized devices.

Ultra-compact non-volatile Mach-Zehnder switch enabled by a high-mobility transparent conducting oxide

Jorge Parra, Irene Olivares, Francisco Ramos, and Pablo Sanchis Kilders

DOI: 10.1364/OL.388363 Received 16 Jan 2020; Accepted 11 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: Compact and broadband non-volatile silicon devices are mainly absorption-based. Hence, access to low-loss non-volatile phase shifters is still a challenge. Here, this problem is addressed by using a high-mobility transparent conducting oxide such as cadmium oxide as a floating gate in a flash-like structure. This structure is integrated in a Mach-Zehnder interferometer switch. Results show an active length of only 30 μm to achieve a π phase shift. Furthermore, an extinction ratio of 20 dB and insertion loss as low as 1 dB may be attained. Thedevice shows an optical broadband reponse and can be controlled with low-power pulses in the nanosecond range. These results open a new way for enabling compact silicon based phase shifters with non-volatile performance

Reshaping enhancement of gold nanorods by femtosecond double-pulse laser

Dezhi Zhu, Jianfeng Yan, and Jiawang Xie

DOI: 10.1364/OL.384326 Received 26 Nov 2019; Accepted 11 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: Gold nanorod (Au NR) is an attractive material due to its superior physical and chemical properties. Various applications in diagnostics and biomedicine have been demonstrated. Single-pulse laser is commonly used to reshape nanoparticles in solvent, however, the laser-material reaction mechanisms underlying nanoparticles reshaping remain unclear. Here we report a reshaping of Au NRs by ultrafast pump-probe-like double-pulse laser irradiation to understand the reaction dynamics. We demonstrate the enhancement of double pulse–induced reshaping, which provides an opportunity to design new Au NR structures. It shows that the reshaping enhancement is dependent on the delay time (τs) between a pair of separated pulses. The absorption peak wavelength of the Au NRs exposed to the shaped double-pulse was lower than that using a single-pulse of the same total fluence when τs was less than the electron-phonon relaxation time. This phenomenon was mainly attributed to changes in electronic heat transport and electron-phonon coupling.

Snapshot spatial-temporal compressive imaging

Mu Qiao, Xuan Liu, and Xin Yuan

DOI: 10.1364/OL.386238 Received 18 Dec 2019; Accepted 11 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: We build a compressive imaging system that can reconstruct high-speed videos of two field-of-views by capturing a single frame. A single digital micromirror device is employed to implement the high-speed modulation of both scenes. We utilize the deep denoising priors into the plug-and-play framework for efficient reconstruction. Two high-speed scenes, each with size 650x650x20 are reconstructed from a single two-dimensional measurement.

Compression and Denoising of Time-Resolved Light Transport

Yun Liang, Mingqin Chen, Zesheng Huang, Diego Gutierrez, Adolfo Muñoz, and Julio Marco

DOI: 10.1364/OL.383130 Received 15 Nov 2019; Accepted 11 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: The ability to obtain time-resolved light transport data is a valuable asset in transient imaging. Exploiting temporal information of light propagation captured at ultra-fast frame rates has enabled applications such as reconstruction of complex hidden geometry, or vision through scattering media. However, these applications usually require high-dimensional and high-resolution transport data that can introduce significant performance and storage constraints. Additionally, due to different sources of noise in both captured and synthesized data, the signal becomes significantly degraded over time, compromising the quality of the results. In this work we tackle these issues by proposing a method that extracts meaningful sets of features to accurately represent time-resolved light transport data. By means of these features, our method is able to reduce the size of time-resolved transport data up to a factor of 32, while significantly mitigating variance in both temporal and spatial dimensions.

Ghost imaging for detecting trembling with random temporal changing

Xianwei Huang, Suqin Nan, Wei Tan, Yanfeng Bai, and Xiquan Fu

DOI: 10.1364/OL.388640 Received 17 Jan 2020; Accepted 11 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: The imaging environment can be destablized for moving object and imaging platform, thus leading to random trembling of the detection which is negative to the object reconstruction. In this Letter, we have experimentally investigated ghost imaging for detecting trembling with random temporal changing, and an improvement method based on the temporal property of imaging process is proposed. It is demonstrated thatthis method is effective to address the image degradation due to the trembling disturbance and obtain a higher quality image of the object. The results provide a promising approach to deal with the image degradationcaused by the unstable environment and can find potential applications of ghost imaging in remote sensing.

Half-Ring Point Spread Functions

Jacob Wirth, Abbie Watnik, and Grover Swartzlander

DOI: 10.1364/OL.376860 Received 20 Sep 2019; Accepted 11 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: Point spread function engineering has been applied to design a reversible diffuser. Our design comprises single or multiple half-ring irradiance patterns satisfying two objectives: reduced peak irradiance in the focal plane, and high image fidelity of the numerically reconstructed image. Optical elements producing such half-ring point spread functions (HR-PSF's) may be useful for applications such as sensor protection or smart diffusers. Experimental and numerical techniques were used to demonstrate three orders of magnitude of suppression of the peak irradiance. Finally, we found a general power-law trend between the Strehl ratio and the light suppression factor.

Label-free characterization of different kinds of cells using optoelectrokinetic-based microfluidics

Wenfeng Liang, Xieliu Yang, Junhai Wang, Yuechao Wang, Hemin Zhang, Wenguang Yang, and Lianqing Liu

DOI: 10.1364/OL.384883 Received 04 Dec 2019; Accepted 10 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: We report a novel method to rapidly characterize different kinds of cells and drug-treated cancer cells using a label-free biomarker of self-rotation in an optoelectrokinetics (OEK) -based microfluidic platform. OEK incorporates optics and electrokinetics into microfluidics, thereby offering a contact-free, label-free and rapid approach to the cellular manipulation community. Self-rotational behaviors of four different kinds of cells were experimentally investigated by the frequency-sweeping of an AC bias potential in an optically-induced non-uniform and irrotational electric field. The results revealed that these kinds of cells displayed a gaussian distribution versus the AC frequency as well as different self-rotational speeds under the same conditions. Furthermore, the peak self-rotational speed varied from one kind of cells to another, with that of cancer cells higher than that of normal cells. In addition, MCF-7 cells treated by various concentrations of drug showed remarkably different self-rotational speeds. This finding suggests a high potential of developing a new label-free biomarker to rapidly distinguish different kinds of cancer cells and quantitatively monitor the response of cancer patients to various treatments.

Manipulating cold atoms through a high-resolution compact system based on a multimode fiber.

Nicolas Vitrant, Kilian Müller, Sébastien Garcia, and Alexei Ourjoumtsev

DOI: 10.1364/OL.385857 Received 18 Dec 2019; Accepted 10 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: To manipulate cold atoms in spatially constrained quantum engineering platforms, we developed a lensless optical system with a ~1 µm resolution and a transverse size of only 225 µm. We use a multimode optical fiber with a high numerical aperture, which directly guides light inside the ultra-high-vacuum system. Spatial light modulators allow us to generate control beams at the in-vacuum fiber end by digital optical phase conjugation. As a demonstration, we use this system to optically transport cold atoms towards the in-vacuum fiber end, to load them in optical microtraps and to re-cool them in optical molasses. This work opens new perspectives for setups combining cold atoms with other optical, electronic or opto-mechanical systems with limited optical access.

Radially polarized symmetric Airy beam

chuangjie xu, hongcheng hu, Yujun Liu, and Dongmei Deng

DOI: 10.1364/OL.389753 Received 03 Feb 2020; Accepted 10 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: In this letter, we have introduced a new kind of radially polarized beam called the radially polarized symmetric Airy beam (RPSAB). Comparing to the linearly polarized symmetric Airy beam (SAB), the hollow focus spot of the RPSAB enables it to trap the micro particle whose refractive index is lower than that of the surrounding medium. And the focus intensity of the RPSAB is nearly about three times higher than the one of SAB under the same conditions. Besides, we also have presented the on axis and off axis radially polarized symmetric Airy vortex beam (RPSAVB). In the on axis case, we find the maximum intensity of the RPSAVB is about twice higher than the one of RPSAB. For the off axis case, we prove that slight misalignment of the vortex and the RPSAB enables the guiding of the vortex into one of the self-accelerating channels, same as the symmetric Airy vortex beam (SAVB). Our results may expand the applications of RPSAB in laser cutting, metal processing, nanofocusing and three dimensional trapping of metallic Raleigh particles.

PFNet: An unsupervised deep network for polarization image fusion

Junchao Zhang, Jianbo Shao, Jianlai Chen, Degui Yang, Buge Liang, and Rongguang Liang

DOI: 10.1364/OL.384189 Received 22 Nov 2019; Accepted 10 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Image fusion is the key step to improve the performance of object detection in polarization images. We propose an unsupervised deep network to address the polarization image fusion issue. The network learns an end-to-end mapping for fused image from intensity and degree of linear polarization images, without ground-truth of fused images. Customized architecture and loss function are designed to boost the performance. Experimental results show that our proposed network outperforms other state-of-the-art methods in terms of visual quality and quantitative measurement.

Band-extended angular spectrum method for accurate diffraction calculation in a wide propagation range

Hao Zhang, Wenhui Zhang, and Guofan Jin

DOI: 10.1364/OL.385553 Received 11 Dec 2019; Accepted 10 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Band-extended angular spectrum method (ASM) is proposed for both near and far fields diffraction calculation with high accuracy. Due to the aliasing problem of the transfer function (TF), ASM is not suitable for far field diffraction calculation. For band-limited ASM, the non-aliased bandwidth of the TF would shrink rapidly with the increase of the propagation distance, which would reduce the calculation accuracy in far field. For the proposed band-extended ASM, the non-aliased bandwidth is significantly extended by rearranging the sampling points in the spatial frequency domain. Therefore, more frequency components of the TF contribute to the wave field calculation, leading to a much wider propagation range and a higher computation accuracy.

Orthogonality-breaking polarimetric sensing modalities for selective polarization imaging

François PARNET, Jonathan Staes, Julien FADE, Noé Ortega-Quijano, and Mehdi Alouini

DOI: 10.1364/OL.387651 Received 10 Jan 2020; Accepted 10 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Polarimetric sensing/imaging by orthogonality breaking is a microwave-photonics-inspired optical remote sensing technique that was shown to be particularly suited to characterize dichroic samples in a direct and single-shot way. In this work we expand the scope of this approach in order to gain sensitivity on birefringent and/or purely depolarizing materials by respectively introducing a circular or a linear polarization analyzer in the detection module. We experimentally validate the interest of these two new induced orthogonality-breaking modalities in the context of infrared active imaging.

Nonlinear multimode interference-based dual-color mode-locked fiber laser at 1.5 and 2 μm

Changxi Yang, Kangjun Zhao, Yan Li, and Xiaosheng Xiao

DOI: 10.1364/OL.388314 Received 15 Jan 2020; Accepted 10 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: We present a dual-color-soliton fiber laser at two different wavebands by nonlinear multimode interference. A saturable absorber (SA) with single mode – multimode – single mode fiber (SMF-MMF-SMF) structure is placed in the common branch shared by two sub cavities. Saturable absorption effects can be simultaneously satisfied at 1.5 and 2 μm at proper length of MMF. Dual-color solitons can still be remained even by slightly tuning the length of MMF. The periodical characteristic of this SA provides a flexible choice of MMF length, making it simple for simultaneous mode locking (SML) at two separate wavebands in practice. Our approach not only paves the way for SML at two or more wavebands by MMF, but also could lead to significant applications in pump-probe spectroscopy.

Incoherent broadband cavity enhanced absorption spectroscopy (IBBCEAS)-based strategy for direct measurement of aerosol extinction in lidar blind zone

Weidong Chen, Lingshuo Meng, Gaoxuan Wang, Patrick Augustin, Marc Fourmantin, Qian Gou, Eric Fertein, Tong Nguyen Ba, Cécile Coeur, and Alexandre Tomas

DOI: 10.1364/OL.389093 Received 24 Jan 2020; Accepted 10 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: In this letter, the development of a custom-designed incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) and its application to in situ measurement of aerosol extinction near ground surface is described, in an effort to address the issue of missing data in the lidar blind zone in the first hundreds of meters of the observation range. Combined measurements of aerosol extinction at the same location using lidar remote sensing and in situ IBBCEAS operating in the UV spectral region around 370 nm showed results with a good correlation (R2=0.90) between the two measurement techniques. This work highlights a new strategy for near-end lidar calibration, using a ground-based compact and robust IBBCEAS located at the lidar measurement site, to determine the vertical profile of the aerosol extinction coefficient with a higher accuracy.

Spectroscopy and high-power laser operation of monoclinic Yb3+:MgWO4 crystal

Weidong Chen, Pavel Loiko, Mengting Chen, Josep Maria Serres, Magdalena Aguilo, Francesc Diaz, Haifeng Lin, Zhang Ge, lizhen zhang, zhoubin lin, Patrice Camy, Shi-Bo Dai, Zhenqiang Chen, Uwe Griebner, Yongguang Zhao, Wang Li, Valentin Petrov, and Xavier Mateos

DOI: 10.1364/OL.389627 Received 04 Feb 2020; Accepted 10 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: Monoclinic (wolframite-type) monotungstate crystals are promising for rare-earth-doping. We report on polarized room- and low-temperature spectroscopy, and efficient high-power laser operation of such a Yb3+:MgWO4 crystal featuring high stimulated-emission cross-section (σSE = 6.2×10-20 cm2 at 1056.7 nm for light polarization E || Ng), large Stark splitting of the ground-state (765 cm-1), large gain bandwidth (26.1 nm for E || Ng) and strong Raman response (most intense mode at 916 cm-1). A diode-pumped Yb3+:MgWO4 laser generated 18.2 W at ~1056 nm with a slope efficiency of ~89% and a linearly polarized laser output.

Mid IR single-cycle light bullet self-reconstruction after an air gap under single-pulse femtosecond filamentation in LiF

Sergey Chekalin, Alexander Dormidonov, Valery Kandidov, and Viktor Kompanets

DOI: 10.1364/OL.382734 Received 11 Nov 2019; Accepted 09 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: The results of investigation of extremely compressed wave packet — ‘light bullet’ (LB) penetration through an air gap under single-pulse femtosecond Mid IR filamentation in LiF are presented. It is revealed by the laser coloration method and from numerical simulations that a single-cycle LB, which was formed before an air gap up to 0.5 mm width, completely recovered after passing some distance in LiF after the gap. This distance increases nonlinearly with the gap width and LB pathway before the gap. LB in the air gap has a strongly convergent wave front with focusing radius of 20 – 100 μm and its divergence after the waist is considerably less than that of Gaussian beam.

22.7 W mid-infrared supercontinuum generation in fluorotellurite fibers

Zhenrui Li, Zhixu Jia, C Yao, Zhipeng Zhao, Nan li, Ming-lie Hu, Yasutake Ohishi, WP Qin, and Guanshi Qin

DOI: 10.1364/OL.383642 Received 21 Nov 2019; Accepted 09 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: In this letter, we demonstrated 22.7 W mid-infrared supercontinuum (SC) generation in all solid fluorotellurite fibers. All solid fluorotellurite fibers based on TeO2-BaF2-Y2O3 and TeO2 modified fluoroaluminate glasses are fabricated by using a rod-in-tube method. By using a 0.6 m long fluorotellurite fiber with a core diameter of 11 μm as the nonlinear medium and a high power 1.93–2.5 μm SC fiber laser as the pump source, we obtain 22.7 W SC generation from 0.93 to 3.95 μm in the fiber for a pump power of 39.7 W. The 10 dB bandwidth is about 1633 nm, and the corresponding spectral range is from 1890 to 35 nm. The optical-to-optical conversion efficiency is about 57.2 %. Our results show that all solid fluorotellurite fibers are promising nonlinear media for constructing high power mid-infrared SC light sources.

Coherent mid-infrared supercontinuum generation in tapered suspended-core As39Se61 fibers pumped by few optical cycles Cr:ZnSe laser

Stanislav Leonov, Yuchen Wang, Vladimir Shiryaev, Gennady Snopatin, Boris Stepanov, Victor Plotnichenko, Edoardo Vicentini, Alessio Gambetta, Nicola Coluccelli, Cesare Svelto, Paolo Laporta, and Gianluca Galzerano

DOI: 10.1364/OL.386429 Received 24 Dec 2019; Accepted 09 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: We report on efficient supercontinuum generation in tapered suspended-core As39Se61 fibers pumped by a femtosecond mode-locked Cr:ZnSe laser. The supercontinuum spectrum spans the mid-infrared spectral region from 1.4 to 4.2 µm and its spectral coherence has been proved by heterodyning with a single-frequency narrow-linewidth Er-fiber laser at 1.55 µm, measuring a beat note with 27 dB signal to noise ratio in a resolution bandwidth of 100 kHz. The intensity stability of the supercontinuum radiation has been also characterized by relative intensity noise measurements.

Topological photonic crystal fibers and ring resonators

Laura Pilozzi, Daniel Leykam, Zhigang Chen, and Claudio Conti

DOI: 10.1364/OL.387043 Received 30 Dec 2019; Accepted 09 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: By an exact recursive approach we study photonic crystal fibers and resonators with topological features induced by the Aubry-Andre-Harper modulation of the cladding. We find non trivial gaps and edge states at the interface between regions with different topological invariants. The proposed structures show topological protection against symmetry-preserving local perturbations that do not close the gap and sustain strong field localization and energy concentration at a given radial distance. As topological light guiding and trapping devices, they may bring about many opportunities for both fundamentals and applications unachievable with conventional optical devices.

An integrated in-fiber coupler for whispering-gallery mode microsphere resonator

Dongning Wang, Xin Liu, and Xinlei Cui

DOI: 10.1364/OL.388166 Received 15 Jan 2020; Accepted 08 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: An integrated in-fiber coupler is proposed and demonstrated which can excite whispering-gallery mode (WGM) in a microsphere resonator. This device is fabricated firstly by using femtosecond laser micromachining and fusing splicing technique to create an inner air-cavity with a suspended fiber-core, then an open micro-channel is drilled on the top of the inner air-cavity, finally a microsphere is placed inside the air-cavity, in contacting with the suspended fiber-core, to excite the WGM through the evanescent field. In the transmission spectrum of the device, the slopes of the two asymmetric Fano resonance lines of 41.12 dB / nm and -18.46 dB / nm, respectively, and a symmetrical Lorentz line with a quality (Q)-factor of 9.32×103 can be obtained simultaneously. Such an in-fiber WGM microsphere resonator has the advantages of compact structure, convenient operation and high durability.

Polarization-controlled bifunctional metasurface for structural color printing and beam deflection

Yun Huang, Jia Zhu, Shengxiao Jin, Meizhang Wu, Xiaoyu Chen, and Wengang Wu

DOI: 10.1364/OL.387408 Received 13 Jan 2020; Accepted 07 Feb 2020; Posted 14 Feb 2020  View: PDF

Abstract: We proposed a polarization-controlled bifunctional metasurface composed of arrayed trapezoidal nanoantennas. Under orthogonal-polarized incidence, different types of gap-surface plasmons are generated, regulating the intensity and phase respectively. Thus, structural color printing and beam deflection functions are achieved on a miniaturized chip. The color printing function works from 400 nm to 800 nm, exhibiting subwavelength-scale chromatic image with broad gamut. The beam deflection function works from 360 nm to 540 nm, mapping light to the first diffraction order with the anomalous angle from 40.4° to 76.6°. The proposed bifunctional metasurface could serve as a key component in integrated optics systems and will find many other wide-ranging applications in optical and biological areas.

Optical phase of inhomogeneous Jones matrices: Retardance and ortho-transmission states

Julio Gutierrez-Vega

DOI: 10.1364/OL.387644 Received 08 Jan 2020; Accepted 07 Feb 2020; Posted 07 Feb 2020  View: PDF

Abstract: We determine the optical phase ψ (dynamic and geometric) introduced by a system described by an inhomogeneous Jones matrix. We show that there are two possible scenarios: (a) ψ has a finite range [ψmin, ψmax]. We calculate both limits and their corresponding polarization states analytically. (b) ψ spans the full range [-π, π]. This scenario leads to the existence of two input polarization states whose output states are orthogonal. We call these states ortho-transmission states (OTSs) and find them analytically. We study the inverse problem of designing an optical system with OTSs given by the user.

Compact high-efficiency four-mode vortex beam generator within the telecom C-band

Yuntao Zhu, Heyun Tan, Nan Zhou, Lifeng Chen, Jian Wang, and Xinlun Cai

DOI: 10.1364/OL.385878 Received 16 Dec 2019; Accepted 07 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: Vortex beams, carrying orbital angular momentum (OAM), have attracted a great deal of attention over the past few years. A compact, high-efficiency, broadband vortex beam generator is necessarily required in wide-ranging OAM-based applications. Here, we demonstrate a high-efficiency silicon-integrated vortex beam generator based on superposed holographic fork gratings. A metal mirror is used to enhance emission efficiency by reflecting the power leaking down to the substrate back to air. Experimental characterization confirms the emission efficiency of the generator increases by ~5 dB. Moreover, the generator shows the preferable features of broadband, polarization diversity and compact footprint.

Vector beams with controllable rotating local linear polarization during propagation

Hui-Tian Wang, Jia-Qi Lv, Xiaolei Wang, Guan-Lin Zhang, Chenghou Tu, and Yongnan Li

DOI: 10.1364/OL.385069 Received 04 Dec 2019; Accepted 07 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Based on the amplitude-phase joint modulation method, vector beams with controllable rotating local linear polarizations during propagation are generated, which in fact achieves the periodic arrangement of polarizations located in the equator of Poincare sphere in the longitudinal direction. In particular, the rotation direction and rate of polarizations for the vector beam can be controlled easily, by changing the radial indices and the intensity ratio of two superposed beams. A rotation angle of ~800 degrees has been achieved after a propagation distance of 120 mm, which corresponds to a rotation rate of ~6.7 degree/mm, which has been raised more 4 times than the previous works.

On-chip polarization-insensitive Fourier transform spectrometer

Huijie Wang, Wei Shi, and Qifeng Li

DOI: 10.1364/OL.385205 Received 06 Dec 2019; Accepted 07 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Chip-scale monolithic Fourier transform spectrometers (FTSs) offer great potential for inexpensive, high-resolution and high-robustness spectroscopic applications in a wide variety of scenarios. Having attracted considerable attention, spatial heterodyne FTSs (SH-FTSs) are featured with a simple and stable configuration composed of an array of Mach-Zehnder interferometers (MZIs) with linearly increasing optical path differences. Due to the strong waveguide birefringence, MZIs on the popular silicon-on-insulator platform are polarization-sensitive, raising the challenge of polarization control of incident light. In this letter, we propose and demonstrate a polarization-insensitive SH-FTS in which a two-dimensional grating coupler is introduced to split an arbitrary state of polarization into two orthogonal polarization components that are both coupled into the TE mode but propagate in opposite directions in the arrayed MZIs. The two orthogonal polarization components are finally recombined in photo-detection without polarization-dependent losses. An edge-coupling configuration using a polarization splitter-rotator is also proposed. This polarization-insensitive strategy enables a higher optical throughput as well as better versatility, free from the polarization limitation.

Gaussian-like transverse-mode profile characteristics of high-power large-area red-emitting VCSELs

Zhihua Huang, Michael Zimmer, Michael Jetter, and Peter Michler

DOI: 10.1364/OL.386525 Received 27 Dec 2019; Accepted 07 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: We demonstrate a large-area red-emitting VCSEL structure with a significant improvement of the uniformity in the charge carrier distribution by adopting a Si-doped Al₀.₂₀GaInP current spreading layer and a bottom disk contact. The new structure emitting at 670 nm with a bottom disk contact diameter of 20 μm was compared with the conventional oxide-confined top-emitting structure with a similar aperture size. The maximum output peak power increased from 8.8 mW to 22.5 mW under pulsed mode operation at room temperature. The transverse mode of the far field improved from a strong multiple mode pattern to a Gaussian-like profile. The corresponding divergence angle of the far-field pattern at the injection current of 2 times threshold reduced from 16. ° to 10.89°.

Maximizing absorption and scattering by spherical nanoparticles

Sergey Bozhevolnyi, Khachatur Nerkararyan, and Torgom Yezakyan

DOI: 10.1364/OL.387046 Received 02 Jan 2020; Accepted 07 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: The absorption and scattering resonances of metal nanostructures are often assumed to be defined by the same condition of localized surface plasmon resonance. Using an electrostatic approximation, we demonstrate that the absorption and scattering cross sections of spherical nanoparticles reach their maxima at different wavelengths, which in turn differ from that defined by the Fröhlich condition. These differences originate from and are proportional to the material dispersion and absorption. Our results provide the design guidelines for maximizing absorption and scattering of spherical nanoparticles, and are thus of special importance for applications, where the efficiency of radiation absorption or scattering is crucial.

Extremely robust femtosecond written fiber Bragg gratings for ytterbium doped fiber oscillator with 5 kW output power

Ria G. Krämer, Friedrich Möller, Christian Matzdorf, Thorsten A. Goebel, Maximilian Strecker, Maximilian Heck, Daniel Richter, Marco Plötner, Thomas Schreiber, Andreas Tünnermann, and Stefan Nolte

DOI: 10.1364/OL.389427 Received 29 Jan 2020; Accepted 06 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: We present highly robust fiber Bragg gratings in passive large mode area fibers for kilowatt fiber laser systems. The gratings were inscribed directly through the fiber coating using near infrared femtosecond laser pulses and then implemented in an all-fiber ytterbium doped single mode oscillator setup reaching up to 5 kW signal output power. The untreated cooled fiber Bragg gratings showed thermal coefficients as low as 1 K kW-1, proving excellent qualification for the implementation into robust high power fiber laser setups.

Statistical mechanics of weakly nonlinear optical multimode gases

Konstantinos Makris, Fan Wu, Pawel Jung, and Demetrios Christodoulides

DOI: 10.1364/OL.387863 Received 15 Jan 2020; Accepted 06 Feb 2020; Posted 07 Feb 2020  View: PDF

Abstract: By utilizing notions from statistical mechanics, we developa general and self-consistent theoretical frameworkcapable of describing any weakly nonlinear opticalmultimode system involving conserved quantities.We derive the fundamental relations that govern thegrand canonical ensemble through maximization of theGibbs entropy at equilibrium. In this classical pictureof statistical photo-mechanics, we obtain analytical expressionsfor the probability distribution, the grandpartition function, and the relevant thermodynamic potentials.Our results universally apply to any otherweakly nonlinear multimode bosonic system.

Promotion of pulse peak power by halving the repetition rate based on vector soliton.

Shijie Wang, Lei Liao, Ying-bin Xing, Haiqing Li, Jinggang Peng, Luyun Yang, Nengli Dai, and Jinyan Li

DOI: 10.1364/OL.384348 Received 26 Nov 2019; Accepted 06 Feb 2020; Posted 07 Feb 2020  View: PDF

Abstract: We report on an all-fiber mode-locked repetition-rate-switch pulse operation based on polarization rotation vector soliton in Yb-doped fiber laser (YDFL). The polarization controller (PC) in fiber loop and polarization isolator at output port are incorporated into the laser resonator as the switch of repetition rate. By adjusting the PC in cavity, the mode-locking can be switched between the fundamental repetition rate and a half of it with pulse width tiny change. And the halved pulse exhibits unique properties: a huge promotion in energy and peak power. As the best of our knowledge, that’s the first all-fiber seed resource with passive switch of repetition rate based on vector soliton.

Ultra-high capacity for three-dimensional optical data storage inside transparent fluorescent tape

Xu Peng Yuan, Miao Zhao, Xinjun Guo, Yao Li, Yang Yu, Zongsong Gan, and Hao Ruan

DOI: 10.1364/OL.387278 Received 03 Jan 2020; Accepted 06 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: In this paper, we show that an ultra-large capacity for three-dimensional optical data storage inside transparent fluorescent tape using two-photon absorption photobleaching method. We can obtain transparent fluorescent tape by the means of simple dip way. We successfully demonstrate recording and reading of six layers of binary data bits with lateral separation of 2 μm and longitudinal layer separation of 3μm. Thus, this result leads to a storage density of approximately 80 Gbits/cm3. Therefore, we can realize authentic ultra-high capacity optical data storage using pretty long transparent fluorescent tape in the future likes magnetic tape and fundamentally solve the data explosion disaster further.

Comparison studies of excitonic properties and multiphoton absorption of near infrared-I emitting Cu-doped InP and InP/ZnSe nanocrystals

Tingchao He, Huan Liu, Junzi Li, Shuyu Xiao, Wenbo Hu, Xin Qiu, Xiaodong Lin, and Yang Gao

DOI: 10.1364/OL.384876 Received 02 Dec 2019; Accepted 06 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: Cu-doped InP and InP/ZnSe nanocrystals (NCs) with near infrared-I (NIR-I) emission were prepared and characterized. Femtosecond transient absorption spectra revealed the epitaxial growth of a ZnSe diffusion barrier onto the Cu-doped InP core can amplify its exciton-dopant coupling strength, with the energy transfer times of ~220 ps for Cu:InP NCs and ~183 ps for Cu:InP/ZnSe NCs, respectively. Importantly, the Cu:InP NCs exhibit much larger two- and three-photon absorption cross-sections, reaching ~10162 GM at 1030 nm and ~1.06×10−77 cm6 s2 photon-2 at 1600 nm, compared with Cu:InP NCs.

Photonic generation of tunable dual-chirp microwave waveforms using a dual-beam optically injected semiconductor laser

Pei Zhou, Hao Chen, Nianqiang Li, Renheng Zhang, and Shilong Pan

DOI: 10.1364/OL.385527 Received 09 Dec 2019; Accepted 06 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: An approach to generating dual-chirp microwave waveforms (DCMWs) is proposed and experimentally demonstrated. The proposed scheme consists of a typical semiconductor slave laser, which is subject to a dual-beam optical injection from two master lasers with one being positively detuned (ML1) and the other negatively detuned (ML2) from the slave laser. Under proper injection conditions, the slave laser operates in the so-called Scenario B of dual-beam injection. After optical-to-electrical conversion, a dual-frequency microwave signal can be generated with one of its two frequencies increasing linearly and the other decreasing linearly as the ML1’s injection strength is increased. By incorporating an injection strength controller, a DCMW with a large time-bandwidth product can be generated. In the experimental demonstration, a DCMW in a temporal period of 1 μs has been obtained, which simultaneously offers an up-chirp (13.4 – 20.2 GHz) and a down-chirp (27.3 – 20.5 GHz), and its frequency tunability has been achieved by simply adjusting the injection parameters. Furthermore, auto-ambiguity function of the generated DCMW has also been investigated, which proves that the proposed scheme has the ability to improve the range-velocity resolution, and, thus, is very promising in modern radar systems.

High-speed and Broadband Digital Receiver Based on Optical Sampling Pulse Waveform Matching

Sitong Wang, Yiwei Sun, Liang Hu, Jianping Chen, and Guiling Wu

DOI: 10.1364/OL.385657 Received 10 Dec 2019; Accepted 06 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: We propose a high-speed and broadband photonic digital receiver which can realize the matched filtering of the digital signal through shaping the optical sampling pulse according to the specific waveform of the transmitted digital signal. The receiver’s filtering response is matched with the spectrum of digital signal’s specific waveform, and the instantaneous signal to average noise ratio of filtered signal is maximized at the sampling points. The principle of proposed receiver is theoretically analyzed, and experimentally verified. The weak digital signals with different signal-to-noise ratio are detected and correctly distinguished in experiments.

Demonstration of a simple technique for controllable orbital rotation of light-absorbing particles in air

Alexey Porfirev, Anna Dubman, and Denis Porfiriev

DOI: 10.1364/OL.386907 Received 26 Dec 2019; Accepted 06 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: The rotation of optically trapped particles is used in many applications for the realization of different micromechanical devices, such as micropumps, microrotors, and microgyroscopes, as well as for the investigation of particle interactions. Although for transparent micro-objects in both liquid media and vacuum, the rotation can easily be realized by transfer of the spin angular or orbital angular momentum from the light to the object, in the case of light-absorbing micro-objects in gaseous media, such transfers are insignificant in comparison with the thermal effects arising from the photo- and thermophoresis phenomena initiating the movement of trapped particles in a laser beam. Currently proposed methods using a single focused laser beam, tapered-ring optical traps, or single and multiple bottle beams have various limitations—for example, the inability to control the direction of the rotation of trapped particles or the low rotation frequency and small rotation angles. Here, we propose a simple method for the realization of the orbital rotation of airborne light-absorbing particles. The method is based on a combination of a circular diaphragm and a rotating cylindrical lens, enabling the generation of linear optical bottle beams. Our results show the flexibility and reliability of the proposed technique, allowing such laser traps to be used in various optical systems for the manipulation of micro-objects with different dimensions and shapes.

Digital implementation of TCSPC

Sucbei Moon, Byungjun Park, and young jae Won

DOI: 10.1364/OL.384461 Received 27 Nov 2019; Accepted 05 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: We present a digital instrumentation method of time-correlated single-photon counting (TCSPC). Pulsed signal of a single-photon sensitive photodetector is digitized by a high-speed analog-to-digital converter and digitally processed for determination of the photon detection times. We found that our digitally implemented TCSPC (dTCSPC) provides a smart way of discriminating valid photon pulses for reliable measurement of fluorescence lifetimes and time-resolved spectroscopy.

High-power and frequency-stable ultraviolet laser performance in space for the wind lidar on Aeolus

Oliver Lux, Denny Wernham, Paolo Bravetti, Phil McGoldrick, Olivier Lecrenier, Wolfgang Riede, Alessandro D'Ottavi, Valeria De Sanctis, Marc Schillinger, Jérémie Lochard, Jon Marshall, Christian Lemmerz, Fabian Weiler, Linda Mondin, Alessandra Ciapponi, Thomas Kanitz, Anders Elfving, Tommaso Parrinello, and Oliver Reitebuch

DOI: 10.1364/OL.387728 Received 08 Jan 2020; Accepted 05 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: Global acquisition of atmospheric wind profiles using a spaceborne direct-detection Doppler wind lidar is being accomplished since the launch of ESA’s Aeolus mission. One key part of the instrument is a single-frequency, ultraviolet laser which emits nanosecond laser pulses into the atmosphere. High output energy and frequency stability ensure sufficient signal-to-noise ratio of the backscatter return and accurate determination of the Doppler frequency shift induced by the wind. This paper discusses the design of the laser transmitter for the first Doppler wind lidar in space and its performance during the first year of the Aeolus mission, providing valuable insights for upcoming lidar space missions.

Photonic digital-to-analog conversion using a blue frequency chirp in a semiconductor optical amplifier

Takuya Okada, Ryuichi Kobayashi, Wang Rui, Masaki Sagara, and Motoharu Matsuura

DOI: 10.1364/OL.386541 Received 24 Dec 2019; Accepted 05 Feb 2020; Posted 07 Feb 2020  View: PDF

Abstract: In this paper, we present a photonic digital-to-analog conversion (DAC) technique based on blue-chirp spectral slicing using a semiconductor optical amplifier (SOA). Because the gain change in an SOA leads to a refractive-index change based on the change in intensity of the input data signal, probe signals experience a dynamic frequency shift to a shorter-wavelength side called blue-chirp. After passing through the SOA, the probe signals corresponding to the logic level of the input digital signal are extracted by filtering only the blue-chirp component of the probe signals using rectangular-shape filters. In this study, we experimentally demonstrate a 10-Gb/s, 2-bit photonic DAC from a 10-Gb/s digital signal with various data patterns to a four-level amplitude signal assuming an analog signal. In addition, we evaluate the resolution performance of the photonic DAC in terms of differential and integral nonlinearities and effective number of bits.

Vector beams in planar photonic crystal cavities with rotating air holes

Xiao mei Gao, Lechen Yang, Fang Bo, Jiafang Li, Guoquan Zhang, and Jingjun Xu

DOI: 10.1364/OL.381458 Received 29 Oct 2019; Accepted 04 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: We report a method to generate angularly polarized vector beams with a topological charge of one by rotating air holes to form two-dimensional (2D) photonic crystal (PC) cavities. The mode volume and resonance wavelength of these cavities are tuned from 0.33(λ/n)3 to 12(λ/n)3 and in a wide range of 400 nm, respectively, by controlling the range of fixed air holes near the center of the structure. As a benefit, the half maximum divergence angles of the vector beam can be widely changed from 90° to ~60°. By adjusting the “phase” of the air holes in the PC cavities, optical vector beams with different far-field morphology are obtained. The scheme not only provides an alternative method to generate optical vector beams, but also provides an effective strategy to control far-field morphology and polarizations, which holds promising applications such as optical microscopy and micro-manipulation.

Higher-order QAM Data Transmission Using a High-Coherence Hybrid Si/III-V Semiconductor Laser

Kaiheng Zou, Zhewei Zhang, Peicheng Liao, Huolei Wang, Yinwen Cao, Ahmed Almaiman, Ahmad Fallahpour, Fatemeh Alishahi, Naresh Satyan, George Rakuljic, Moshe Tur, Amnon Yariv, and Alan Willner

DOI: 10.1364/OL.383137 Received 19 Nov 2019; Accepted 04 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: We experimentally demonstrate the use of a high-coherence hybrid Si/III-V semiconductor laser as the light source for a transmitter generating 20-Gbaud 16- and 64- quadrature amplitude modulated (QAM) data signals over an 80-km single-mode fiber (SMF) link. The hybrid Si/III-V laser has a measured Schawlow-Townes linewidth of ~ 10 kHz, which is achieved by storing modal optical energy in low-loss Si rather than the relatively lossy III-V materials. We measure a received bit error rate (BER) of 4.1×10-3 when transmitting the 64-QAM data over an 80-km SMF using the hybrid Si/III-V laser. Furthermore, we measure a BER of < 1×10-4 with the Viterbi-Viterbi digital carrier phase recovery method when transmitting the 16-QAM data over an 80-km SMF using the hybrid Si/III-V laser. This performance is achieved at power penalties lower than those obtained with an exemplary distributed feedback laser, and slightly higher than those with an exemplary narrow-linewidth external cavity laser.

Linear and nonlinear optical properties of co-sputtered Ge-Sb-Se amorphous thin films

Tomas Halencovic, JAN GUTWIRTH, Tintu Kuriakose, Marek Bouska, Mathieu Chauvet, Gilles Renversez, Petr Nemec, and Virginie Nazabal

DOI: 10.1364/OL.386775 Received 02 Jan 2020; Accepted 04 Feb 2020; Posted 13 Feb 2020  View: PDF

Abstract: Amorphous thin films, co-sputtered using GeSe4 and Sb2Se3 targets, were investigated for potential applications in the field of nonlinear optics. Depending on the sputtered film composition, linear optical properties were studied by ellipsometry. Kerr coefficient and two-photon absorption coefficient were estimated using Sheik-Bahae’s formalism for co-sputtered films of GeSe4-Sb2Se3 compared to GeSe2-Sb2Se3 pseudo-binary system and As2Se3 as reference. Kerr coefficient was found within the range of 4.9-21×10-18 m2W-1. Quantitatively by means of figure of merit at 1.55 μm, thin films with compositions of Ge7Sb25Se68 and Ge9Sb20Se71 having estimated Kerr coefficient of about 10.1×10-18 m2W-1 and 13.4×10-18 m2W-1 should be considered for the future nonlinear optical integrated platforms. Such compositions being close to (GeSe4)50(Sb2Se3)50 pseudo-binary (i.e. Ge7.5Sb25.0Se67.5) provides just the trade-off between high Kerr coefficient and low optical losses related to two-photon absorption.

Multiband 5G NR system with photonic-assisted RF amplification

Ramon Borges, Eduardo Lima, Alexandre Ferreira, Danilo Spadoti, Marcelo Abreu, Luciano Mendes, and Arismar Cerqueira Sodre Junior

DOI: 10.1364/OL.386606 Received 23 Dec 2019; Accepted 04 Feb 2020; Posted 11 Feb 2020  View: PDF

Abstract: We propose and report the implementation of a multiband and photonically-amplified 5G new radio (5G NR) system based on radio over fiber (RoF) technology and four-wave mixing (FWM) nonlinear effect. A piece of highly nonlinear fiber has been employed to stimulate FWM, which enables photonics-assisted RF amplification up to millimeter waves (mm-waves). Experimental results demonstrate uniform and stable 15 dB ultrawideband gain for a 4G and three 5G signals, in the frequency range from 780 MHz to 26 GHz, coexisting in the transport network. The obtained digital performance has efficiently met the 3GPP requirements, demonstrating the applicability of the proposed approach for using fiber- optic links to either distribute and jointly amplify 4G and 5G signals in the optical domain.

Refractive twisted microaxicons

Stanislav Krasnov, Svetlana Khonina, Andrey Ustinov, Sergey Degtyarev, Alexey Porfirev, Aleksandr Kuchmizhak, and Sergey Kudryashov

DOI: 10.1364/OL.386223 Received 17 Dec 2019; Accepted 04 Feb 2020; Posted 04 Feb 2020  View: PDF

Abstract: Complex-shaped light fields with a specially designed intensity-, phase- and polarization distributions are highly demanded for various applications including optical tweezers, laser material processing and lithography. Here, we proposed novel optical element formed by twisting of a conic surface, a twisted microaxicon, allowing to controllably generate high-quality spiral-shape intensity patterns. Performance of the proposed element was analyzed both analytically and numerically using ray approximation and rigorous finite difference time domain (FDTD) solution of Maxwell’s equation. The main geometric parameters, an apex cone angle and a degree of twisting, were considered to control and optimize the generated spiral-shaped intensity patterns. 3D structure of such microaxicon cannot be described by an unambiguous height function; therefore, it has no any diffraction analogue in the form of a thin optical element. Such element can be produced via direct laser ablation of transparent targets with structured laser beams or direct laser writing via two-photon photopolymerization and can be used in various micro- and nanooptical applications.

Optical microwave generation with ultra-stable Fabry-Perot cavity in a laser diode self-injection loop

Fasong Zheng, Fang Fang, Weiliang Chen, Shaoyang Dai, Kun Liu, and Tianchu Li

DOI: 10.1364/OL.388989 Received 23 Jan 2020; Accepted 04 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: The optical microwave generation scheme with an ultra-stable Fabry-Perot cavity (USC) in a self-injection loop of laser diode (LD) is proposed and experimentally demonstrated for the first time. Using an USC as the mode selector, an injection light is obtained with coherent and equidistant comb-like modes, which are separated by integer multiples of the free spectral range (FSR) of the USC in frequency domain. After injecting back to the LD, the lasing modes are referenced to these comb-like modes, and the microwaves with frequencies of integer multiples of the USC FSR are obtained from the beating signal of detection photodiode. For the microwaves at frequencies of one FSR and two FSRs, the signal-to-noise ratios (SNRs) are better than 60 dB and the 3-dB linewidths are below 16 Hz. The phase noises and the frequency stability of the generated microwave are also investigated experimentally.

GeSn Resonant-Cavity-Enhanced Photodetectors for Efficient Photodetection at the 2μm Wavelength Band

Guo-En Chang, Cheng-Hsun Tsai, BO-JUN HUANG, Richard Soref, Gregory Sun, and Henry Cheng

DOI: 10.1364/OL.381960 Received 06 Nov 2019; Accepted 04 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: The 2-µm wavelength band has recently gained increased attention for potential applications in next-generation optical communication. Here we present an investigation of GeSn resonant-cavity-enhanced photodetectors (RCEPDs) on silicon-on-insulator substrates for efficient photodetection at the 2-µm wavelength band. Narrow-bandgap GeSn alloys are used as the active layer to extend the photodetection range to cover the 2-µm wavelength band, and the optical responsivity is significantly enhanced by theresonant cavity effect as compared to a reference GeSn photodetector. Temperature-dependent experiments demonstrate that the GeSn RCEPDs can have a wider photodetection range and higher responsivity in the 2-µm wavelength band at higher temperatures because of the bandgap shrinkage. These results suggest that our GeSn RCEPDs are promising for CMOS-compatible, efficient, uncooled optical receivers in the 2-µm wavelength band for a wide range of applications.

Real-time measurement of complex fast signals by bandwidth compression in frequency shifting loops

Hughes Guillet de Chatellus, Côme Schnebelin, Luis Romero Cortes, and Jose Azana

DOI: 10.1364/OL.385000 Received 03 Dec 2019; Accepted 04 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: We report coherent time-to-frequency mapping in frequencyshifting loops (FSLs). We show that whenseeded by a temporal signal shorter than the inverseof the frequency shift per roundtrip, the optical spectrumat the FSL output consists of a periodic replica ofthe input waveform, whose amplitude and phase aremapped into the frequency domain. We provide anexperimental demonstration of this phenomenon, andshow how this simple set-up enables to monitor fastnon-repetitive input signals with a detection chain twoorders of magnitude slower than the input signal.

Generation of optical triangular-shaped pulse train with variable symmetry by using an I/Q modulator

Jing Li, Chuangye Wang, Li Pei, Tigang NIng, Jingjing Zheng, yujian li, and ruisi he

DOI: 10.1364/OL.386910 Received 27 Dec 2019; Accepted 04 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: A setup for optical triangular-shaped pulse train with variable symmetry is proposed. The key component is an I/Q modulator. By properly setting the three variables, the optical intensity can be expressed by the sum of infinite sinusoidal harmonics, which indicates the possibility to approximate the asymmetrical waveform. It is found that the scheme is capable for generating optical triangular-shaped pulse train with tunable symmetrical coefficient (20%≤δ≤80%) and low fitting error (η≤5%).

Inverse design of plasmonic metasurfaces by convolutional neural network

Ronghui Lin, Yanfen Zhai, Chenxin Xiong, and Xiaohang Li

DOI: 10.1364/OL.387404 Received 21 Jan 2020; Accepted 03 Feb 2020; Posted 06 Feb 2020  View: PDF

Abstract: Artificial neural networks have shown effectiveness in the inverse design of nanophotonic structures, however, the numerical accuracy and algorism efficiency are not analyzed adequately in previous reports. In this letter, we demonstrate the convolutional neural network as an inverse design tool to achieve high numerical accuracy in plasmonic metasurfaces. A comparison of the convolutional neural networks and the fully connected neural networks show that convolutional neural networks have higher generalization capabilities. We share practical guidelines for optimizing the neural network and analyzed the hierarchy of accuracy in the multi-parameter inverse design of plasmonic metasurface. A high inverse design accuracy of ±8 nm for the critical geometrical is demonstrated.

Effects of magnetodipole corrections on the spectra of spheroidal whispering gallery mode resonators interacting with a dielectric nanoparticle

Lev Deych and Lan Yang

DOI: 10.1364/OL.385993 Received 13 Dec 2019; Accepted 03 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: Interaction between whispering-gallery-mode (WGM) resonators and nanoparticles is an area of active interest in fundamental understanding of nanoparticle-induced spectral modifications of the WGM resonances in sensing applications. Existing theories of this phenomenon assumed that nanoparticles can be described taking into account only electrodipole contribution to the field of the nanoparticle. In this paper we explore theoretically effects of the magnetodipole contribution to the nanoparticle's field and show that this contribution might become significant even in situations when electrodipole approximation is expected to remain valid.

Optical Vortex Braiding by Perturbation of a Bessel Beam

Andrew Voitiv, Jasmine Andersen, Mark Siemens, and Mark Lusk

DOI: 10.1364/OL.388032 Received 13 Jan 2020; Accepted 03 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: We theoretically propose and experimentally demonstrate the braiding of optical vortices in a laser beam with more than 2π rotation by superposing Bessel modes with a plane wave. The amount of braiding is limited only by the numerical aperture of the system and the resolution of the digital holography used to generate the modes. We present experimental measurement of three complete braids.

Proposal for detecting ring current via electron vortices

Qingbin Zhang, Run Wang, Chen Ran, Wei Cao, and Peixiang Lu

DOI: 10.1364/OL.388516 Received 17 Jan 2020; Accepted 03 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: In an intense circularly polarized laser field, the excitation of the atoms shows a strong dependence on the orbital helicity. The resonant excitation starting from the ground state with m=-1 occurs much more easily in the left-handed circularly polarized (LCP with m=+1) pulse than in the right-handed circularly polarized (RCP with m=-1) pulse, vice versa. Here we numerically demonstrate that the orbital helicity dependent excitation leads to the photoelectron vortex pattern in polarization plane being sensitive to the sequence of the two counter-rotating circularly polarized (CRCP) pulses, which enables us to detect the ring currents associate with different quantum states. This results also provide an effective way for controlling the rotational symmetry of the electron vortex.

Spatially entangled Airy photons

Ohad Lib and Yaron Bromberg

DOI: 10.1364/OL.388692 Received 22 Jan 2020; Accepted 03 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: Over the past decade, Airy beams have been the subject of extensive research, leading to new physical insights and various applications. In this letter, we extend the concept of Airy beams to the quantum domain. We generate entangled photons in a superposition of two-photon Airy states via spontaneous parametric down conversion, pumped by a classical Airy beam. We show that the entangled Airy photons preserve the intriguing properties of classical Airy beams, such as free acceleration and reduced diffraction, while exhibiting nonclassical anti-correlations. Finally, we discuss the advantages offered by entangled Airy photons for high dimensional free-space quantum communications.

A 3D High Resolution Generative Deep-learning Network for Fluorescence Microscopy Image

Shaoqun Zeng, Hang Zhou, Ruiyao Cai, Tingwei Quan, Shijie Liu, Shiwei Li, Qing Huang, and Ali Erturk

DOI: 10.1364/OL.387486 Received 06 Jan 2020; Accepted 02 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: Microscopic fluorescence imaging serves as a basic tool in many research areas including biology, neuroscience, medicine and chemistry. With the help of optical clearing, large volume imaging of mouse brain or even whole body has been enabled. However, constrained by the physical principles of optical imaging, volume imaging has to balance imaging resolution and speed. Here, we develop a new 3D deep learning network based on dual generative adversarial network (dual-GAN) framework for recovering high resolution (HR) volume images from high speed acquired low resolution (LR) volume images. This framework can free from precise image registration process and meanwhile guarantee the predicted HR volume image faithful to its corresponding LR volume image. The results demonstrated our method can recover 20 /1.0-NA volume images from coarse registered 5 /0.16-NA volume images collected by light-sheet microscopy. This method would provide great potential in applications which require high resolution volume imaging.

Optical Circular Dichroism Engineering in Chiral Metamaterials utilizing a Deep Learning Network

Tian Jiang, Zilong Tao, Jie You, Jun Zhang, Xin Zheng, and HENGZHU LIU

DOI: 10.1364/OL.386980 Received 30 Dec 2019; Accepted 02 Feb 2020; Posted 05 Feb 2020  View: PDF

Abstract: Here, a deep learning (DL) algorithm based on deep neural networks is proposed and employed to predict the chiroptical response of two-dimensional (2D) chiral metamaterials. Specifically, these 2D metamaterials contain nine types of left-handed nanostructure arrays, including U-like, T-like and I-like shapes. Both the traditional rigorous coupled wave analysis (RCWA) method and DL approach are utilized to study the circular dichroism (CD) in the higher-order diffraction beams. One common feature of these chiral metamaterials is that they all exhibit the weakest intensity but the strongest CD response in the third-order diffracted beams. Our work suggests that the DL model can predict CD performance of 2D chiral nanostructure with a computational speed that is four orders of magnitude faster than RCWA but remain high accuracy. The DL model introduced in this work shows great potentials in exploring various chiroptical interactions in metamaterials and accelerating the design of hyper-sensitive photonic devices.

Harmonically-decoupled gradient light interference microscopy of transparent specimens

Michael Fanous, Gabriel Popescu, Chenfei Hu, Mikhail Kandel, hsuanyu chen, Xiaoxu Lu, and wang yi

DOI: 10.1364/OL.379732 Received 04 Oct 2019; Accepted 01 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: Differential phase sensitive methods, such as Nomarski microscopy, play an important role in quantitative phase imaging (QPI) due to their compatibility with partially coherent illumination and excellent optical sectioning ability. In this work, we propose a new system to retrieve differential phase information from transparent samples. It is based on a 4f optical system with an amplitude type spatial light modulator (SLM), which removes the need for traditional DIC optics and specialized phase-only SLMs. The proposed system is both easy to implement as add-on to existing microscopes and cost efficient. We demonstrate the principle of HD-GLIM using standard samples, as well as static and dynamic biospecimens

Reconstruction of an Electromagnetic Gaussian Schell-model Source from Far-zone Intensity Measurements

Taco Visser and David Kuebel

DOI: 10.1364/OL.385644 Received 11 Dec 2019; Accepted 01 Feb 2020; Posted 03 Feb 2020  View: PDF

Abstract: An electromagnetic Gaussian Schell-model source that produces a random beam may be characterized by eight independent quantities. We show how far-zone measurements of the Stokes parameters, together with the Hanbury Brown-Twiss coefficient, allows one to determine all the source parameters. This method provides a new tool to identify distant sources.

LED array reflectance microscopy for scattering-based multi-contrast imaging

Weiy Song, Alex Matlock, Sipei Fu, Xiaodan Qin, Hui Feng, Christopher Gabel, Lei Tian, and Ji Yi

DOI: 10.1364/OL.387434 Received 09 Jan 2020; Accepted 01 Feb 2020; Posted 10 Feb 2020  View: PDF

Abstract: LED array microscopy is an emerging platform for computational imaging with significant utility for biological imaging. Existing LED array systems often exploit transmission imaging geometries of standard brightfield microscopes that leave the rich backscattered field undetected. This backscattered signal contains high-resolution sample information with superb sensitivity to subtle structural features that make it ideal for biological sensing and detection. Here, we develop an LED array reflectance microscope capturing the sample’s backscattered signal. In particular, we demonstrate multimodal brightfield, darkfield, and differential phase contrast imaging on fixed and living biological specimens including Caenorhabditis elegans (C. elegans), zebrafish embryos, and live cell cultures. Video-rate multimodal imaging at 20Hz records real-time features of freely-moving C. elegans and the fast beating heart of zebrafish embryo. Our new reflectance mode is a valuable addition to the LED array microscopic toolbox.

Efficient terahertz polarization conversion with hybrid coupling of chiral metamaterial

Jing Wang, Hao Tian, Shuai Li, Li Li, Guanchao Wang, Jiaojiao Gao, Wenpeng Guo, and Zhongxiang Zhou

DOI: 10.1364/OL.388722 Received 20 Jan 2020; Accepted 31 Jan 2020; Posted 03 Feb 2020  View: PDF

Abstract: We propose an ultrathin terahertz waveplate of bilayer chiral metamaterial for cross-polarization conversion at asymmetric transmission. The chiral metamaterial is constructed with hybrid-coupling plasmonic resonators of concentric ring and double-split ring. The terahertz metamaterial can efficiently convert the y-polarized wave into the x-polarized wave with the cross-polarized transmittance over 97% and the polarization conversion ratio of 99% in simulation. The asymmetric transmission parameter, defined by the difference between two opposite propagating transmittances, can be as high as 0.9. The operation frequency and efficiency are geometrically adjustable with the ring size by exploiting hybrid coupling effect of electric and magnetic resonances. The presented metamaterial enables the functionality of nonreciprocal terahertz waveplate with high isolation.

Ultra-high-sensitivity refractive index sensor based on dual-microfiber coupler structure with Vernier effect

Pengfei Wang, Yuxuan Jiang, yating yi, and Gilberto Brambilla

DOI: 10.1364/OL.385345 Received 12 Dec 2019; Accepted 31 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: We demonstrate a novel refractive index sensor based on the Vernier effect in dual microfibre coupler (MFC) structures. The sensor sensitivity was studied both theoretically and experimentally. The numerical results show that by tracing the wavelength shifts of the envelope formed by the Vernier effect, the sensitivity can be improved by several times compared to that obtained for normal coupler-based sensors. In this paper, two MFCs with a width and free spectral range (FSR) of ~3.5 µm and 6 nm, respectively, were fabricated. Based on the sensitivity of 5820 nm/RIU for a single coupler, we experimentally achieved an ultra-high sensitivity of 126,540 nm/RIU using dual MFCs by the Vernier effect, which shows good agreement with numerical simulations. The proposed all-fibre refractive index sensor has the advantages of high sensitivity and low cost, and can find applications in chemical and biological detection as well as electronic/magnetic field measurement.

Asymmetric coherence gratings

Olga Korotkova and Zhangrong Mei

DOI: 10.1364/OL.388093 Received 13 Jan 2020; Accepted 31 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: We introduce a class of partially coherent, Schell-type sources whose degree of coherence is represented by a finite series of complex-valued functions. The significance of implementing such a series is due to the fact that one can manipulate the weighting coefficients of the series with terms having a computationally trivial linear phase of the degree of coherence for obtaining the radiated beams of the same complexity as could only be previously achieved with analytically intractable non-linear phases. Our examples illustrate new opportunities for modeling asymmetric coherence gratings and lattices.

Spontaneous Parametric Down Conversion in a Doubly Resonant One-dimensional Photonic Crystal

Viola Introini, Michael Steel, John Sipe, Lukas Helt, and Marco Liscidini

DOI: 10.1364/OL.385741 Received 12 Dec 2019; Accepted 30 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: We study spontaneous parametric down conversion (SPDC) in a one-dimensional photonic crystal designed to operate in a doubly resonant configuration, where the frequencies of the pump and the generated photons are both tuned to band-edge resonances. We investigate the spectral correlations of the generated photons as a function of the spectral width of the pump, and demon- strate that in this structure the SPDC generation rate can scale with L5, where L is the structure length. We show that such an unusual scaling can be simply con- nected with the scaling of second harmonic generation in the same structure, illustrating the general link be- tween spontaneous and stimulated parametric nonlin- ear processes.

Instabilities and intermittence in a dissipative soliton-similariton laser using a scalar iterative map

Fanchao Meng, Coraline Lapre, Cyril Billet, Goëry Genty, and John Dudley

DOI: 10.1364/OL.386110 Received 16 Dec 2019; Accepted 30 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: Numerical simulations of a dissipative soliton-similariton laser are shown to reproduce a wide range of instabilities seen in recent experiments. The model uses a scalar nonlinear Schrodinger equation map with gain and saturable absorption, and regions of stability and instability are readily identified as a function of gain and saturable absorber parameters. Studying pulse evolution over multiple roundtrips reveals spectral instabilities linked with soliton molecule internal motion, soliton explosions, chaos, and intermittence. For the particular case of soliton molecules, the relative phase variation seen in the spectrum is shown to be due to differences in nonlinear phase evolution between the molecule components over multiple roundtrips. These results contribute to understanding the essential physics of dissipative soliton instabilities.

Coupled Edge Modes Supported by a Microwave Metasurface

Julia de Pineda, Alastair Hibbins, and J. Sambles

DOI: 10.1364/OL.384639 Received 29 Nov 2019; Accepted 30 Jan 2020; Posted 03 Feb 2020  View: PDF

Abstract: Microwave metasurfaces comprised of overlapping layers of circular patches arranged in an hexagonal array are found to support edge modes akin to edge plasmons. The coupling of these edge modes across small gaps between two such arrays are explored.

High power, twin-band mid-infrared PPMgLN optical parametric oscillator pumped at 1.679 μm

Yong Wang, peng yuefeng, and Jianing Zhang

DOI: 10.1364/OL.388781 Received 23 Jan 2020; Accepted 29 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: We report on a high-power, twin-band mid-infrared (MIR) periodically poled MgO-doped LiNbO3 (PPMgLN) optical parametric oscillator (OPO) pumped by a near-infrared pulsed OPO with 20 kHz of repetition rate. The pump source has central wavelength of 1.679 μm and linewidth of 0.12 nm. The MIR OPO can be tuned from 2.74 to 2.80 μm for signal and from 4.34 to 4.19 μm for idler through raising the oven temperature from 30 to 200℃, respectively. Under 100 °C oven temperature, the OPO yielded maximum total output power of 15.4 W (2.76 μm signal plus 4.29 μm idler) with single-pass pumping configuration. The beam quality M2 was measured to be <2.5 for signal and <4 for idler.

Underwater 3D deblurring-gated range-intensity correlation imaging

Minmin Wang, Xinwei Wang, Liang Sun, Yuqing Yang, and Yan Zhou

DOI: 10.1364/OL.385983 Received 17 Dec 2019; Accepted 29 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: 3D range-gated imaging has great potentials in underwater target detection, navigation, and marine scientific research due to good backscatter suppression. However, in turbid water apparent backscatter noise leads to bad range resolution and accuracy in 3D reconstruction. To solve this problem, a 3D deblurring-gated range-intensity correlation imaging method is proposed based on light propagation property in water. In the method, only the water attenuation coefficient and a reference image are needed to calculate the depth-noise maps (DNM) of target gate images at different ranges. By subtracting the DNMs from target gate images, new gate images with less noise can be obtained, and then 3D images with high range resolution and accuracy are reconstructed. To prove the feasibility of the proposed method, experiments have been performed in pools at different water conditions.

Experimental observation of internally-pumped parametric oscillation and quadratic comb generation in a χ² whispering-gallery-mode microresonator

Ian Hendry, Luke Trainor, Yiqing Xu, Stephane Coen, Stuart Murdoch, Harald Schwefel, and Miro Erkintalo

DOI: 10.1364/OL.385751 Received 12 Dec 2019; Accepted 29 Jan 2020; Posted 30 Jan 2020  View: PDF

Abstract: We report on the experimental observation of internally-pumped parametric oscillation in a high-Q lithium niobate microresonator under conditions of natural phase-matching. Specifically, launching near-infrared pump light around 1060 nm into a z-cut congruent lithium niobate microresonator, we observe the generation of optical sidebands around the input pump under conditions where second-harmonic generation is close to natural phase-matching. We find that a wide range of different sideband frequency shifts can be generated by varying the experimental parameters. Under particular conditions, we observe the cascaded generation of several equally-spaced sidebands around the pump -- the first steps of optical frequency comb generation via cavity-enhanced second-harmonic generation.

Slow light using magnetic and electric Mie resonances

Søren Raza

DOI: 10.1364/OL.386781 Received 30 Dec 2019; Accepted 29 Jan 2020; Posted 30 Jan 2020  View: PDF

Abstract: The ability to slow down light leads to strong light-matter interaction, which is important for a number of optical applications such as sensing, nonlinear optics and optical pulse manipulation. Here, we show that a dramatic reduction of the speed of light can be realized through the interference of electric and magnetic dipole resonances in Mie-type resonators made of a dielectric material with a high refractive index. We present a general theory which links the maximal speed reduction of light to the resonator radiation losses and then consider a specific realization based on silicon nanodisk arrays.

Multi-layer full-field phase imaging using continuous-wave terahertz ptychography

Dayong Wang, Bing Li, Lu Rong, Fangrui Tan, John Healy, Jie Zhao, and Yunxin Wang

DOI: 10.1364/OL.384589 Received 29 Nov 2019; Accepted 29 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: Due to the unique propagation properties of terahertz (THz) waves, THz phase imaging is widely investigated to retrieve the absorption and phase information of dielectric two-dimensional thin samples as well as multiple stacked samples. In this letter, we apply the 3D-PIE (three-dimensional ptychographic iterative engine) algorithm for continuous-wave THz full-field multi-layer phase imaging. The complexed-valued transmittance function of two-layer polypropylene thin plates and the corresponding probe function are reconstructed respectively, which are immune from superposition. The phenomenon of field-of-view enlargement at the second object layer is observed and analyzed. This lensless compact imaging method can be potentially used for THz three-dimensional imaging.

Light-efficient beamsplitter for Fourier-domain full-field optical coherence tomography

Egidijus Auksorius

DOI: 10.1364/OL.383823 Received 21 Nov 2019; Accepted 29 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: Fourier-domain full-field optical coherence tomography (FD-FF-OCT) is currently the fastest volumetric OCT imaging technique. However, it wastes almost 75% of light, including 50% of OCT signal, because it uses a 50/50 beamsplitter in the standard implementation. Here, a design of a light-efficient beamsplitter is presented that losses almost no light when implemented in FD-FF-OCT. It is based on pupil engineering and a small highly asymmetric beamsplitter. The presented signal-to-noise ratio (SNR) analysis demonstrates almost 4 times improvement over the conventional design. In addition, it is shown that the light-efficient beamsplitter can be used to suppress specular reflections from a sample and potentially reduce the autocorrelation noise, which can further improve the SNR.

The statistical distribution of polarization-dependent-loss in systems characterized by the hinge model

Mark Shtaif and Segev Zarkovsky

DOI: 10.1364/OL.385525 Received 10 Dec 2019; Accepted 29 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We study the statistics of polarization dependent loss in systems where this phenomenon is dominated by discrete optical components. Exact expressions for the probability distribution of the polarization-dependent loss and of the overall link attenuation are reported here for the first time.

Quantitative strain sensing in a multimode fiber using dual frequency speckle pattern tracking

Matthew Murray and Brandon Redding

DOI: 10.1364/OL.383569 Received 17 Nov 2019; Accepted 29 Jan 2020; Posted 30 Jan 2020  View: PDF

Abstract: We report an amplitude-measuring multimode fiber sensor capable of making quantitative strain measurements and extracting the algebraic sign of the strain. The Rayleigh-based sensor probes the fiber with pulses of alternating optical frequency and records the backscattered speckle patterns on a high-speed camera. We show that measuring the change in the speckle pattern induced by a change in optical frequency provides a form of in-situ calibration, enabling the sensor to recover the magnitude and algebraic sign of the strain. The sensor, which can be positioned anywhere along 2km of fiber, has a linear strain response, a 10 kHz bandwidth, and a strain noise of 10.2 pε/√Hz.

Low-loss and broadband fiber-to-chip coupler by 3D fabrication on silicon photonic platform

ZHI-HAO LUO, Fei Xie, Yao-Yu Cao, Siyuan Yu, Lifeng Chen, and Xinlun Cai

DOI: 10.1364/OL.386550 Received 23 Dec 2019; Accepted 29 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We propose and demonstrate a low loss fiber-to-chip vertical coupler on the silicon photonic platform by using a 3D two-photon fabrication method. Such a coupler significantly reduces insertion loss, measured to be 1 dB, and provides a wide working wavelength range for both TE and TM polarizations over the entire C-band. Moreover, a large tolerance for misalignment of the coupling fiber, up to 4.5 μm for a 1 dB loss, enables the development of relaxed alignment techniques.

Shear-unlimited common-path speckle interferometer

Jie Dong, Shengjia Wang, Ali Yetisen, Xingchen Dong, Franziska Pöller, Nicholas Ong, Martin Jakobi, Zhanwei Liu, Felix Salazar-Bloise, and Alexander Koch

DOI: 10.1364/OL.382893 Received 12 Nov 2019; Accepted 28 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: A single-aperture common-path speckle interferometer with an unlimited shear amount is developed. This unlimited shear amount is introduced when a Wollaston prism is placed near the Fourier plane of a common-path interferometer, which is built by using a quasi-4f imaging system. The fundamentals of the shear amount and the spatial carrier frequency generation are analyzed mathematically, and the theoretical predictions are validated by a static experiment. Mode-I fracture experiments through the three-point bending are conducted to prove the feasibility and the capability of this method in full-field strain measurement with various shear amounts. A remarkable feature of this setup is that no tilt is required between the optical elements to produce the unlimited shear amount in off-axis holography.

Dual-chirp microwave waveform transmitter with elimination of power fading for one-to-multi base stations fiber transmission

Wei Li, Sha Zhu, xiaojie fan, Ming Li, and Ninghua Zhu

DOI: 10.1364/OL.386474 Received 19 Dec 2019; Accepted 28 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We present a photonic approach to generate and transmit a dual-chirp microwave waveform with anti-dispersion performance. Traditionally, the microwave signal generated based on double-sideband (DSB) modulation suffers from power fading significantly. We propose a DSB-based dual-chirp microwave waveform transmitter which can eliminate the chromatic dispersion-induced power fading (CDIP) over fiber transmission. The CDIP elimination rather than compensation ensures that the working bandwidth of the dual-chirp waveform is not limited by the periodical power fading. The proposed signal modulation scheme makes the signal transmitter free from the direct current (DC) bias drifting of the modulator. Moreover, thanks to the phase modulation, the generated waveform is background free. The proposed dual-chirp waveform transmitter features compact structure, polarization independent and CDIP elimination, which has great potential in radars for one-to-multi base stations fiber transmission.

Computational aberration correction in spatiotemporal optical coherence (STOC) imaging

Dawid Borycki, Egidijus Auksorius, Piotr Wegrzyn, and Maciej Wojtkowski

DOI: 10.1364/OL.384796 Received 13 Dec 2019; Accepted 28 Jan 2020; Posted 31 Jan 2020  View: PDF

Abstract: Spatiotemporal optical coherence (STOC) imaging is a new technique for suppressing coherent crosstalk noise in Fourier-domain full-field optical coherence tomography (FD-FF-OCT). In STOC imaging, the time-varying inhomogeneous phase masks modulate the incident light to alter the interferometric signal. Resulting interference images are then processed as in standard FD-FF-OCT and averaged incoherently or coherently to produce crosstalk-free volumetric OCT images of the sample. Here, we show that coherent averaging is suitable when phase modulation is performed for both interferometer arms simultaneously. We explain the advantages of coherent over incoherent averaging. Specifically, we show that modulated signal, after coherent averaging, preserves lateral phase stability. This enables computational phase correction to compensate for geometrical aberrations. Ultimately, we employ it to correct for aberrations present in the image of the photoreceptor layer of the human retina that reveals otherwise invisible photoreceptor mosaic.

Enhanced spectral resolution for broadband coherent anti-Stokes Raman spectroscopy

Takuro Ideguchi, Faris Sinjab, Kazuki Hashimoto, Xuanqiang Zhao, and Yu Nagashima

DOI: 10.1364/OL.388624 Received 17 Jan 2020; Accepted 28 Jan 2020; Posted 03 Feb 2020  View: PDF

Abstract: The spectral resolution of broadband Fourier-transform coherent anti-Stokes Raman spectroscopy is limited by the maximum optical path length difference that can be scanned within a short time in an interferometer. However, alternatives to the Fourier-transform exist which can bypass this limitation with certain assumptions. We apply one such approach to broadband coherent Raman spectroscopy using interferometers with short delay line (low Fourier spectral resolution) and large delay line (high Fourier spectral resolution). With this method, we demonstrate broadband coherent Raman spectroscopy of closely spaced vibrational bands is possible using a short delay line interferometer, with superior spectral resolution to the longer delay line instrument. We discuss how this approach may be particularly useful for more complex Raman spectra, such as those measured from biological samples.

High-power back-to-back dual-absorption germanium photodetector

Xiuli Li, Linzhi Peng, zhi Liu, Xiangquan Liu, Jun Zheng, Yuhua Zuo, Chunlai Xue, and Buwen Cheng

DOI: 10.1364/OL.388011 Received 14 Jan 2020; Accepted 28 Jan 2020; Posted 05 Feb 2020  View: PDF

Abstract: A high-power germanium photodetector is designed and fabricated using a cold-wall ultrahigh vacuum chemical vapor deposition. A back-to-back dual-absorption structure improves high-power characteristics by reducing the space-charge effect. Compared to a typical p-i-n photodetector, the saturated photocurrent of the back-to-back dual-absorption photodetector is improved from 16.2 mA to 21.3 mA at -3 V. At bias voltage of -1 V, the dark current is 1.31 µA. The optical responsivities are 0.31 A/W and 0.52 A/W at 1550 nm and 1310 nm, respectively. The 3-dB bandwidth of 4.14 GHz is achieved at -3 V. Theoretically, the 3-dB bandwidth can be further optimized in future device fabrication.

Multistable circular currents of polariton condensates trapped in ring potentials

Xuekai Ma, Franziska Barkhausen, and Stefan Schumacher

DOI: 10.1364/OL.386250 Received 18 Dec 2019; Accepted 27 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We demonstrate the formation and trapping of different stationary solutions, oscillatory solutions, and rotating solutions of a polariton condensate in a planar semiconductor microcavity with a built-in ring-shaped potential well. Multistable ring shaped solutions are trapped in shallow potential wells. These solutions have the same ring shaped density distribution but different topological charges, corresponding to different orbital angular momentum (OAM) of the emitted light. For stronger confinement potentials, besides the fundamental modes, higher excited (dipole) modes can also be trapped. If two modes are excited simultaneously, their beating produces a complex oscillation and rotation dynamics. When the two modes have the same OAM, a double-ring solution forms for which the density oscillates between the inner and the outer ring. When the two modes have different OAM, a rotating solution with a crescent-shaped density and fractional OAM is created.

Octave-spanning, continuous wave supercontinuum generation with record power using standard telecom fibers pumped with power combined fiber lasers

Arun S, Vishal Choudhury, V Balaswamy, and V R Supradeepa

DOI: 10.1364/OL.384690 Received 02 Dec 2019; Accepted 27 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We have demonstrated a record output power of ~72 W, octave spanning, nearly single mode, continuous-wave (CW) supercontinuum with a bandwidth of ~1050 nm using standard telecom fiber as the nonlinear medium in an all-fiber architecture. We have utilized the recently proposed nonlinear power combining architecture by which power scaling is achieved using multiple independent Ytterbium (Yb) lasers operating at different wavelengths. In this work, Raman conversions in the fiber assist in combining multiple input laser lines into one single wavelength which then undergoes supercontinuum generation. The architecture is based on the recently proposed grating free, cascaded Raman lasers based on distributed feedback. Here, all Raman conversions are well seeded, thereby enhancing the efficiency of supercontinuum generation to ~44%. In this work, we have obtained power spectral densities (PSD) of >3mW/nm from 850-1350 nm and a high PSD of >100mW/nm from 1350-1900nm. Here, we have also investigated the power-combined supercontinuum generation for different pump wavelength combinations demonstrating the flexibility of this technique.

Metasurface based contact lenses for color vision deficiency

Sharon Karepov and Tal Ellenbogen

DOI: 10.1364/OL.384970 Received 05 Dec 2019; Accepted 27 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We embed large-scale, plasmonic metasurfaces into off-the-shelf rigid-gas-permeable contact lenses and study their ability to serve as visual aids for color vision deficiency, specifically deuteranomaly. The effect of the metasurfaces-based contact lenses on the color perception was simulated using CIE color spaces and conventional models of the human color-sensitive photoreceptors. Comparison between normal color vision, uncorrected and corrected deuteranomaly by the proposed element, demonstrates the ability offered by the nanostructured contact lens to shift back incorrectly perceived pigments closer to the original pigments. The maximal improvement in the color perception error before and after the proposed correction for deuteranomaly is up to a factor of ~10. In addition, an Ishihara-based color test was also simulated, showing the restore of contrast achieved by the element, for deuteranomaly conditions.

Adaptive over-the-air RF self-interference cancellation using signal-of-interest driven regular triangle algorithm

Lizhuo Zheng, shilin xiao, Zhiyang Liu, Mable Fok, Jiafei Fang, Hang Yang, Ming Lu, zhiyi zhang, and Weisheng Hu

DOI: 10.1364/OL.385640 Received 12 Dec 2019; Accepted 27 Jan 2020; Posted 27 Jan 2020  View: PDF

Abstract: An optically-enabled RF self-interference cancellation system is demonstrated for over-the-air in-band full duplex transmission, based on a signal-of-interest (SOI) driven regular triangle algorithm. Since the goal of a self-interference cancellation system is to retrieve the SOI that is masked by the in-band interference signal, using the SOI quality as the driven parameter for optimizing the self-interference cancellation performance is a natural and effective way to allow the system to adapt to changes and obtain the best cancellation performance. Since regular triangle algorithm has short iteration time, bursts of pseudo-random binary sequence would be used between real data transmission for optimizing the self-interference cancellation performance. The adaptive regular triangle algorithm optimizes the cancellation setting such that the in-band interference can be cancelled to a minimum, i.e. down to the noise floor. During the over-the-air experiment, 22 dB of cancellation depth is obtained over a 300 MHz bandwidth at 18.35 GHz without the need of digital self-interference cancellation.

High-energy self-mode-locked Cr: fosterite laser near the soliton blowup threshold

Anatoly Ivanov, Grigory Martynov, Alexandr Lanin, Andrey Fedotov, and Aleksei Zheltikov

DOI: 10.1364/OL.384850 Received 02 Dec 2019; Accepted 27 Jan 2020; Posted 27 Jan 2020  View: PDF

Abstract: At the level of peak powers needed for a Kerr-lens mode-locked operation of solid-state soliton short-pulse lasers, a periodic perturbation induced by a spatially localized pulse amplification in a laser cavity can induce soliton instability with respect to resonant dispersive-wave radiation, eventually leading to soliton blowup and pulse splitting of the laser output. Here, we present an experimental study of a high-peak-power self-mode-locking Cr: forsterite laser, showing that, despite its complex, explosion-like buildup dynamics, this soliton blowup can be captured and quantitatively characterized via an accurate cavity-dispersion- and gain-resolved analysis of the laser output. We demonstrate that, with a suitable cavity design and finely tailored balance of gain, dispersion, and nonlinearity, such a laser can be operated in a subcritical mode, right beneath the soliton blowup threshold, providing an efficient source of sub-100-fs 15 – 20-MHz-repetition-rate pulses with energies as high as 33 nJ.

Active modulation of intracavity laser intensity with PDH locking for photoacoustic spectroscopy

Zhen Wang, Haoyun Wei, Yan Li, Ruifeng Kan, and Wei Ren

DOI: 10.1364/OL.386523 Received 23 Dec 2019; Accepted 25 Jan 2020; Posted 27 Jan 2020  View: PDF

Abstract: We report a novel method of active intracavity intensity modulation for cavity-enhanced photoacoustic spectroscopy (PAS) without the need for any external optical modulators. Based on the Pound-Drever-Hall (PDH) locking technique, a dither is added to the PDH error signal to periodically vary the locking point between the laser frequency and optical cavity within a sub-MHz frequency range. While significantly enhancing the intracavity laser intensity, the optical cavity also acts as an intensity modulator. As a proof-of-principle, we demonstrated PAS of C₂H₂ by placing a photoacoustic cell (Q-factor ~10) inside a Fabry-Pérot cavity (finesse ~628) and adopting the proposed intracavity intensity modulation scheme. By detecting the weak C2H2 line at 6412.73 cm-1, the sensor achieves a normalized noise equivalent absorption (NNEA) coefficient of 1.5×10-11 cm-1WHz-1/2. This method enables the continuous locking of laser frequency and optical cavity, and achieves the intracavity intensity modulation with an adjustable modulation depth as well.

Chirality Breakdown in the Presence of Multiple Exceptional Points and Specific Mode Excitation

Harsh K. Gandhi, Arnab laha, sibnath dey, and Somnath Ghosh

DOI: 10.1364/OL.383749 Received 18 Nov 2019; Accepted 25 Jan 2020; Posted 27 Jan 2020  View: PDF

Abstract: The dynamical parametric encirclement around a second-order Exceptional Point (EP) enables the time-asymmetric nonadiabatic evolution of light which follows the chirality of the underlying system. Such light dynamics in the presence of multiple EPs and the corresponding chiral aspect is yet to be explored. Here, we report a gain-loss assisted four-mode supported optical waveguide that hosts a parameter space to encircle multiple EPs dynamically. In the presence of multiple EPs, we establish a unique nonadiabatic behavior of light, where beyond the chiral aspect of the system, light is switched to a particular mode irrespective of the choice of input mode. Proposed scheme certainly opens a step-forward approach in light manipulation through guided wave devices to facilitate next-generation integrated photonic systems.

Simultaneous generation of ultrabroadband noise-like pulses and intracavity third harmonic at 2 μm

Yuhang Li, Yi Kang, Xin Guo, and Limin Tong

DOI: 10.1364/OL.384768 Received 02 Dec 2019; Accepted 25 Jan 2020; Posted 07 Feb 2020  View: PDF

Abstract: We demonstrated that ultrabroadband noise-like pulses (NLPs) spanning from below 1600 nm to beyond 00 nm can be generated in Tm-doped fiber laser enabled by an optical microfiber. Meanwhile, pronounced red light around 660 nm was also observed, which was attributed to the intracavity third harmonic generation (THG) of ultrashort pulses by harnessing the intermodal phase matching in the optical microfiber. As far as we know, it is the first time to simultaneously observe the ultrabroadband NLPs and the intracavity THG in ultrafast fiber lasers, and it is anticipated that this ultrabroadband NLPs can be adopted as a compact broadband source for optical spectroscopy around 2 μm and a wonderful potential seed for supercontinuum generation in the mid-infrared. Moreover, the THG of the intracavity high peak power NLPs in ultrafast fiber lasers provides a new kind of fiber-format visible light sources.

Ultrafast ultrasound imaging of surface acoustic waves induced by laser excitation compared with acoustic radiation force

Lingyi Zhao, Don Vanderlaan, Heechul Yoon, Jingfei Liu, Stanislav Emelianov, and Changhui Li

DOI: 10.1364/OL.383932 Received 21 Nov 2019; Accepted 25 Jan 2020; Posted 18 Feb 2020  View: PDF

Abstract: We report a study comparing two generation mechanisms and their resulting effects on surface acoustic wave (SAW) phenomenon. The two generation mechanisms studied, optical perturbation and acoustic radiation force (ARF), were investigated with the use of high frame rate ultrasound imaging to track the propagation of induced surface acoustic waves. First, we compared ARF-induced surface acoustic waves with laser-induced surface acoustic waves generated by local laser beam irradiation of the uniformly absorbing phantom, where light was preferentially absorbed at the surface. Then we compared the spectral dependence of SAWs generated by ARF versus pulsed laser light, using the same temporal duration of excitation for both generation mechanisms. Differences in the SAW bandwidth were anticipated due to the ability to focus laser light into a smaller area. Last, we compared the two methods effect on wave generation and propagation when the wave’s origin was underneath the phantom’s surface. For the optical generation mechanism, we also investigated the relationship between shear wave amplitude and optical fluence. For optical perturbation, we used a photoabsorber located inside the phantom.

Raman tensor of layered MoS2

Ying Ding, Wei Zheng, Mingge Jin, Yanming Zhu, Ruinan Zhu, Zeguo Lin, and Feng Huang

DOI: 10.1364/OL.384524 Received 28 Nov 2019; Accepted 23 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: Raman tensor, one of the basic physical properties of MoS2, is rarely reported. Here, angle-resolved polarized Raman scatterings on basal and cross planes of layered MoS2 were carried out by using the geometry configuration of parallel polarization, and the Raman tensors of three optical vibration modes were systematically studied. As a polar vibration mode, the differential polarizability of A1g mode corresponding to the Raman tensor along c-direction is larger than that along a-direction. And it is also larger than that formed by E2g and E1g modes. All the experimental results above are beneficial to strengthen the understanding of inelastic light scattering process of MoS2.

Compact High-Extinction Tunable CROW filters for Integrated Quantum Photonic Circuits

Rakesh Ranjan Kumar, Xinru WU, and Hon Tsang

DOI: 10.1364/OL.384187 Received 25 Nov 2019; Accepted 23 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: We describe the use of cascaded second order coupled-resonator optical waveguide (CROW) tunable filters to achieve one of the highest reported combined extinction ratios of 147 ± 2 dB. The CROW filters were used to remove the pump photons in spontaneous four-wave mixing (SFWM) in a silicon waveguide. The SFWM generated quantum correlated photons which could be measured after the cascaded CROW filters. The CROW filters offer a compact foot-print for use in monolithic quantum photonic circuits.

Mass-producible micro-optical elements by injection compression molding and focused ion beam structured titanium molding tools

Simon Ristok, Marcel Roeder, Simon Thiele, Mario Hentschel, Thomas Guenther, André Zimmermann, Alois Herkommer, and Harald Giessen

DOI: 10.1364/OL.385599 Received 16 Dec 2019; Accepted 23 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: We demonstrate mass production compatible fabrication of polymer-based micro Fresnel lenses by injection compression molding. The extremely robust titanium molding tool is structured with high precision by focused ion beam milling. In order to achieve optimal shape accuracy in the titanium we use an iterative design optimization. The inverse Fresnel lens structured into the titanium is transferred to polymers by injection compression molding, enabling rapid mass replication. We show that the optical performance of the molded diffractive Fresnel lenses is in good agreement with simulations, rendering our approach suitable for applications which require compact and high quality optical elements in large numbers.

Optical vortex rotation and propagation from a spiral phase plate resonator with surface reflective coating

Yisa Rumala

DOI: 10.1364/OL.387081 Received 30 Dec 2019; Accepted 21 Jan 2020; Posted 24 Jan 2020  View: PDF

Abstract: A spiral phase plate resonator (SPPR) is created by depositing reflective coating on the surfaces of a single conventional spiral phase plate (SPP) for the first time. Optical transmission through the SPPR on the output plane of the device is measured to give sharp Fabry-Perot resonances as a function of beam roll angle for the first time. Similar measurements are performed for the reflected light emerging from the input plane of the SPPR device. Varying the light frequency going into the SPPR changes the orientation of the angular pattern (Fabry-Perot resonances) to give the rotational constant of the device, in agreement with theory. The optical mode profile is measured after the beam has propagated beyond the plane of the SPPR device while remaining in the diffraction nearfield, thus revealing new features in the transmitted optical beam. These new results have important implications in developing the SPPR for microscopy, imaging, angle measurement, rotational scanning, and LiDAR.

Brillouin scattering induced by shear acoustic mode in a step-index fiber

Min Cao, Lin Huang, Min Tang, an Mi, and Guobin Ren

DOI: 10.1364/OL.386478 Received 19 Dec 2019; Accepted 21 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: We present the mechanism of backward Brillouin scattering induced by shear acoustic mode (SAM) in a step-index fiber (SIF). Unlike a longitudinal acoustic mode (LAM) with negligible transverse displacement, a SAM has both considerable transverse and longitudinal displacements. During the light-sound coupling process, the fundamental and high-order SAMs can be guided and excited, ultimately generating a Brillouin gain spectrum with multi-peak structure in a frequency range around 6GHz. The interaction characteristics of the optical force with the displacement of all excited SAMs determine a partial cancellation effect, which is of great importance for the coupling coefficient of the optical-acoustic modes. The SAMs induced backward Brillouin scattering would provide a promising new approach for application such as multi-parameter sensing.

Phase Retrieval with Fast Convergence Employing Parallel Alternative Projections and Phase Reset for Coherent Communications

Haoshuo Chen, Hanzi Huang, Nicolas Fontaine, and Roland Ryf

DOI: 10.1364/OL.385435 Received 10 Dec 2019; Accepted 21 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Phase-retrieval (PR) receivers can reconstruct complex-valued signals using only direct detection without the use of any optical carriers.We propose two PR receiver solutions with faster and better convergence.First, we demonstrate a PR receiver based on parallel alternative projections that are produced by propagating the signal through an array of dispersive elements of increasing length followed by direct detection.Fast convergence and high retrieved phase accuracy are achieved using a modified Gerchberg–Saxton (GS) algorithm that uses each projection as an intensity constraint.We experimentally reconstruct a 30-Gbaud QPSK signal after 55-km single-mode fiber transmission using the proposed solutions with a reduced number of iterations.

Impact of Laser Flicker Noise and Linewidth on 64 to 96 Gbaud/DP-nQAM Metro Coherent Optical Links

Rui Zhang, Konstantin Kuzmin, WenJr Jiang, Giorgio Giaretta, Tadatoshi Tomimoto, Yi Weng, Gee-Kung Chang, and Winston Way

DOI: 10.1364/OL.386267 Received 06 Jan 2020; Accepted 21 Jan 2020; Posted 22 Jan 2020  View: PDF

Abstract: We experimentally investigate the impact of laser flicker noise and linewidth on 64Gbaud/DP-64QAM, 96Gbaud/DP-32QAM and 64 or 96Gbuad/DP-16QAM links. To give a more practical viewpoint, the examined flicker noise closely follows that of an industry forum (OIF 400ZR). We have found that higher modulation order (e.g., 64QAM) is sensitive to phase noise from the linewidth and flicker noise, even in back to back case. Significant OSNR and cycle slip rate penalties can also be observed with a transmission distance >200km for both 64QAM and 32QAM signals, which mainly comes from equalization enhanced phase noise. Moreover, with the increasing of transmission distances, the effective linewidth of a tunable laser with a higher flicker noise and higher linewidth (210KHz) increases significantly, while it remains unchanged for an ECL with 47-kHz linewidth. The result indicates the importance of more stringent flicker noise and linewidth requirement for future ultra-baud rate transmissions.

Nitrous Oxide detection at 5.26 µm with a compound glass Antiresonant Hollow-Core Optical Fiber

Piotr Jaworski, Karol Krzempek, Grzegorz Dudzik, Pier Sazio, and Walter Belardi

DOI: 10.1364/OL.383861 Received 21 Nov 2019; Accepted 21 Jan 2020; Posted 30 Jan 2020  View: PDF

Abstract: Laser-based gas sensors utilizing various light-gas interaction phenomena have proved their capacity for detecting different gases. However, achieving reasonable sensitivity, especially in the mid-Infrared, is crucial. Improving sensors detectivity usually requires incorporating multipass cells, which increase the light-gas interaction path-length at a cost of reduced stability. An unconventional solution comes with the aid of hollow-core fibers. In such fiber light is guided inside an air-core, which when filled with the analyte gas can serve as a low-volume and robust absorption cell. Here, we report on the use of a borosilicate Antiresonant Hollow-Core Fiber for laser-based gas sensing. Due to its unique structure and guidance, this fiber provides low-loss, single-mode transmission >5 µm. The feasibility of using the fiber as a gas cell was verified by detecting NO at 5.26 µm with a minimum detection limit of 20 ppbv.

Plasma Mirror focal spot quality for glass andaluminum mirrors for laser pulses up to 20picoseconds

Brandon Edghill, Pierre Foresteier-Colleoni, Jaebum Park, Alexander Rubenchik, Farhat Beg, and Tammy Ma

DOI: 10.1364/OL.385326 Received 16 Dec 2019; Accepted 20 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: High intensity short pulse lasers are being pushed furtheras applications continue to demand higher laser intensities.Uses such as radiography and laser-driven particle accelerationrequire these higher intensities to produce the necessaryx-ray and particle fluxes. Achieving these intensitieshowever is limited by the damage threshold of costlyoptics and the complexity of target chambers. This is evidencedby the Advanced Radiographic Capability (ARC)short pulse laser, at the National Ignition Facility (NIF) atthe Lawrence Livermore National Laboratory, producingfour high energy 1 kJ laser pulses at 30 ps pulse durationbeing limited to an intensity of 10^18 W/cm^2 by the large focalspot size of 100 µm. Due to the setup complexity ofNIF, changing the location of the final focusing parabola inorder to improve the focal spot size is not an option. Thisleads to the possible use of disposable ellipsoidal plasmamirrors (PMs) placed within the chamber, close to the targetin an attempt to refocus the four ARC beams. However,the behavior of plasma mirrors at these relatively longpulse durations (tens of ps) is not well characterized. Theresults from the COMET laser at the Jupiter Laser Facility(JLF) carried out at 0.5 to 20 ps pulse durations on flat mirrorsare presented as a a necessary first step towards focusingcurved mirrors. The findings show defocusing at longerpulse durations and higher intensities, with less degradationwhen using aluminum coated mirrors.

Single-shot ultrafast visualization and measurement oflaser matter interactions in flexible glass usingfrequency domain holography

Dennis Dempsey, Garima Nagar, Christopher Renskers, Rostislav Grynko, Jay Sutherland, and Bonggu Shim

DOI: 10.1364/OL.382645 Received 11 Nov 2019; Accepted 19 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: We perform single-shot Frequency Domain Holography (FDH) to measure the ultrafast spatio-temporal phase change induced by the optical Kerr effect andplasma in flexible Corning® Willow® Glass during femtosecond laser matter interactions. We measure the nonlinear index of refraction (n₂) to be (3.6±0.1)x10¯¹⁶ cm²/W and visualize the plasma formation and recombinationon femtosecond time scales in a single shot. To compare with the experiment, we carry out numerical simulations by solving the Nonlinear Envelope Equation(NEE).

Effective Rabi dynamics of Rydberg atoms and robust high-fidelity quantum gates with a resonant amplitude-modulation field

Jin-Lei Wu, Shi-Lei Su, Yan Wang, Jie Song, Yan Xia, and Yongyuan Jiang

DOI: 10.1364/OL.386765 Received 24 Dec 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: With a resonant amplitude-modulation field on two Rydberg atoms, we propose a Rydberg antiblockade (RAB) regime, where the Rabi oscillation between collective ground and excited states is induced. A controlled-Z gate can be yielded through a Rabi cycle. Further, several common issues of implementing the RAB gates are solved to some certain extent. The robustness against the control error and the fidelity are enhanced by using a shaped pulse. The requirement of control precision of the strength of the Rydberg-Rydberg interaction is relaxed. In addition, breaking the RAB restrains the atomic excitation and therefore enhances the gate robustness against the atomic decay.

Ultra-wideband far-infrared absorber based on anisotropically-etched doped silicon

Xiaolong You, Aditi Upadhyay, Yongzhi Cheng, Madhu Bhaskaran, Sharath Sriram, Christophe Fumeaux, and Withawat Withayachumnankul

DOI: 10.1364/OL.382458 Received 06 Nov 2019; Accepted 18 Jan 2020; Posted 21 Jan 2020  View: PDF

Abstract: Far-infrared absorbers exhibiting wideband performance are in great demand in numerous applications, including imaging, detection, and wireless communications. Here, a non-resonant far-infrared absorber with an ultra-wideband operation is proposed. This absorber is in the form of inverted pyramid cavities etched into moderately doped silicon. By means of a wet-etching technique, the crystallinity of silicon restricts the formation of the cavities to a particular shape, in an angle that favors impedance matching between lossy silicon and free space. Far-infrared waves incident on this absorber experience multiple reflections on the slanted lossy silicon side walls, and being dissipated towards the cavity bottom. The simulation and measurement results confirm that an absorption beyond 90% can be sustained from 1.25 to 5.00 THz. Furthermore, the experiment results suggest that the absorber can operate up to at least 21.00 THz with a specular reflection less than 10% and negligible transmission.

Si₃N₄-Chip-based versatile photonic RF waveforms generator with a wide tuning range of repetition rate

zihang zhu, Yang Liu, Moritz Merklein, ziqian zhang, David Marpaung, and Benjamin Eggleton

DOI: 10.1364/OL.383876 Received 21 Nov 2019; Accepted 17 Jan 2020; Posted 22 Jan 2020  View: PDF

Abstract: We demonstrate a Si₃N₄-chip-based photonic approach to generate versatile radio-frequency (RF) waveforms with a large tuning range of repetition rates. The amplitude and phase of the RF-phase-modulated signal are spectrally manipulated to synthesize Fourier coefficients of the desired RF waveforms, by controlling the resonance conditions and frequencies of Si3N4 optical ring resonators. Full-duty-cycle triangular, square, and sawtooth waveforms with widely-tunable repetition rates from 1 to 13 GHz were experimentally generated.

Mid-infrared frequency comb with 6.7 W average power based on difference frequency generation

Anthony Catanese, Jay Rutledge, Myles Silfies, Xinlong Li, Henry Timmers, Abijith Kowligy, Alexander Lind, Scott Diddams, and Thomas Allison

DOI: 10.1364/OL.385294 Received 11 Dec 2019; Accepted 17 Jan 2020; Posted 29 Jan 2020  View: PDF

Abstract: We report on the development of a high-power mid-infrared frequency comb with 100 MHz repetition rate and 100 fs pulse duration. Difference frequency generation is realized between two branches derived from an Er:fiber comb, amplified separately in Yb:fiber and Er:fiber amplifiers. Average powers of 6.7 W and 14.9 W are generated in the 2.9 μm idler and 1.6 μm signal, respectively. With high average power, excellent beam quality, and passive carrier-envelope phase stabilization, this light source is a promising platform for generating broadband frequency combs in the far infrared, visible, and deep ultraviolet.

Producing anomalous uniform periodic nanostructures on Cr thin films by femtosecond laser irradiation in vacuum

Fei Wang, Bo Zhao, Yuhao Lei, Jianjun Yang, and Chunlei Guo

DOI: 10.1364/OL.382322 Received 07 Nov 2019; Accepted 17 Jan 2020; Posted 30 Jan 2020  View: PDF

Abstract: We report the producing unprecedentedly uniform periodic structures on chromium (Cr) thin films in vacuum conditions with irradiation of femtosecond laser pulses. In sharp contrast to the observations in air circumstance, the achieved surface structures of the ablated groove arrays are surprisingly found to have not only an extraordinarily uniform distribution but also a deep-subwavelength period of 360 nm. The measured both width and depth of the ablated periodic grooves are 150 nm and 120 nm, respectively, showing large depth-to-width ratio and sharp-edge profiles. Remarkably, such well-organized nanostructures can be enabled to robustly extend into an infinitely long range via the sample scanning and even have a large-area production with a cylindrical lens. Raman spectral analyses reveal that the regular formation of such nanostructures benefits from avoiding the material oxidation and thermal disturbance of the air plasma on the sample surface

Overcoming standard quantum limit using momentummeasuring interferometer

Sankar Davuluri and Yong Li

DOI: 10.1364/OL.385092 Received 04 Dec 2019; Accepted 16 Jan 2020; Posted 24 Jan 2020  View: PDF

Abstract: We show that the back-action noise in the momentum measurement of a damped forced oscillator can be suppressed because of the damping. Using this principle, we design a back-action suppressed interferometer in which the signal is a function of momentum of atoms in a harmonic trap. Having suppressed the back-action noise, we show that the quantum noise limited sensitivity of this interferometer can overcome the standard quantum limit of force sensing even at frequencies much smaller than the eigen frequency of the harmonic trap.

Conical refraction mode of an optical resonator

Yuri Loiko, Alex Turpin, Grigorii Sokolovskii, and Edik Rafailov

DOI: 10.1364/OL.387182 Received 02 Jan 2020; Accepted 14 Jan 2020; Posted 23 Jan 2020  View: PDF

Abstract: The fundamental mode of a conical refraction resonator, i.e. an optical cavity where light experiences conical refraction (CR) from a biaxial crystal, is experimentally demonstrated in the plano-concave cavity configuration. We have discovered that the fundamental CR mode is characterized by the polarization and intensity structures of CR beams between the plane mirror and CR crystal and it resembles the fundamental Gaussian mode with homogeneous polarization between the crystal and concave mirror. We theoretically explained this fundamental CR mode using the dual cone model and symmetry of the CR phenomenon.

Detecting WDM Visible Light Signals by a SingleMulti-Color Photodiode with MIMO Processing

Alessandro Messa, Giulio Cossu, Marco Presi, Stefan Schidl, Kerstin Schneider-Hornstein, Horst Zimmermann, and Ernesto Ciaramella

DOI: 10.1364/OL.385641 Received 10 Dec 2019; Accepted 14 Jan 2020; Posted 15 Jan 2020  View: PDF

Abstract: For the first time, we experimentally demonstrate that MIMO processing allows using a single photodiode to detect simultaneously an RGB (red, green and blue) VLC signals. The photodiode has a triple junction and, when it is illuminated by a WDM signal, the junctions produce inherently three photocurrents that are unusable to detect any of the WDM signals. However, by means of linear MIMO processing, we are able to recover the transmitted signals exactly. Bit error ratio measurements confirm the effectiveness of the proposed solution. This opens a new scenario for practical WDM-VLC systems.

2.9 µm lasing from Ho3+/Pr3+ co-doped AlF3 based glass fiber pumped by a 1150 nm laser

Pengfei Wang, Shunbin Wang, Jiquan Zhang, Shijie Jia, Gilberto Brambilla, and Niannian Xu

DOI: 10.1364/OL.384216 Received 28 Nov 2019; Accepted 13 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Ho3+/Pr3+ co-doped AlF3 based glass fibers were fabricated by using a rod-in-tube method based on the matrix glass composition of AlF3 - BaF2 - CaF2 - YF3 - SrF2 - MgF2 - LiF - ZrF4 - PbF2. Under the pump of a 1150 nm Raman fiber laser, 2.9 µm laser was observed in a 19 cm long Ho3+/Pr3+ co-doped AlF3 based glass fiber with an output power of 157 mW and a slope efficiency of 10.4%. Ho3+/Pr3+ co-doped AlF3 based glasses were fabricated to investigate the deactivation effects of Pr3+ ions on the Ho3+: 5I7 level. Our results showed that the Ho3+/Pr3+ co-doped AlF3 based glass fibers are potential gain media for ~ 2.9 µm lasers.

4 Mb/s under 3-m transmission distance using quantum dot light-emitting diode and NRZ-OOK modulation

Hua Xiao, Rui Wang, Gui He, Zhijian Lv, Zhaojun Liu, and Kai Wang

DOI: 10.1364/OL.386175 Received 24 Dec 2019; Accepted 12 Jan 2020; Posted 17 Jan 2020  View: PDF

Abstract: We realize signal transmission with a miniature light source fabricated by a 4-mm2 red-emissive CdSe/ZnS QLED in visible light communication (VLC). The light emitted from the 60°-designed QLED transmit in free space with data rate of 4 Mb/s at 3-m transmission distance by using a simple modulation scheme of none-return-to-zero on-off keying. The maximum data rate of 2.5 Mb/s with BER under the forward-error-correction (FEC) limit is achieved within the optical angles of ± 20°. The influences caused by voltage, distance, and optical angle of emitting light are taken into consideration during communication. The performance of the QLED based light source is excellent among all solution-processed devices both in efficiency, luminance, bandwidth, transmission speed, and distance. Additionally, this is the first time to realize QLED communication, and our results should be instructive for further application of QLEDs in VLC.

A light field camera study of near ground turbulence anisotropy and observation of small outer-scales

Chensheng Wu, Daniel Paulson, John Rzasa, and Christopher Davis

DOI: 10.1364/OL.386444 Received 19 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: Understanding turbulence effects on laser beam propagation is critical to the emerging design, study, and test of many long range free space optical (FSO) communication and directed energy (DE) systems. Conventional studies make the prevalent assumption of isotropic turbulence, while more recent results suggest anisotropic turbulence for atmospheric channels within a few meters elevation above the ground. As countless FSO systems have been and continue to be deployed in such channels, analysis of anisotropic modelings has become one of the fastest growing areas in FSO research. This in turn motivates new tools which can distinguish anisotropic characteristics to improve both modeling accuracy and physical interpretations. Wavefront sensors such as Shack-Hartmann sensors, interferometers, and plenoptic sensors have been devised and used in experiments, however they all require rigid alignments that lack resilience against temperature gradient build-up and beam wander. We find that by using a light field camera (LFC) which extracts perturbation of individual light rays, the wave structure function of turbulence can be retrieved with high reliability. Furthermore, we find through experiments that the outer scales of near ground turbulence tend to be a magnitude smaller than conventional theoretical assumptions, which agrees with new findings by others but is absent in current theoretical modelings. As a result, we believe that the LFC is an ideal candidate in the frontier of turbulence research, which is both commercially available and easy to adapt to turbulence experiments.

Photonic generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading

Wu Zhang, Qinggui Tan, Xiaojun Li, and Di Wang

DOI: 10.1364/OL.386138 Received 16 Dec 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: A novel photonic approach for the generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading is proposed. By switching between the carrier suppressed double sideband (CS-DSB) modulation with reverse quadrature phase difference and carrier suppressed single sideband (CS-SSB) modulation, background-free phase-coded microwave pulses with frequency doubling of the LO signal can be obtained. Thanks to the CS-DSB/CS-SSB modulation, the generated microwave pulses can be directly transmitted via single mode fiber (SMF) without the influence of the periodic power fading. Since no filter is utilized, the proposed scheme owns a wide frequency tunable range. An experiment is conducted to verify the proposed approach. Two 1-Gbit/s phase-coded microwave pulses with center frequencies of 6 GHz and 12 GHz are generated and successfully transmitted through a 25-km SMF. The pulse compression ratio (PCR) and main-to-sidelobe ratio (MSR) after the transmission are measured as 13 and 8.84 dB, respectively.

Indocyanine green provides absorption and spectral contrast for optical coherence tomography at 840 nm in vivo

Conrad Merkle, Marco Augustin, Danielle Harper, and Bernhard Baumann

DOI: 10.1364/OL.380051 Received 10 Oct 2019; Accepted 10 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: In recent years, there has been growing interest in the application of exogenous contrast agents to supplement the traditional strengths of optical coherence tomography (OCT) and provide additional biological information. Here we present how indocyanine green, a common fluorescent contrast agent approved by the United States Food and Drug Administration, can provide absorption and spectral contrast for OCT imaging in the mouse eye in vivo. We further demonstrate high stability of spectral contrast measurements for the long term monitoring of contrast agents in spite of fluctuations in intensity.

Photovoltaic splitting of water microdroplets on a y-cut LiNbO₃:Fe crystal coated with oil-infused hydrophobic insulating layers

Xiong Zhang, Kaifang Gao, Zuoxuan Gao, Zhitao Zan, Lihong shi, Xiaohu Liu, Mengtong Wang, Hongjian Chen, and Wenbo Yan

DOI: 10.1364/OL.385212 Received 06 Dec 2019; Accepted 09 Jan 2020; Posted 14 Jan 2020  View: PDF

Abstract: We demonstrate a successful photovoltaic splitting of water microdroplets on a y-cut LiNbO₃:Fe substrate coated with an oil-infused hydrophobic layer. The temporal evolution of the microdroplet contact angle upon a central illumination and the distinct behaviors of two sub-droplets during a following boundary illumination reveal that both electrowetting and electroosmotic effects induced by the dipolar photovoltaic potential on the substrate contribute to the water micro-droplet splitting. The reciprocal relationship between the splitting time and the illumination intensity verifies the inherent photovoltaic nature of the water microdroplet splitting. The splitting time is found to be linearly dependent on the initial microdroplet size. These points are quite important to the practicalization of LN-based microfluidic chips in the biological field.

Thermal-induced luminescence enhancement of BAC-P in bismuth-doped phosphogermanosilicate fibers

Zhao Qiancheng, Qun Hao, Yanhua Luo, and GangDing Peng

DOI: 10.1364/OL.386293 Received 18 Dec 2019; Accepted 09 Jan 2020; Posted 09 Jan 2020  View: PDF

Abstract: The thermal quenching effect has been systematically investigated in Bi-doped phosphogermanosilicate fiber with varying thermal conditions. For the first time, the activation of phosphor-related bismuth active center (BAC-P) is achieved by thermal quenching at 400 °C with a heating time of 10 min, evidenced by the enhanced luminescence of BAC-P (~1.3 times) at 1300 nm. The experimental results reveal that a relatively low heating temperature with prolonged heating time stimulates the growth of BAC-P, whereas higher operating temperatures (≥500 °C) result in irreversible destruction of BAC-P. The underlying mechanism for the thermally-stimulated process of BAC-P is also analyzed and discussed.

Flexible Spectrum Sharing of Two Asynchronous QAM Signals using Power Division Multiplexing

Peicheng Liao, Kaiheng Zou, Huibin Zhou, Ahmad Fallahpour, Nanzhe Hu, Yinwen Cao, Ahmed Almaiman, Fatemeh Alishahi, Changjing Bao, Moshe Tur, and Alan Willner

DOI: 10.1364/OL.383931 Received 20 Nov 2019; Accepted 09 Jan 2020; Posted 13 Jan 2020  View: PDF

Abstract: We numerically and experimentally report the flexible spectrum sharing of two asynchronous quadrature amplitude modulated (QAM) signals using power division multiplexing. We show that a hybrid QAM signal is generated when two QAM signals with different power are superposed. By exploiting successive interference cancellation, a 20 Gbaud “strong” signal combined with a 9- or 4-Gbaud “weak” can both be recovered sequentially with bit-error rate (BER) performance below the forward error correction (FEC) threshold. In addition, we show the dependence of system performance on the power ratio between the strong and weak signals. These two signals can contain different baudrates, pulse shapes and modulation formats.

Asymmetric Chiroptical Effect from Chiral Medium Filled Golden Slit Grating on Substrate

Ming Yong, Chen Feiliang, Xuannan Wu, Lin Pang, fuhua Gao, and Yidong Hou

DOI: 10.1364/OL.384773 Received 03 Dec 2019; Accepted 06 Jan 2020; Posted 07 Jan 2020  View: PDF

Abstract: In this work, we report a giant and robust asymmetric chiroptical effect (ACOE) in the Chiral medium filled golden slit Grating on glass Substrate (CMGSG-GS). This ACOE comes from the influence of interface asymmetry on the electromagnetic cross-coupling in CMGSG-GS, and is inherently different with that reported in the Faraday medium and the planar anisotropic chiral metamaterials. Both of the polarization eigenstate and the transmission matrix are highly-depended on the metal structure used in CMGSG-GS. The polarization eigenstates of CMGSG-GS are two co-rotating elliptical states with ellipticity of nearly 0, and keeps mainly unchanged for opposite directions. The transmission matrices of opposite directions are normal matrices, which do not show any symmetric law although the geometry of CMGSG-GS owns a high rotational symmetry. The reported ACOE gives a measureable physical parameter to reveal the events happening at interface.

Time-moduated non-reciprocal metasurface absorber for surface waves

Aobo Li, Yun Bo Li, Jiang Long, Ebrahim Forati, Zhixia Du, and Daniel Sievenpiper

DOI: 10.1364/OL.382865 Received 03 Dec 2019; Accepted 17 Dec 2019; Posted 23 Dec 2019  View: PDF

Abstract: We have investigated a magnet-free, non-reciprocal surface wave absorber based on high impedance surfaces (HIS) using a spatial-temporal modulation approach. By controlling embedded switches with a travelling wave, the HIS metasurface is modulated to break the time and spatial symmetry, which enables surface waves to propagate in one direction, but be absorbed when propagating in the reverse direction. The non-reciprocity has been demonstrated by an EM-circuit co-simulation. We envision that this could be possibly applied in future communication systems that preferably transmit unidirectionally but absorb interference from the reverse direction caused by reflections or other devices.

Ultrafast laser ablation assisted spatially-selective attachment of fluorescent sensors onto optical fibres

Vikram Kamaljith, Michael Tanner, Harry Wood, Kerrianne Harrington, Debaditya Choudhury, Mark Bradley, and Robert Thomson

DOI: 10.1364/OL.381018 Received 22 Oct 2019; Accepted 08 Dec 2019; Posted 14 Feb 2020  View: PDF

Abstract: A robust method to selectively attach specific fluorophores to the individual cores of a multicore fibre is demonstrated. The method is based on the use of ultrafast laser pulses to nanostructure the facet of the fibre core, followed by amine functionalization and sensor conjugation. This surface machining protocol not only enables precise spatial selectivity, but also facilitates high deposition densities of the sensor moieties. As a proof of concept, the successful deposition of three different fluorophores onto selected cores of a multicore-fibre is demonstrated. The protocol was developed to include attachment of a fluorescence based pH sensor using the ratiometric carboxynapthofluorescein.

Superlensing Plano-Convex-Microsphere (PCM) lens for direct laser nano marking and beyond

Bing Yan, Liyang Yue, James Monks, Xibin Yang, daxi xiong, Chunlei Jiang, and Zengbo Wang

DOI: 10.1364/OL.380574 Received 17 Oct 2019; Accepted 07 Dec 2019; Posted 03 Jan 2020  View: PDF

Abstract: A high-performance all-dielectric lens, formed by integrating a conventional Plano-Convex lens with a high-index Microsphere lens (PCM), was developed for far-field super-resolution applications. The PCM lens features a resolution of ~λ/3.5 in air with a working distance ~2 μm away from the lens. The super-resolution properties were theoretically and experimentally verified, and utilized for direct laser nano writing of arbitrary patterns and nanostructures on various substrates for the first time. This work can be naturally extended to other super-resolution applications including imaging, sensing, trapping and more, with potentials to develop the next-generation low-cost direct laser nano marking machine and super-resolution imaging nanoscope

Mueller-Jones matrices as representing conformal Lorentz transformations

Tiberiu Tudor

DOI: 10.1364/OL.383444 Received 21 Nov 2019; Accepted 05 Dec 2019; Posted 06 Dec 2019  View: PDF

Abstract: It is shown that Mueller-Jones matrices represent conformal Lorentz transformations. Thus the necessary and sufficient condition of a polarization device to be deterministic is to be describable by a conformal Lorentz transformation

Broadband amplitude and frequency demultiplexer for terahertz frequencies using parallel-plate-waveguides technology

Arturo Hernandez, Daniel Mittleman, and Emma Pickwell-MacPherson

Doc ID: 379411 Received 07 Oct 2019; Accepted 17 Nov 2019; Posted 22 Nov 2019  View: PDF

Abstract: In this work we report a novel broadband frequency/polarization demultiplexer based on parallel-plates-waveguides (PPWG) for terahertz (THz) frequencies. The fabrication and experimental validation of this polarization sensitive demultiplexer is demonstrated for the range from 0.2 THz to 1 THz. By adding a second demultiplexer stage, a fifty-fifty amplitude splitter is also demonstrated in the same frequency range. The multiplexer is based on a traveling-wave antenna, exhibiting strong mechanical robustness. This unique device exhibits three splitting mechanisms in the same device; amplitude, polarization and frequency splitting. This is a significant improvement for the next generation of THz passive components for communication purposes.

Electric Field Vector Measurements Via Nanosecond Electric Field Induced Second Harmonic Generation

Tat Loon Chng, Maya Naphade, Benjamin Goldberg, Igor Adamovich, and Svetlana Starikovskaya

Doc ID: 374833 Received 15 Aug 2019; Accepted 19 Oct 2019; Posted 21 Oct 2019  View: PDF

Abstract: Electric field induced second harmonic generation, or E-FISH, has received renewed interest as a non-intrusive tool for probing electric fields in gas discharges and plasmas using ultrashort laser pulses. An important contribution of this work lies in establishing that the E-FISH method works effectively in the nanosecond regime, yielding field sensitivities of about a kV/cm at atmospheric pressure from a 16 ns pulse. This is expected to broaden its applicability within the plasma community, given the wider access to conventional nanosecond laser sources. A Pockels-cell-based pulse-slicing scheme, which may be readily integrated with such nanosecond laser systems, is shown to be a complementary and cost-effective option for improving the time resolution of the electric field measurement. Using this scheme, a time resolution of ~3 ns is achieved, without any detriment to the signal sensitivity. This could prove invaluable for non-equilibrium plasma applications, where time resolution of a few nanoseconds or less is often critical. Finally, we take advantage of the field vector sensitivity of the E-FISH signal to demonstrate simultaneous measurements of both the horizontal and vertical components of the electric field.

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