Expand this Topic clickable element to expand a topic
OSA Publishing

Early Posting

Accepted papers to appear in an upcoming issue

OSA now posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

Imaging of cortical structures and microvasculature using extended-focus optical coherence tomography at 1.3 μm

Paul James Marchand, Daniel Szlag, Jérôme Extermann, Arno Bouwens, David Nguyen, Markus Rudin, and Theo Lasser

Doc ID: 319781 Received 15 Jan 2018; Accepted 16 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: Extended-focus optical coherence tomography (xf- OCT) is a variant of optical coherence tomogra- phy (OCT) wherein the illumination and/or detection modes are engineered to provide a constant diffraction- less lateral resolution over an extended depth of field (typically 3 to 10× the Rayleigh range). xf-OCT systems operating at 800 nm have been devised and used in the past to image brain structures at high resolution in-vivo but are limited to ∼ 500 μm in penetration depth due to their short illumination wavelength. Here, we present an xf-OCT system optimized to image deeper within the cortex by using a longer illumination central wave- length of 1310 nm. The system offers a lateral resolu- tion of 3 μm and 6.5 μm, over a depth of 900 μm and >1.5 mm using a 10× and 5× objective respectively, in air. We characterize the system’s resolution using mi- crobeads embedded in PDMS and demonstrate its capa- bilities by imaging the cortical structure and microvas- culature in anesthesized mice to a depth of ∼ 1 mm. Fi- nally, we illustrate the difference in penetration depths obtainable with the new system and an xf-OCT system operating at 800 nm.

Resolution enhancement for fiber bundle imaging using maximum a posteriori estimation

Jianbo Shao, Rongguang Liang, Kobus Barnard, and Wei-chen Liao

Doc ID: 319803 Received 16 Jan 2018; Accepted 15 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: We propose a new framework to jointly improve spatial resolution and remove fixed structural patterns for coherent fiber bundle imaging systems. Our forward model integrates shifts under homography with the fiber point spread function to express multiple low resolution images in terms of a single high resolution one. We compute a maximum a posteriori estimate of the high resolution image under a smoothing prior using conjugate gradient descent. This results in a higher resolution image computed from multiple random-shifted captures. We test our method on data synthesized from the USAF target, data from the measurement of a transmissive USAF target, and the measurement data from the lens tissue. In the case of the resolution target and 16 low resolution captures, spatial resolution is enhanced by a factor of 2.8.

Cathodoluminescence hyperspectral analysis of whispering gallery modes in active semiconductor wedge resonators

Pierre Guilleme, julie Stervinou, Tony Rohel, Charles Cornet, Stéphane Balac, David Gachet, Fabrice Mahé, Yannick Dumeige, and Yoan Léger

Doc ID: 319846 Received 17 Jan 2018; Accepted 15 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: Whispering gallery mode resonators are key devices for integrated photonics. Despite their generalization in fundamental and applied science, information on spatial confinement of light in these structures is mostly retrieved from purely spectral analysis. In this work, we present a detailed spectral and spatial characterization of whispering gallery modes in active semiconductor microdisk resonators by use of hyperspectral cathodoluminescence. By comparing our experimental findings to finite element simulations, we demonstrate that the combination of spectral and spatial measurements enables unique identification of the modes and even reveals specific features of the microresonator geometry such as a wedge profile.

Microwave-assisted Rydberg electromagnetically induced transparency

THIBAULT VOGT, christian gross, Thomas Gallagher, and wenhui Li

Doc ID: 321121 Received 01 Feb 2018; Accepted 15 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: We demonstrate electromagnetically induced transparency (EIT) in a four-level cascade-like system, where the two upper levels are Rydberg states coupled by a microwave field. A two-photon transition consisting of an off-resonant microwave field and an off-resonant optical field forms an effective coupling field to induce transparency of the probe light. We characterize the Rabi frequency of the effective coupling field, as well as the EIT microwave spectra. The results show that microwave assisted EIT allows us to efficiently access Rydberg states with relatively high orbital angular momentum l=3, which is promising for the study of exotic Rydberg molecular states.

Ultra-low noise dual-frequency VECSEL at telecom wavelength using fully correlated pumping

Hui Liu, Gregory Gredat, Syamsundar De, Ihsan Fsaifes, Aliou LY, Remy Vatre, Ghaya Baili, Sophie Bouchoule, Fabien Bretenaker, and Fabienne Goldfarb

Doc ID: 323393 Received 15 Feb 2018; Accepted 15 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: An ultra-low intensity and beatnote phase noise dual-frequency vertical-external-cavity surface-emitting laser is built at telecom wavelength. The pump laser is realized by polarization combining two single-mode fibered laser diodes in a single-mode fiber, leading to a 100 % in-phase correlation of the pump noises for the two modes. The relative intensity noise is lower than -140 dB/Hz, and the beatnote phase noise is suppressed by 30 dB, getting close to the spontaneous emission limit. The role of the imperfect cancellation of the thermal effect resulting from unbalanced pumping of the two modes in the residual phase noise is evidenced

Subwavelength light confinement and enhancement enabled by dissipative dielectric nanostructures

Kaichen Dong, Yang Deng, Xi Wang, Kyle Tom, Zheng You, and Jie Yao

Doc ID: 320196 Received 22 Jan 2018; Accepted 15 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: Dissipative loss in optical materials is considered one of the major challenges in nano-optics. Here we show that, counter-intuitively, large imaginary part of material permittivity contributes positively to subwavelength light enhancement and confinement. The Purcell factor and fluorescence enhancement of dissipative dielectric bowtie nanoantennas, such as Si in ultraviolet (UV), are demonstrated to be orders of magnitude higher than their lossless dielectric counterparts, which is particularly favorable in deep UV applications where metals are plasmonically inactive. The loss facilitated field enhancement is the result of large material property contrast and electric field discontinuity. These dissipative dielectric nanostructures can be easily achieved with a great variety of dielectrics at their Lorentz oscillation frequencies, thus having the potential to build a completely new material platform boosting light-matter interaction over broader frequency ranges, with advantages such as bio-compatibility, CMOS compatibility and harsh environment endurance.

Recording, erasing and rewriting of ripples on metal surfaces by ultrashort laser pulses

Kongyu Lou, Jing Qian, Danyang Shen, Hening Wang, Teng Ding, Guande Wang, Ye Dai, and Quan-Zhong Zhao

Doc ID: 321383 Received 08 Feb 2018; Accepted 15 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: Recording, erasing and rewriting of ripples are achieved by applying femtosecond laser pulses on tungsten surfaces. Ripples, oriented perpendicular to the polarization direction of the writing beam, can be recorded on metal surface by exposing the sample to a series of linearly polarized pulses. When applying the second series of pulses with varied polarization direction, on the same place, the original ripples can be erased and new ripples are rewritten with the orientation perpendicular to the polarization of the second group of pulses. The simulation shows that when original ripples exist, laser intensity is focused above the grooves with polarization parallel to original ripples, which can erase the ripples. However, when the polarization is perpendicular to the existed ripples, laser intensity is almost confined in the grooves, which accelerates the formation of ripples.

An array of high slope efficiency waveguide lasers in a Yb doped glass chip operating at 1 μm

Fiorina Piantedosi, George Chen, Tanya Monro, and David Lancaster

Doc ID: 323426 Received 19 Feb 2018; Accepted 14 Mar 2018; Posted 14 Mar 2018  View: PDF

Abstract: Ultrafast laser inscribed waveguide lasers can lead to highly efficient compact integrated optical devices. This Letter reports an average lasing efficiency of 65% from a multi-waveguide 2.5 mol.% Ytterbium-doped ZBLAN (ZrF₄/BaF₂/LaF₃/AlF₃/NaF) chip in extended-cavity configuration. A maximum output power of 750 mW with a lasing efficiency of 68% is achieved. Monolithic end-coupled configuration achieved a maximum output power of 784 mW with a lasing efficiency of 70%. The lasing wavelength is tuned from 1001 nm to 1045 nm in a Littrow configured cavity. A beam propagation factor of lowest order transverse mode output was routinely achieved with an M² of 1.15.

Dynamic Mechanical Analysis on Fused Polymer Optical Fibers: Towards Sensor Applications

Arnaldo Leal Junior, Anselmo Frizera-Neto, Maria Pontes, Paulo Antunes, Nélia Alberto, Maria de Fátima F. Domingues, Heeyoung Lee, Ryohko Ishikawa, Yosuke Mizuno, Kentaro Nakamura, Paulo Andre, and Carlos Marques

Doc ID: 318089 Received 19 Dec 2017; Accepted 13 Mar 2018; Posted 14 Mar 2018  View: PDF

Abstract: This letter presents, for the first time, the dynamic mechanical analysis of a polymer optical fiber (POF) that was previously submitted to the catastrophic fuse effect. The variation of the fiber Young’s modulus was evaluated with respect to the increase of temperature, humidity, and frequency of strain cycles. The data obtained for the fused POF are compared with the ones for the same POF without the fuse effect. Results show the feasibility of the fused POF for sensor applications such as strain and acceleration, since it presents temperature sensitivity 2 times lower and Young’s modulus 2.3 times lower than those obtained with the bare fiber. The Young’s modulus variation with the humidity is 8×10-4 GPa/%RH, which is also lower than the one of non-fused POF. In addition, the fused POF presented variation of its dynamic modulus with the increase of the frequency 3.5 times lower than non-fused POF. These results pave the way for future applications of fused POFs as sensing elements.

Giant enhancement of Faraday rotation due to electromagnetically induced transparency in all-dielectric magneto-optical metasurfaces

Aristi Christofi, Yuma Kawaguchi, Andrea Alu, and Alexander Khanikaev

Doc ID: 320060 Received 18 Jan 2018; Accepted 13 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: In this paper we introduce a new class of Fano-resonant all-dielectric metasurfaces for enhanced, high figure of merit magneto-optical response. The metasurfaces are formed by an array of magneto-optical Ce:BIG nano-disks embedded into a low-index matrix. The strong field enhancement in the magneto-optical disks, which results in two orders of magnitude enhancement of Faraday rotation, is achieved by engineering two, electric and magnetic, resonances. It is shown, that while enhancement of rotation also takes place for spectrally detuned resonances, the resonant excitation inevitably results in stronger reflection and low figure of merit of the device. We demonstrate that this can be circumvented by overlapping electric and magnetic resonances of the nano-disks, yielding a sharp electromagnetically induced transparency (EIT) peak in the transmission spectrum, which is accompanied by gigantic Faraday rotation. Our results show that one can simultaneously obtain a large Faraday rotation enhancement along with almost 100% transmittance in an all-dielectric metasurface as thin as 300 nm. A simple analytical model based on coupled-mode theory is introduced to explain the effects observed in first-principle Finite Element Method (FEM) simulations.

Single-shot phase-shifting incoherent digital holography with multiplexed checkerboard phase gratings

Teruyoshi Nobukawa, Tetsuhiko Muroi, Yutaro Katano, Nobuhiro Kinoshita, and Norihiko Ishii

Doc ID: 319570 Received 22 Jan 2018; Accepted 13 Mar 2018; Posted 13 Mar 2018  View: PDF

Abstract: A single-shot phase-shifting incoherent digital holography with multiplexed checkerboard phase gratings is proposed for acquiring holograms of moving objects. The gratings presented here play the following three roles: dividing beams, modulating the curvature of spherical beams, and introducing different phase shifts. With the gratings of our proposed method, four individual holograms of a spatially incoherent light are formed on an image sensor. It is therefore possible to simultaneously capture four holograms and implement a phase-shifting technique. A proof-of-principle experiment was conducted to show the feasibility of the proposed method.

The impact of mechanical stress induced in silica vacuum windows on laser-induced damage

Clémence Gingreau, Thomas Lanternier, Laurent Lamaignère, Thierry Donval, Roger Courchinoux, Christophe Leymarie, and Jerome Neauport

Doc ID: 319843 Received 23 Jan 2018; Accepted 13 Mar 2018; Posted 13 Mar 2018  View: PDF

Abstract: At the interface between vacuum and air, optical Windows must keep their optical properties despite being subjected to mechanical stress. In this paper we investigate the impact of such stress on the laser induced damage of fused silica windows at the wavelength of 351nm in the nanosecond regime. Different stress values, from 1 to 30MPa, both tensile and compressive were applied. No effect of the stress on the laser induced damage was evidenced.

Quantized pseudomodes for plasmonic cavity-QED

Stephen Hughes, Andreas Knorr, and Marten Richter

Doc ID: 315411 Received 11 Dec 2017; Accepted 13 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: We present a quantized pseudomode theory for solving system-level cavity-QED with quantum emitters coupled to plasmonic resonators. Complimented by the Green function response of a silver nanoparticle and an exact solution for the input-output coupling, we study the non-Markovian coupling regimes of multi-mode vacuum Rabi oscillationsand coherently pumped multi-photon resonances from the higher lying ladder states of collective plasmon modes. The nonlinear pumping regime also facilitates pronounced population inversion of a single two-level atom through multiphoton resonances, dominated by dark plasmons.

High-order orbital angular momentum mode generator based on twisted photonic crystal fiber

Cailing Fu, Shen Liu, Ying Wang, Zhiyong Bai, Jun He, Changrui Liao, Yan Zhang, Feng Zhang, Bin Yu, shecheng Gao, Zhaohui Li, and Yiping Wang

Doc ID: 319689 Received 17 Jan 2018; Accepted 13 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: High-order orbital angular momentum (OAM) modes, i.e., OAM+5 and OAM+6, were generated and demonstrated experimentally by twisting a solid-core hexagonal photonic crystal fiber (PCF) during hydrogen-oxygen flame heating. Leaky orbital resonances in the cladding depend strongly on the twist rate and length of the helical PCF. Moreover, the generated high-order OAM mode is linearly polarized. The secret of the successful observation of high-order modes is that leaky orbital resonances in the twisted PCF cladding have a high coupling efficiency of more than 20 dB.

Gas spectroscopy with integrated frequency monitoring through self-mixing in a terahertz quantum-cascade laser

Rabi Chhantyal-Pun, Alex Valavanis, James Keeley, Pierluigi Rubino, Iman Kundu, Yingjun Han, Paul Dean, Lian He Li, Giles Davies, and Edmund Linfield

Doc ID: 323151 Received 15 Feb 2018; Accepted 12 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4-THz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional THz instrumentation (detectors, mixers or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630 MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0-800-mm round-trip delay-line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path, and show that a limiting absorption coefficient of ~1e-4 / cm is resolvable.

High-precision 3-D rough surface measurement using two-wavelength digital holography referenced by optical frequency comb

Dahi Abdelsalam and Takeshi Yasui

Doc ID: 315397 Received 11 Dec 2017; Accepted 12 Mar 2018; Posted 16 Mar 2018  View: PDF

Abstract: High-stable, two-wavelength, digital holography referenced by optical frequency comb is presented. The technique is demonstrated by mixing scattered waves from external cavity laser diode and a mode-locked Er-doped fiber laser light via a 2D diffraction grating. The mixed beat signal is stabilized at 30MHz by a sensitive proportional integral derivative controller. The stabilized continuous-waves are employed to investigate large stepped structures with 10^-12 uncertainty.

Single shot multi-wavelength phase retrieval with coherent modulation imaging

cheng liu, Xue Dong, Xingchen Pan, and Jianqiang Zhu

Doc ID: 319319 Received 09 Jan 2018; Accepted 12 Mar 2018; Posted 13 Mar 2018  View: PDF

Abstract: A single shot multi-wavelength phase retrieval method is proposed by combing the common coherent modulation imaging (CMI) and low rank mixed-state method together. A radiation beam consisting of multi wavelengths to be measured is incident on random phase plate to form a speckle pattern, which is the incoherent superposition of diffraction patterns of each wavelength. The complex amplitude including both the modulus and phase of each wavelength can be reconstructed simultaneously from the recorded diffraction intensity using the low rank mixed-state algorithm. The feasibility of this proposed method was verified with visible light. This proposed method remarkably extends the application range of common CMI and can find important applications in the fields of wave diagnostics and frequency conversion analysis etc. More importantly, this study hints that very possibly CMI can use table x-ray sources containing several wavelengths to do phase retrieval.

Reconfigurable all-optical on-chip MIMO 3-mode demultiplexing based on multi-plane light conversion

Rui Tang, Takuo Tanemura, Samir Ghosh, Keijiro Suzuki, Ken Tanizawa, Kazuhiro Ikeda, Hitoshi Kawashima, and Yoshiaki Nakano

Doc ID: 319288 Received 12 Jan 2018; Accepted 11 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: We present the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output (MIMO) 3-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference (MMI) couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable 3-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3 dB and modal crosstalk of less than -10 dB over -nm optical bandwidth. Error-free mode demultiplexing of 40-Gbps non-return-to-zero signal is also demonstrated.

Graphene electrodes for lithium-niobate electro-optic devices

Zeshan Chang, Wei Jin, and Kin Chiang

Doc ID: 321219 Received 01 Feb 2018; Accepted 11 Mar 2018; Posted 13 Mar 2018  View: PDF

Abstract: We propose and demonstrate the use of graphene electrodes for lithium-niobate (LiNbO3) electro-optic (EO) devices to exempt the need of incorporating a buffer layer between the waveguide and the electrodes. Using graphene electrodes, our experimental mode converter based on an EO-generated long-period grating in a LiNbO3 waveguide shows a reduction in the half-π voltage by almost three times, compared with the conventional electrode design using metal. With the buffer layer exempted, the device fabrication process is also significantly simplified. The use of graphene electrodes is an effective approach to enhancing the efficiency of EO devices and, at the same time, reducing their fabrication cost.

Compressed Supercontinuum Probe for Transient Absorption Microscopy

Tessa Calhoun and Kevin Higgins

Doc ID: 320435 Received 23 Jan 2018; Accepted 10 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: Here we combine three optical advancements to transient absorption microscopy in order to access the photodynamics in systems requiring stringent spatial and temporal resolution criteria. First, a broadband visible probe is generated by a commercial photonic crystal fiber. Second, an SLM-based pulse shaper is incorporated to reduce the pulse dispersion and improve temporal resolution. Third, 1.4 numerical aperture objectives for excitation and light collection provide optimal spatial resolution. The result of these improvements is a probe beam that spans 115 nm below 700 nm yet maintains a ~100 fs instrument response at the sample position. We demonstrate the capabilities of this microscope by imaging polystyrene beads in a solution of IR-144 dye revealing aggregated species at the bead surfaces.

65-Yb-doped all-fiber laser using tapered fiber for nonlinearity and dispersion management

peilong yang, Hao Teng, SHAOBO FANG, Zhongqi Hu, Guoqing Chang, Junli Wang, and Zhiyi Wei

Doc ID: 320767 Received 26 Jan 2018; Accepted 09 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: We implement an ultrafast Yb-doped all-fiber laser that incorporates tapered single-mode fibers for managing nonlinearity and dispersion. The tapered fiber placed in the oscillator cavity aims to broaden the optical spectrum of the intra-cavity pulse. At the oscillator output, we use another tapered fiber to perform pulse compression. The resulting 66.1-MHz Yb-doped all-fiber oscillator self-starts and generates 0.4-nJ, 65-fs pulses, which can serve as a compact and robust seed source for subsequent high-power, high-energy amplifiers.

Compact lenless subpixel resolution large field of view microscope

Manon Rostykus, Mattia Rossi, and Christophe Moser

Doc ID: 321141 Received 31 Jan 2018; Accepted 09 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: We report on a method to increase the spatial resolution in a compact lensless microscope. A compact side-illumination is fabricated to illuminate the sample with a collimated beam by diffraction from a volume phase grating. The wavelength of a semi-conductor laser source (VCSEL) is tuned with the injection current to alter the illumination direction by wavelength selective diffraction from the volume phase grating. The angle tuning is such that several subpixel shifted digital inline holograms are obtained. The stack of holograms is then processed in a pixel super resolution reconstruction algorithm. The amplitude of the sample is reconstructed with subpixel resolution over a large field of view (FOV). The technique is demonstrated on a 1951 USAF test target. A resolution of ~2.76μm, over a FOV of ~28mm2 is demonstrated for a device of <2cm height. The original pixel size was 5.2μm demonstrating the subpixel resolution.

Edge enhancement by negative Poincare-Hopf index filters

B.S.Bhargava Ram and Paramasivam Senthilkumaran

Doc ID: 321473 Received 05 Feb 2018; Accepted 09 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: Phase and polarization are interrelated quantities andhence polarization elements that perform like phase elementscan be designed. In this paper we show that apolarizing element producing negative Poincare-Hopf(PH) index beam can be used as a spatial filter to performedge enhancement. Either isotropic or anisotropicedge enhancement can be achieved by polarization selectionof the light that illuminates the sample. A conventionalmicroscope imaging system is modified intoa polarization selective optical Fourier processor. Experimentalresults are presented to show that negative PHindex filters, producing a set of orthogonal polarizationdistribution and their superpositions, can also be usedfor edge enhancement in optical signal processing.

High-gain and wide-band optical amplifications induced by coupled excited state of organic dye molecules co-doped in polymer waveguide

Youtaro Higase, Shinya Morita, Toshiyuki Fujii, Shun Takahashi, Kenichi Yamashita, and Fumio Sasaki

Doc ID: 323052 Received 12 Feb 2018; Accepted 09 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: We investigated optical gain and lasing characteristics of a polymer thin film co-doped with Rhodamine 6G (R-6G) and DCM dye molecules. By the co-doping of these dyes, the optical gain coefficient and its spectral range were enhanced and widened, respectively. These results cannot be accounted for by the linear combination of the optical gain properties of single-doped films. Owing to this high-gain and wide-band optical amplification property, a distributed feedback lasing with a low threshold of 70 μJ/cm2 as well as a widely tunable wavelength range of 587 – 613 nm was achieved. We attribute this amplification property of the co-doped film, which cannot be achieved with the single-doped films, to the appearance of coupled excited state between the two molecules.

Miniaturized single-fiber-based needle probe for combined imaging and sensing in deep tissue

Jiawen Li, Erik Schartner, Stefan Musolino, Bryden Quirk, Rodney Kirk, Heike Ebendorff-Heidepriem, and Robert McLaughlin

Doc ID: 323306 Received 23 Feb 2018; Accepted 08 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: The ability to visualize structure whilst simultaneously measuring chemical or physical properties of a biological tissue has the potential to improve our understanding of complex biological processes. We report the first miniaturized single-fiber-based imaging+sensing probe capable of simultaneous optical coherence tomography (OCT) imaging and temperature sensing. An OCT lens is fabricated at the distal end of a double clad fiber, including a thin layer of rare-earth doped tellurite glass to enable temperature measurements. The high refractive index of the tellurite glass enables a common-path interferometer configuration for OCT, allowing easy exchange of probes for biomedical applications. The simultaneous imaging+sensing capability is demonstrated on rat brains.

Mid-infrared frequency comb generation via cascaded quadratic nonlinearities in quasi-phase-matched waveguides

Abijith Kowligy, Alexander Lind, Dan Hickstein, David Carlson, Henry Timmers, Nima Nader, Flavio Cruz, Gabriel Ycas, Scott Papp, and Scott Diddams

Doc ID: 320558 Received 25 Jan 2018; Accepted 08 Mar 2018; Posted 08 Mar 2018  View: PDF

Abstract: We experimentally demonstrate a simple configuration for mid-infrared (MIR) frequency comb generation in quasi-phase-matched lithium niobate waveguides using the cascaded-χ² nonlinearity. With nanojoule-scale pulses from an Er:fiber laser, we observe octave-spanning supercontinuum in the near-infrared with dispersive-wave generation in the 2.5--3 μm region and intra-pulse difference-frequency generation in the 4--5 μm region. By engineering the quasi-phase-matched grating profiles, tunable, narrow-band MIR and broadband MIR spectra are both observed in this geometry. Finally, we perform numerical modeling using a nonlinear envelope equation, which shows good quantitative agreement with the experiment---and can be used to inform waveguide designs to tailor the MIR frequency combs. Our results identify a path to a simple single-branch approach to mid-infrared frequency comb generation in a compact platform using commercial Er:fiber technology.

87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb:YLF amplifier

Yi Hua, Wei Liu, Michaël Hemmer, Luis Zapata, Gengji Zhou, Damian Schimpf, Tino Eidam, Jens Limpert, Andreas Tünnermann, Franz Kärtner, and Guoqing Chang

Doc ID: 319283 Received 17 Jan 2018; Accepted 08 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-µJ pulse energy constituting a powerful seed source for cryogenically-cooled ultrafast Yb:YLF amplifiers.

Efficient frequency conversion for cubic harmonic generation at 266 nm in centrosymmetric α-BBO crystal

Mengjing Shi, Zhang Ge, Li Bingxuan, Renfu Li, Tao Yan, Min Luo, and Ning Ye

Doc ID: 322585 Received 13 Feb 2018; Accepted 07 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: Direct third-harmonic generation (THG) is a third-order nonlinear process without the restriction to the symmetric characteristic of crystals. It is of great interest for setting up a optical parametric oscillator which can be used in multi-photon quantum correlation. To obtain the pure and strong THG, we proposed to elect the centrosymmetric crystal with delocalized π bond as the nonlinear media during frequency conversion process. And an unprecedented cubic harmonic energy 37.6 μJ (average power ~ 37.6 mW, conversion efficiency ~ 2.5%) at 266 nm generated in α-BBO crystal (not the β-BBO), indicating direct THG in practical applications. These results also demonstrated a succinct and efficient way to generate deep-UV laser and another development direction of nonlinear crystals in UV region.

Full Stokes Temporal Imaging

Moti Fridman, Hamootal Duadi, and avi Klein

Doc ID: 315597 Received 12 Dec 2017; Accepted 07 Mar 2018; Posted 08 Mar 2018  View: PDF

Abstract: We developed a full Stokes temporal imaging system which measures the Stokes vector of ultrafast signals as a function of time. The system is based on a time-lens array where each time-lens in the array projects the signal on a different state of polarization.

Chaotic Brillouin Optical Correlation Domain Analysis

Jianzhong Zhang, Mingtao Zhang, Mingjiang Zhang, Yi Liu, Changkun Feng, Yahui Wang, and Yun-cai Wang

Doc ID: 319230 Received 09 Jan 2018; Accepted 07 Mar 2018; Posted 08 Mar 2018  View: PDF

Abstract: We propose and experimentally demonstrate a chaotic Brillouin optical correlation-domain analysis (BOCDA) system for distributed fiber sensing. The utilization of a chaotic laser with low coherence state ensures high spatial resolution. The experimental results demonstrate a 4-cm spatial resolution over a 906-m measurement range. The uncertainty in the measurement of the local Brillouin frequency shift is  1.2 MHz. The analysis of the expected spatial resolution and signal-to-noise ratio is also given.

Side scattering shadow and energy concentration effects of epsilon-near-zero media

Jiawen Song, jie luo, and Yun Lai

Doc ID: 321283 Received 01 Feb 2018; Accepted 06 Mar 2018; Posted 12 Mar 2018  View: PDF

Abstract: By analytically and numerically investigating the near-field scattering properties of particles composed of epsilon-near-zero (ENZ) media, we reveal a unique scattering phenomenon which is hereby denoted as the side scattering shadow and energy concentration effect. The scattering of ENZ particles leads to the emergence of near-field “shadows”, where the electric fields are significantly decreased to zero, on two sides of the particle perpendicular to the incident direction. At the same time, the electric fields inside the particle are enhanced to a certain extent, indicating concentration of wave energy. These extraordinary behaviors can be well explained through the Mie scattering theory. Moreover, such unique effects robustly exist for isotropic and anisotropic ENZ particles of almost arbitrary shapes and with loss. We further demonstrate interesting applications of this effect, including the “quenching” of surface plasmons and the guiding of electromagnetic energy in the deep subwavelength scale. Our findings may inspire novel applications in nanophotonics.

Graphene-based Broadband Terahertz Detector Integrated with Square-spiral Antenna

Wanlong Guo, Wang Lin, Xiaoshuang Chen, Changlong Liu, WeiWei Tang, Cheng Guo, Jin Wang, and Wei Lu

Doc ID: 315510 Received 19 Dec 2017; Accepted 06 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: The raising interest in terahertz (THz) radiation (loosely defined 0.1~10THz frequency range) for the application-oriented issues in everyday life, requires progressive development of fast, sensitive, portable photodetectors. In this work, a broadband graphene-based THz detector with both good integrability and sensitivity at room temperature is proposed. It is based on the CVD-grown graphene in integrated with a square-spiral metal antenna, which on one hand improves the efficiency for electromagnetic coupling, and on the other hand facilitates the hot-electron photo-thermoelectric process for the photon detection. Sensitivity over 28V/W at room temperature and noise-equivalent power of less than 0.35nW/Hz0.5 are demonstrated in referenced to the incident power. The presented results appealingly open an alternative way to realize chip-level graphene-based THz optoelectronics with good scalability and expected performance for targeted THz applications.

Thermal noise limited higher-order mode locking of a reference cavity

Zeng Xiaoyi, Yanxia Ye, Xiaohui Shi, Zhiyuan Wang, Ke Deng, Jie Zhang, and Zehuang Lu

Doc ID: 319582 Received 12 Jan 2018; Accepted 06 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: Higher-order mode locking has been proposed to reduce the thermal noise limit of reference cavities. By locking a laser to the HG02 mode of a 10-cm long all ULE cavity, and measure its performance with the three-cornered-hat method among three independently stabilized lasers, we demonstrate a thermal noise limited performance of a fractional frequency instability of 4.9×10^(−16). The results match the theoretical models with higher-order optical modes. The achieved laser instability improves the all ULE short cavity results to a new low level.

Double Anisotropic Coherent Backscattering of Light

Philipp Krauter, Christian Zoller, and Alwin Kienle

Doc ID: 321341 Received 02 Feb 2018; Accepted 06 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: A double anisotropic coherent backscattering cone was found. In contrast to the (single) anisotropic coherent backscattering, which was observed in liquid crystals, here, the long axis of the elongated structures changes its orientation with angular distance. We compared our results with the 2-dimensional Fourier transform of spatially resolved reflectance measurements and found a good agreement which is predicted by the reciprocity thesis. Furthermore, a Monte Carlo model was applied to reproduce successfully the results of the experiment, whereas the double anisotropy is not predicted by diffusion models.

Efficient middle‐infrared generation in LiGaS2 by simultaneous spectral broadening and difference-frequency generation

Peter Baum, Bo-Han Chen, and Tamas Nagy

Doc ID: 319975 Received 17 Jan 2018; Accepted 06 Mar 2018; Posted 13 Mar 2018  View: PDF

Abstract: We report a surprisingly broadband and efficient mid-infrared pulse generation in LiGaS2 (Langasite, LGS) by invoking a simultaneous interplay of intra-pulse difference frequency generation, self-phase-modulation and dispersion. This cascaded mechanism expands the output bandwidth and output power at the same time. With 30-fs driving pulses centered at 1030 nm wavelength we obtain a broadband middle-infrared (MIR) spectrum of 8-11 µm with an LGS crystal as thick as 4 mm, which is 8 times longer than the walk-off length.

Tuning selective reflection of light by surface anchoring in cholesteric cells with oblique helicoidal structures

Olena Iadlovska, GRAHAM Maxwell, Greta Babakhanova, George Mehl, Christopher Welch, Sergij Shiyanovskii, and Oleg Lavrentovich

Doc ID: 318731 Received 29 Dec 2017; Accepted 06 Mar 2018; Posted 14 Mar 2018  View: PDF

Abstract: Selective reflection of light by oblique helicoidal cholesteric (ChOH) can be tuned in a very broad spectral range by an applied electric field. In this work, we demonstrate that the peak wavelength of the selective reflection can be controlled by surface alignment of the director in sandwich cells. The peak wavelength is blue-shifted when the surface alignment is perpendicular to the bounding plates and red-shifted when it is planar. The effect is explained by the electric field redistribution within the cell caused by spatially varying heliconical ChOH structure. The observed phenomenon can be used in sensing applications.

Tunable Dispersion Compensation of Quantum Cascade Laser Frequency Combs

Johannes Hillbrand, Pierre Jouy, Mattias Beck, and Jérôme Faist

Doc ID: 321401 Received 19 Feb 2018; Accepted 06 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: Compensating for group velocity dispersion is an important challenge to achieve stable mid-infrared quantum cascade lasers (QCL) frequency combs with large spectral coverage. We present a tunable dispersion compensation scheme consisting of a planar mirror placed behind the back facet of the QCL. Dispersion can be either enhanced or decreased depending on the position of the mirror. We demonstrate that the fraction of the comb regime in the dynamic range of the laser increases considerably when the dispersion induced by the Gires-Tournois interferometer compensates the intrinsic dispersion of the laser. Furthermore, it is possible to tune to offset frequency of the comb with the Gires-Tournois interferometer while the repetition frequency is almost unaffected.

440 mW continuous-wave single-longitudinal-mode Er:Yb:Lu₂Si₂O₇ microchip laser at 1537 nm

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

Doc ID: 325022 Received 28 Feb 2018; Accepted 05 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: An Er:Yb:Lu₂Si₂O₇ microchip laser was constructed by placing an 1.2 mm-thick, Y-cut Er:Yb:Lu₂Si₂O₇ microchip between two 1.2 mm-thick sapphire crystals, in which input and output mirrors were directly deposited onto one face of each crystal. End-pumped by a continuous-wave 975.4 nm diode laser, a 1564 nm multi-longitudinal-mode laser with a maximum output power of 940 mW and slope efficiency of 20% was realized at an absorbed pump power of 5.5 W when the transmission of output mirror was 2.2%. When the transmission of output mirror was increased to 6%, a 1537 nm single-longitudinal-mode laser with a maximum output power of 440 mW and slope efficiency of 12% was realized at an absorbed pump power of 4.3 W. The results indicate that the Er:Yb:Lu₂Si₂O₇ crystal is a promising microchip gain medium to realize a single-longitudinal-mode laser.

Tailoring a nanofiber for enhanced photon emission and coupling efficiency from single quantum emitters

jinjin Du, wenfang li, and Sile Nic Chormaic

Doc ID: 319751 Received 17 Jan 2018; Accepted 04 Mar 2018; Posted 08 Mar 2018  View: PDF

Abstract: We present a novel approach to enhance the spontaneous emission rate of single quantum emitters in an optical nanofiber-based cavity by introducing a narrow air-filled groove into the cavity. Our results show that the Purcell factor for single quantum emitters located inside the groove of the nanofiber-based cavity can be at least six times greater than that for such an emitter on the fiber surface when using an optimized cavity mode and groove width. Moreover, the coupling efficiency of single quantum emitters into the guided mode of this nanofiber-based cavity can reach up to ~80% with only 35 cavity-grating periods. This new system has the potential to act as an all-fiber platform to realize efficient coupling of photons from single emitters into an optical fiber for quantum information applications.

A high-power 671-nm laser by second-harmonic generation with 93% efficiency in external ring cavity

Xingyang Cui, Qi Shen, MEI-CHEN YAN, Chao Zeng, Tao Yuan, Wen-Zhuo Zhang, Xing-Can Yao, Zhi Peng, Xiao Jiang, Yu-Ao Chen, and Jian-Wei Pan

Doc ID: 321386 Received 02 Feb 2018; Accepted 04 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: Second-harmonic generation (SHG) is useful for obtaining single-frequency continuous-wave laser sources at various wavelengths for applications ranging from biology to fundamental physics. Using an external power-enhancement cavity is an effective approach to improve the frequency conversion efficiency. However, thermal effects limit the efficiency, particularly, in high-power operation. Reducing thermal effects is therefore important when designing a cavity. This study reports the use of an external ring cavity for SHG, yielding 5.2-W, 671-nm laser light with a conversion efficiency of 93.8 ±0.8%, which is a new record of conversion efficiency for an external ring cavity. It is achieved using a 10-mm-length periodically poled potassium titanyl phosphate (PPKTP) crystal and a 65-um-radius beam waist in the cavity so as to minimize thermal dephasing and thermal lensing.Furthermore, a method is developed to determine the conversion efficiency more accurate based on measuring the pump depletion using a photodiode detector and a maximum pump depletion up to 97% is recorded. In this method, the uncertainty is much less than that achieved in common method by direct measuring with a power meter.

Imaging through scattering layers exceeding memory effect range with spatial-correlation-achieved point-spread-function

Li Long, Quan Li, Shuai Sun, Huizu Lin, WeiTao Liu, and Ping-Xing Chen

Doc ID: 322961 Received 09 Feb 2018; Accepted 04 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: Demands on optical imaging through scattering medium are growing rapidly in many fields. We propose to measure intensity transmission matrices or point-spread-function(PSF) of diffusers via spatial-correlation, with no scanning or interferometric detection required. With measured PSF, we report optical imaging based on memory effect that allows tracking of moving objects through scattering medium. Our technique enlarges the limited effective range of traditional imaging techniques based on memory effect, and substitutes time-consuming iterative algorithms by fast cross-correlation deconvolution method to greatly reduce time comsumption for image reconstructing.

Low-power broadband all-optical switching via intermodal cross-phase modulation in integrated optical waveguides

Niklas Lüpken, Tim Hellwig, Martin Schnack, Jörn Epping, Klaus Boller, and Carsten Fallnich

Doc ID: 320806 Received 26 Jan 2018; Accepted 04 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: We demonstrate the potential of all-optical switches in integrated waveguides based on intermodal cross-phase modulation between transverse modes. For this purpose, the differential phase between two transverse modes of a probe beam was altered by cross-phase modulation with a control beam propagating only in the fundamental mode. A switching behavior was accomplished by spatially filtering the resulting multimode interference of the probe modes, which changed depending on the control beam power. All-optical switching with a contrast of 82% at 1280nm over a frequency range of 4.4THz at 1.6nJ was achieved, representing an improvement by a factor of nearly 2000 compared to similar experiments in graded-index fibers. Additionally, we show that the center wavelength of the switch can be tailored by changing the cross-sectional geometry of the waveguide or the involved probe modes.

Integrated flexible-grid WDM transmitter using an optical frequency comb in microring modulators

YELONG XU, Jiachuan Lin, Raphaël Dubé-Demers, Sophie LaRochelle, Leslie Rusch, and Wei Shi

Doc ID: 320881 Received 29 Jan 2018; Accepted 04 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Advanced optical interconnects require high-speed links, which can be achieved by combining high channel rates with wavelength-division multiplexing (WDM). We report a multi-channel transmitter using cascaded microring modulators (MRMs) in silicon photonics. One MRM works as a flexible-grid optical comb generator, while the others work as channel modulators. With a single-wavelength laser input, we achieve flexible channel spacing (up to 25 GHz) with a tone-to-noise ratio (TNR) above 54 dB at power consumption of several milliwatts. We examine experimentally multi-channel transmission modulating data onto adjacent comb lines without significant signal crosstalk. This is the first demonstration of monolithic integration of a comb generator and multi-channel modulators for ultra-compact, power-efficient WDM photonic interconnects.

Sub-bandgap Photo-Response of Non-doped Black-silicon Fabricated by Nanosecond Laser Irradiation

Chun-Hao Li, Ji-Hong Zhao, Qi-Dai Chen, Jing Feng, and Hong-Bo Sun

Doc ID: 321095 Received 31 Jan 2018; Accepted 04 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Non-doped black silicon (b-Si) is fabricated on the surface layer of near-intrinsic Si substrate by nanosecond (ns) laser direct writing in argon (Ar) atmosphere. The non-doped samples exhibit near-unity sub-bandgap (1100~2500 nm) absorptance of more than 50%. Amazingly, the resistivity of the ns laser irradiated b-Si layer is about five orders of magnitude lower than that of the unprocessed Si substrate. The carrier density of b-Si layer is about 1×10¹⁸ cm¯³ according to the Hall effect measurement. Temperature-dependent Hall effect measurements show that the non-doped b-Si layer exhibits an energy level of 0.026 eV below the conduction band minimum (CBM). At last, Si infrared photodiodes are made based on the difference of carrier concentration between the ns-laser processed b-Si layer and the high resistivity Si substrate. The responsivity of the b-Si photodiode for 1310 nm is up to 256 mA/W at 10 V reverse bias, which is much higher than that of the reported pure Si bulk-structure photodiodes.

Measurement of Rotation Speed Based on Double-Beam Self-mixing Speckle Interference

BINGKUN GAO, Chen Qing, SHUNXIN YIN, peng chen, and Chunlei Jiang

Doc ID: 321269 Received 02 Feb 2018; Accepted 04 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Rotation speed measurement based on double-beam self-mixing speckle interference is presented. In this paper, double -beam is used to measure rotation speed, and the self-mixing speckle signal is analyzed by cross-correlation algorithm. The experimental results show that this approach can measure high speed at a low sampling rate compared with the traditional Doppler frequency shift method.

Spectral shifts of stimulated Rayleigh - Mie scattering in Ag nanoparticle colloids

Alexander Erokhin, Igor Smetanin, Nikolay Bulychev, and Sergey Mikhailov

Doc ID: 314816 Received 01 Dec 2017; Accepted 04 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Stimulated Rayleigh-Mie Scattering~(SRMS) in two photon absorption~(2PA) liquids is realized by Fourier transform-limited pulsed Nd-glass laser. For the first time we have measured anti-Stokes spectral shifts of the SRMS in toluene and hexane colloids of Ag nanoparticles as well as in pure toluene. These values appreciably exceed the Rayleigh line width in those liquids. The Four-wave mixing method is applied both experimentally and theoretically to display the process as Rayleigh-induced parametric generation. We show that the amplification effect is provided predominantly by thermally-induced coherent polarization oscillations while interference-assisted thermal grating provides formation of a self-induced optical cavity inside the interaction region.

A Spectrally Efficient Digitized Radio-over-fiber System with K-means Clustering based Multidimensional Quantization

Lu Zhang, Xiaodan Pang, Oskars Ozolins, Aleksejs Udalcovs, Sergei Popov, shilin xiao, Weisheng Hu, and Jiajia Chen

Doc ID: 318006 Received 22 Dec 2017; Accepted 03 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: We propose a spectrally efficient digitized radio-over-fiber (D-RoF) system by grouping highly-correlated neighboring samples of the analog signals into multidimensional vectors, where the k-means clustering algorithm is adopted for adaptive quantization. A 30-Gbit/s D-RoF system is experimentally demonstrated to validate the proposed scheme, reporting carrier aggregation of up to 40 100-MHz orthogonal frequency division multiplexing (OFDM) channels with quadrate amplitude modulation (QAM) order of 4 and aggregation of 10 100-MHz OFDM channels with QAM order of 16384. The equivalent common public radio interface rates from 37-Gbit/s to 150-Gbit/s are supported. Besides, the error vector magnitude (EVM) of 8% is achieved with the number of quantization bits of 4, and the EVM can be further reduced to 1% by increasing the number of quantization bits to 7. Compared with conventional pulse coding modulation based D-RoF systems, the proposed D-RoF system improves the signal-noise-ratio up to ~9-dB and greatly reduces the EVM given the same number of quantization bits.

Simultaneous viscosity and elasticity measurement using laser speckle contrast imaging

Xiao Chen, yang wang, Jinling Lu, and Pengcheng Li

Doc ID: 318149 Received 20 Dec 2017; Accepted 03 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Viscosity and elasticity are closely related to the status of biological tissues. This letter reports a simultaneous quantitative measurement of these parameters realized with the laser speckle contrast imaging method. Propagation of a Rayleigh wave induced by an acoustic speaker is traced, and the frequency-dependent velocity dispersion is extracted with the frequency-wavenumber spectrum analysis method. The viscosity and elasticity moduli of oil-in-gelatin tissue-mimicking phantoms are calculated by fitting the dispersion curves to the Voigt model. The method is validated by comparing with results obtained by using the conventional mechanical rheometer.

Integrated Catheter for Simultaneous Radiofrequency Ablation and Optoacoustic Monitoring of Lesion Progression

Johannes Rebling, Francisco Javier Oyaga Landa, Xose Luis Dean Ben, Alexandre Douplik, and Daniel Razansky

Doc ID: 319854 Received 22 Jan 2018; Accepted 03 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Radiofrequency catheter ablation is commonly used to eliminate dysfunctional cardiac tissue by heating via an alternating current. Clinical outcomes are highly dependent on careful anatomical guidance, electrophysiological mapping, as well as careful radiofrequency power titration during the procedure. Yet, current treatments rely mainly on the expertise of the surgeon to assess lesion formation, causing large variabilities in the success rate. We present an integrated catheter design suitable for simultaneous radiofrequency ablation and real-time optoacoustic monitoring of the forming lesion. The catheter design utilizes copper-coated multimode light-guides capable of delivering both ablation current and near-infrared pulsed-laser illumination to the target tissue. The generated optoacoustic responses were used to visualize the ablation lesion formation in ex-vivo bovine heart specimen in 3D. The presented catheter design enables the monitoring of ablation lesions with high spatio-temporal resolution while the overall therapy-monitoring approach remains compatible with commercially available catheter designs.

Coherently combined 16 channel multicore fiber laser system

Arno Klenke, Michael Müller, Henning Stark, Fabian Stutzki, Christian Hupel, Thomas Schreiber, Andreas Tünnermann, and Jens Limpert

Doc ID: 319963 Received 19 Jan 2018; Accepted 03 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled aperture beam splitting and combination together with temporal phasing is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses are achieved with combination efficiencies around 80%.

Enhancing THz magnetic near-field induced by metamaterial resonator with a tapered waveguide

Hongsong Qiu, Takayuki Kurihara, Hirofumi Harada, Kosaku Kato, Keisuke Takano, Tohru Suemoto, Masahiko Tani, Nobuhiko Sarukura, Masashi Yoshimura, and Makoto Nakajima

Doc ID: 319260 Received 19 Jan 2018; Accepted 03 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Substantial enhancement of terahertz magnetic near-field achieved by the combination of a tapered metallic waveguide and a metamaterial resonator is demonstrated. The magnetic near-field is probed directly via the magneto-optic sampling with a Tb₃Ga₅O₁₂ crystal. The incident terahertz wave with a half cycle waveform is generated by using the pulse-front tilting method. The magnetic near-field at the resonant frequency is enhanced by more than 30 times through the combination of the waveguide and the resonator. The peak amplitude of the magnetic field with a damped oscillation waveform in the time domain is up to 0.4 T. The resonant frequency can be tuned by adopting different resonator designs. The mechanism of the enhancement is analyzed by performing calculations based on the finite element method. The strong terahertz magnetic near-field enables the excitation of large-amplitude spin dynamics and can be utilized for an ultrafast spin control.

Gain-switched fiber laser at 3.55 μm

Frédéric Jobin, Vincent Fortin, Frédéric Maes, Martin Bernier, and Real Vallee

Doc ID: 319877 Received 25 Jan 2018; Accepted 03 Mar 2018; Posted 15 Mar 2018  View: PDF

Abstract: We report the first demonstration of a gain-switched fiber laser operating near 3.5 μm. A dual-wavelength pumping scheme consisting of a 1976 nm Q-switched fiber system and a continuous-wave 976 nm laser diode was used to gain-switch a monolithic erbium-doped fluorozirconate fiber laser cavity at 3.552 μm. Stable pulses were produced for repetition rates ranging between 15 kHz and 20 kHz and a record peak power of 204 W was achieved at 15 kHz. A quenching phenomenon was also observed at 15 kHz for 1976nm pulse energies beyond 180 µJ.

Micro-hole drilling by tightly focused vector beams

Shugo Matsusaka, Yuichi Kozawa, and Shunichi Sato

Doc ID: 319160 Received 05 Jan 2018; Accepted 02 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: We demonstrated that azimuthally polarized beams offer high speed laser micro-hole drilling compared with radially and linearly polarized beams under a tight focusing condition. The speed was evaluated by piercing time by vector beams for several objective lenses with different numerical aperture (NA). As a result, in the case of NA = 0.63, the piercing time of an azimuthally polarized beam was 2.4 to 3.2 times shorter than that of a linearly polarized beam for all materials tested. Surprisingly, for NA = 0.85, the difference is expanded to about 7.7 times for copper. It is indicated that the number of reflections on the side wall of the hole played a significant role to give rise to the difference in the piercing time depending on the polarization of the light beam.

All-fiber all-normal-dispersion femtosecond laser with nonlinear multimodal interference based saturable absorber

Uğur Tegin and Bulend Ortac

Doc ID: 319516 Received 12 Jan 2018; Accepted 02 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: In this letter, we demonstrate the first all-fiber all-normal dispersion ytterbium-doped oscillator with nonlinear multimodal interference based saturable absorber capable to generate ultrashort dissipative soliton pulses. Additional to functioning as a saturable absorber, the use of multimode fiber segments between single mode fibers also ensures the bandpass filtering via multimode interference reimaging necessary to obtain dissipative soliton mode-locking. The oscillator generates dissipative soliton pulses at 1030 nm with 5.8 mW average power, 5 ps duration and 44.25 MHz repetition rate. Pulses are dechirped to 276 fs via an external grating compressor. All-fiber cavity design ensures high stability and ~70 dB sideband suppression is measured in RF spectrum. Numerical simulations are performed to investigate cavity dynamics and obtained results well-matched with experimental observations. The proposed cavity presents an alternative approach to achieve all-fiber dissipative soliton mode-locking with a simple and low-cost design.

Generation of coherent extreme--ultraviolet radiation from alpha-quartz using 50 fs laser pulses at 1030 nm wavelength and high repetition rates

Tran Trung Luu, Valerio Scagnoli, Susmita Saha, Laura Heyderman, and Hans Jakob Woerner

Doc ID: 319623 Received 17 Jan 2018; Accepted 02 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Coherent extreme--ultraviolet (EUV) radiation using high harmonic generation (HHG) in alpha-quartz is demonstrated from 10 - 200 kHz, using 50 fs laser pulses at the center wavelength of 1030 nm. The EUV radiation extends beyond 25 eV in the non-damaging regime. The number of photons generated in a single harmonic order at 15.6 eV is measured to be ~ 3.5 ± 2.5*10^10 per second which is a first and record value reported to date using EUV HHG from solids. Our work demonstrates one of the first all-solid-state EUV sources based on industrial-grade fiber lasers, enabling the possibility of bringing reliable EUV sources to the mass market.

Broadband and High Efficiency Vortex Beam Generator based on Hybrid Helix Array

Chao Wu, Chaoqun Fang, gong zhijie, Song Zhao, Anqi Sun, Zeyong Wei, and Hongqiang Li

Doc ID: 321312 Received 02 Feb 2018; Accepted 02 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Orbital angular momentum carried by vortex beam provides versatile applications like high-resolution imaging, particle manipulation and optical communications. Generation of vortex beam based on the Pancharatnam-Berry (PB) phase has drawn considerable attentions for its unique spin to orbital conversion features. Despite of the intrinsic frequency independent features of PB phase, broadband and high efficiency circular polarization conversion optical element is needed for broadband high efficiency vortex beam generation. In this work, a broadband and high efficiency vortex beam generator based on PB phase is built with hybrid helix array. Such devices are used to generate vortex beams with arbitrary topological charge, and vortex beams with opposite topological charge can be generated with an opposite handedness beam incident backward. The measured efficiency of our device is above 65% for a wide frequency range with the relative bandwidth of 46.5%.

All-fiber radially/azimuthally polarized lasers based on mode-coupling of tapered fibers

Dong Mao, Zhiwen He, Hua Lu, Li Mingkun, Wending Zhang, cui xiaoqi, Biqiang Jiang, and Jianlin Zhao

Doc ID: 319946 Received 17 Jan 2018; Accepted 02 Mar 2018; Posted 06 Mar 2018  View: PDF

Abstract: We demonstrate a mode converter with an insertion loss of 0.36 dB based on mode-coupling of tapered single-mode fiber and two-mode fiber, and then realize all-fiber flexible cylindrical-vector lasers (CVLs) at 1550 nm. Attributing to the continuous distribution of tangential electric field at taper boundaries, the CVL is switchable between the radially and azimuthally polarized states by adjusting the input polarization. In the temporal domain, the operation is controllable among continuous-wave, Q-switched, and mode-locked statuses by changing the saturable absorber or pump strength. The duration of Q-switched RPB/APB spans from 10.4/10.8 μs to 6/6.4 μs at the pump range of 38 mW to 58 mW, while that of mode-locked pulse varies from 39.2/31.9 ps to 5.6/5.2 ps by controlling the laser bandwidth. The proposed scheme combines the features of cylindrical-vector beam, fiber laser, and ultrafast pulse, providing a special while cost-effective source for practical applications.

Dissipation induced $W$ state in a Rydberg-atom-cavity system

Dong Xiao Li, xq shao, Jin-Hui Wu, and X X Yi

Doc ID: 323301 Received 14 Feb 2018; Accepted 02 Mar 2018; Posted 06 Mar 2018  View: PDF

Abstract: A dissipative scheme is proposed to prepare tripartite $W$ state in a Rydberg-atom-cavity system. It is an organic combination of quantum Zeno dynamics, Rydberg antiblockade and atomic spontaneous emission to turn the tripartite $W$ state into the unique steady state of the whole system. The robustness against the loss of cavity and the feasibility of the scheme are demonstrated thoroughly by the current experimental parameters, which leads to a high fidelity above $98\%$.

AlGaAs-based VECSEL exceeding 4 W of direct emission power in the 740 - 790 nm spectral range

Hermann Kahle, Kostiantyn Nechay, Jussi-Pekka Penttinen, Antti Tukiainen, Sanna Ranta, and Mircea Guina

Doc ID: 315637 Received 19 Dec 2017; Accepted 02 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: An optically pumped vertical-external-cavity surface-emitting laser (VECSEL) for direct emission in the 740 - 790 nm wavelength region is reported. The gain structure is based on 12 AlGaAs quantum wells (QWs). Wavelength tuning between 747 nm to 788 nm and free running operation with a maximum power of 4.24 W (pump power limited) are demonstrated for a heat sink temperature of 14°C. This laser system fills a spectral gap uncovered with VECSEL technology and also represents the most powerful VECSEL within the 700-nm wavelength region.

Terahertz adaptive optics with a deformable mirror

Emmanuel Abraham, Mathilde Brossard, Jean-Francois Sauvage, and Mathias Perrin

Doc ID: 322805 Received 08 Feb 2018; Accepted 01 Mar 2018; Posted 01 Mar 2018  View: PDF

Abstract: We report on the wavefront correction of a terahertz beam using adaptive optics which requires a wavefront sensor able to sense the optical aberrations and a wavefront corrector. The wavefront sensor relies on a direct 2D electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. By measuring the phase variation of the terahertz electric field in the crystal, we were able to minimize the geometrical aberrations of the beam thanks to the action of a deformable mirror. This phase control will open the route to terahertz adaptive optics in order to optimize the THz beam quality for both practical and fundamental applications.

Enhanced broadband terahertz radiation generation near the reststrahlen band in sub-wavelength leaky-mode LiNbO₃ waveguides

Brett Carnio and Abdulhakem Elezzabi

Doc ID: 318374 Received 26 Dec 2017; Accepted 01 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Generation of terahertz (THz) radiation in a compact geometry is crucial for the implementation of on-chip, coherent THz radiation sources. Here, via numerical time domain simulations, we show that LiNbO₃ (LN) waveguides having sub-wavelength core widths can provide high-efficiency optical-to-THz radiation frequency-conversion. By exploiting the nonlinear susceptibility enhancement of LN near its phonon reststrahlen band and utilizing THz leaky mode guidance to minimize reststrahlen band absorption and improve phase matching, we show that broadband (0.2-11.6 THz) electric field pulses of 3.4 kV/cm can be generated at an optical-to-THz conversion efficiency of 2.5×10-4. These sub-wavelength leaky-mode waveguides provide a compact platform for wideband, coherent THz radiation sources.

Broadening the optical bandwidth of quantum cascade lasers using RF noise current perturbations

Tomas Pinto, James Kirkbride, and Grant Ritchie

Doc ID: 321041 Received 02 Feb 2018; Accepted 01 Mar 2018; Posted 07 Mar 2018  View: PDF

Abstract: We report on the broadening of the optical bandwidth of a distributed feedback quantum cascade laser (QCL) caused by the application of radio frequency (RF) noise to the injection current. The broadening is quantified both via Lamb-dip spectroscopy and the frequency noise power spectral density (PSD). The linewidth of the unperturbed QCL (emitting at approx. 5.3 μm) determined by Lamb-dip spectroscopy is 680 ± 170 kHz, and is in reasonable agreement with the linewidth of 460 ± 40 kHz estimated by integrating the PSD measured under the same laser operating conditions. Measurements with both techniques reveal that by mixing the driving current with broadband RF noise the laser lineshape was reproducibly broadened up to approx. 6 MHz with an increasing Gaussian contribution. The effects of linewidth broadening are then demonstrated in the two colour coherent transient spectra of nitric oxide.

Spin-optomechanical coupling between light and a nanofiber torsional mode

Eliot Fenton, adnan khan, Pablo Solano, Luis Orozco, and Fredrik Fatemi

Doc ID: 320540 Received 25 Jan 2018; Accepted 01 Mar 2018; Posted 05 Mar 2018  View: PDF

Abstract: Light that carries linear or angular momentum can interact with a mechanical object giving rise to optomechanical effects. In particular, a photon can transfer its intrinsic angular momentum to an object when the object either absorbs the photon or changes the photon polarization, as in an action/reaction force pair. Here, we demonstrate resonant driving of torsional mechanical modes of a single-mode tapered optical nanofiber using spin angular momentum. The nanofiber torsional mode spectrum is characterized by polarimetry, showing narrow natural resonances (Q≈ 2,000). By sending amplitude-modulated light through the nanofiber, we resonantly drive individual torsional modes as a function of the light polarization. By varying the input polarization to the fiber, we find the largest amplification of a mechanical oscillation (>35 dB) is observed when driving the system with light containing longitudinal spin on the nanofiber waist. These results present optical nanofibers as a platform suitable for quantum spin-optomechanics experiments.

Novel approach for temperature-insensitive strain measurement using a dual-core As₂Se₃-PMMA taper

Song Gao, Chams Baker, Liang Chen, and Xiaoyi Bao

Doc ID: 320522 Received 24 Jan 2018; Accepted 28 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: A temperature-insensitive strain sensor is proposed and demonstrated based on a dual-core As₂Se₃-PMMA taper. Longitudinal and transverse forces on the As₂Se₃ cores are induced by thermal expansion/contraction of the PMMA cladding due to an order of magnitude difference between the thermal expansion coefficients of As₂Se₃ and PMMA. At an optimal PMMA layer thickness, the wavelength shift caused by the thermally-induced forces on the refractive-index of the dual-core fiber cores counterbalances that caused by the thermally-induced fiber length variation leading to temperature insensitive transmission. Temperature-insensitive strain measurement is demonstrated in a dual-core As₂Se₃-PMMA fiber with an As₂Se₃ core diameter of 0.61 μm and a PMMA cladding diameter of 34.4 μm. Thermally-induced forces in hybrid fibers open the path towards the realization of novel sensors and devices that are immune to temperature fluctuations.

Dual-reference digital holographic interferometry for analyzing high density gradients in fluid mechanics

Jean-Michel Desse and François OLCHEWSKY

Doc ID: 319611 Received 12 Jan 2018; Accepted 28 Feb 2018; Posted 06 Mar 2018  View: PDF

Abstract: This paper proposes a dual-reference digital holographic interferometer for analyzing high refractive index encountered in transonic and supersonic flows. For that, a Wollaston prism is inserted in the reference arm in order to simultaneously generate two orthogonally polarized reference waves. As consequence, recorded interferograms contain two crossed and perpendicular interference patterns which give two orders fully separated in the Fourier spectrum. It is then possible to analyze a transparent object regardless the orientation of the refractive index gradient using the two phase maps reconstructed with each of the two first interference orders. Fusion of the phase maps yield a single phase map in which the phase singularities are removed. Experimental results demonstrate the suitability of the proposed approach for analyzing shock waves in the unsteady wake flow around a circular cylinder at Mach 0.75.

All-fiber gas sensor with intracavity photothermal spectroscopy

YAN ZHAO, Wei Jin, Yuechuan Lin, Fan Yang, and HOI LUT HO

Doc ID: 318907 Received 03 Jan 2018; Accepted 28 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: We present an all-fiber intracavity photothermal (IC-PT) spectroscopic gas sensor with a hollow-core photonic bandgap fiber (HC-PBF) gas cell. The gas cell is placed inside a fiber-ring laser cavity to achieve higher laser light intensity in the hollow-core and hence higher PT modulation signal. Experiment with a 0.62-m-long HC-PBF gas cell demonstrated a noise equivalent concentration of 896 ppb acetylene. Theoretical modeling shows that the IC-PT sensor has the potential of achieving sub-ppb (parts-per-billion) acetylene detection sensitivity.

Side lobe apodization in optical pulse compression reflectometry for fiber optic distributed acoustic sensing

juan josé Roselló, Sonia Martin-Lopez, Miguel Gonzalez Herraez, and Alayn Loayssa

Doc ID: 319942 Received 17 Jan 2018; Accepted 28 Feb 2018; Posted 01 Mar 2018  View: PDF

Abstract: We demonstrate a technique to reduce the sidelobes in optical pulse compression reflectometry for distributed acoustic sensing. the technique is based on using a Gaussian probe pulse with linear frequency modulation. This is shown to improve the sidelobe supression by 13 dB compared to the use of square pulses, without any significant penalty in terms of spatial resolution. In addition, a 2.25-dB enhancement in signal-to-noise ratio is calculated compared to the use of receiver-side windowing. The method is tested by measuring 700 Hz vibrations with 140 nε amplitude at the end of a 50 km fiber sensing link with 34 cm spatial resolution, giving a record 147,058 spatially-resolved points.

Deuterated silicon nitride photonic devices for broadband optical frequency comb generation

Jeff Chiles, Nima Nader, Dan Hickstein, Su Peng Yu, Travis Briles, David Carlson, Hojoong jung, Jeffrey Shainline, Scott Diddams, Scott Papp, Sae Woo Nam, and Richard Mirin

Doc ID: 321340 Received 06 Feb 2018; Accepted 28 Feb 2018; Posted 01 Mar 2018  View: PDF

Abstract: We report and characterize low-temperature, plasma-deposited deuterated silicon nitride thin films for nonlinear integrated photonics. With a peak processing temperature less than 300ºC, it is back-end compatible with pre-processed CMOS substrates. We achieve microresonators with a quality factor of up to 1.6 × 10⁶ at 1552 nm, and >1.2 × 10⁶ throughout λ = 1510-1600 nm, without annealing or stress management. We then demonstrate the immediate utility of this platform in nonlinear photonics by generating a 1 THz free spectral range, 900-nm-bandwidth modulation-instability microresonator Kerr comb and octave-spanning, supercontinuum-broadened spectra.

Grayness-dependent emission of dispersive waves from dark solitons in optical fibers

Alexandre Kudlinski, Tomy Marest, Carlos Mas Arabi, Matteo Conforti, Arnaud Mussot, Carles Milian Enrique, and Dmitry Skryabin

Doc ID: 320250 Received 24 Jan 2018; Accepted 27 Feb 2018; Posted 28 Feb 2018  View: PDF

Abstract: We report the experimental observation of dispersive wave emission from gray solitons propagating in the normal dispersion region of an optical fiber. Besides observing for the first time the emission of a dispersive wave from an isolated dark soliton, we show that the dispersive wave frequency and amplitude strongly depends on soliton grayness. This process can be explained by the higher-order dispersion contribution into the phase-matching condition and the grayness of the soliton. Numerical simulations and theoretical predictions are in good agreement with the experiments.

Photonics-enabled wideband microwave burst detection

Yihan Li, Naoya Kuse, and Martin Fermann

Doc ID: 318123 Received 12 Jan 2018; Accepted 27 Feb 2018; Posted 28 Feb 2018  View: PDF

Abstract: In this paper, a novel wideband microwave burst detection system is realized by utilizing photonics-assisted wavelength and time division multiplexing, in conjunction with optical storage. Deploying a coherent electro-optical dual comb and a recirculating optical frequency shifter with ~ 1.28 µs round-rip delay, the proof-of-concept experimental system demonstrates the interrogation of ~ 1 µs RF bursts with up to 8 GHz bandwidth and arbitrary hopping pattern at 1 MHz resolution and a refresh rate of ~ 60 kHz using an 80 MHz RF detection unit. The proposed system can be easily upgraded to higher bandwidth and longer burst time-aperture with suitable hardware.

Microcavity-coupled fiber Bragg grating with tunable reflection spectra and speed of light

Lei Chen, Ya Han, Qian Liu, Yange Liu, Weigang Zhang, and Keng Chou

Doc ID: 319771 Received 15 Jan 2018; Accepted 27 Feb 2018; Posted 07 Mar 2018  View: PDF

Abstract: After a fiber Bragg grating (FBG) is fabricated, the reflection spectrum of the FBG is generally not tunable without mechanical deformation or temperature adjustment. Here we present a microcavity-coupled FBG with tunable reflection lineshape and dispersion using electromagnetically induced transparency. The Fano interference of light in the FBG and the microcavity allows dramatic modification of the reflection spectrum. The phase of the reflected spectrum is tunable continuously between 0 and 2π to produce various Fano lineshapes. The dispersion of the output light is adjustable from normal to abnormal dispersion, consequently providing an adjustable speed of light. Additionally, it allows the FBG to switch from a notch filter to a bandpass filter at the resonant wavelength, which is not possible in a conventional uniform FBG.

Resonant-state expansion for open optical systems:Generalization to magnetic, chiral, and bi-anisotropic materials

Egor Muljarov and Thomas Weiss

Doc ID: 320632 Received 30 Jan 2018; Accepted 27 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: The resonant-state expansion, a recently developed powerful method in electrodynamics, is generalized here for open optical systems containing magnetic, chiral, or bi-anisotropic materials. It is shown that the key matrix eigenvalue equation of the method remains the same, but the matrix elements of the perturbation now contain variations of the permittivity, permeability, andbi-anisotropy tensors. A general normalization of resonant states in terms of the electric and magnetic fields is presented.

Filamentation of Mid-IR pulses in ambient air in the vicinity of molecular resonances

Valentina Shumakova, Skirmantas Alisauskas, Pavel Malevich, Claudia Gollner, Andrius Baltuska, Daniil Kartashov, Aleksei Zheltikov, Alexander Mitrofanov, Aleksandr Voronin, Dmitry Sidorov-Biryukov, and Audrius Pugzlys

Doc ID: 322812 Received 08 Feb 2018; Accepted 26 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: Properties of filaments ignited by multi-millijoule, 90 fs mid-IR pulses centered at 3.9 µm are examined experimentally by monitoring plasma density and losses as well as spectral dynamics and beam profile evolution at different focusing strengths. By softening the focusing from strong (f=0.25 m) to loose (f=7 m) we observe a shift from plasma assisted filamentation to filaments with low plasma density. In the latter case, filamentation manifests itself by beam self-symmetrization and spatial self-channeling. Spectral dynamics in the case of loose focusing is dominated by the non-linear Raman frequency downshift, which leads to the overlap with the CO2 resonance in the vicinity of 4.2 µm. The dynamic CO2 absorption in the case of 3.9 µm filaments with their low plasma content is the main mechanism of energy losses and either alone or together with other nonlinear processes contributes to the arrest of intensity.

Observation of sub-femtosecond fluctuations of the pulse separation in a soliton molecule

Haosen Shi, Youjian Song, Luming Zhao, Ming-lie Hu, and Chingyue Wang

Doc ID: 319667 Received 22 Jan 2018; Accepted 26 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: In this work, we study on the timing instability of a scalar twin-pulse soliton molecule generated by a passively mode-locked Er-fiber laser. Attosecond precision relative timing jitter characterization between the two solitons composing the molecule is enabled by the balanced optical cross-correlation (BOC) method. Jitter spectral density reveals a short term (on the μs to ms timescale) random fluctuation of the pulse separation even in the robust stationary soliton molecules. The rms timing jitter is on the order of femtoseconds depending on the pulse separation and the mode-locking regime. The lowest rms timing jitter is < 1 fs which is observed in the dispersion managed mode-locking regime. Moreover, the BOC method is proved to be capable of resolving the soliton interaction dynamics in various vibrating soliton molecules.

Ultracompact high-resolution photoacoustic microscopy

Qian Chen, Guo Heng, Tian Jin, Weizhi Qi, Huikai Xie, and Lei Xi

Doc ID: 319229 Received 09 Jan 2018; Accepted 26 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: Optical resolution photoacoustic microscopy (ORPAM), benefiting from rich optical contrast, scalable acoustic resolution and deep penetration depth, is of great importance for the fields of biology and medicine. However, limited by the size and performance of reported optical/acoustic scanners, existing portable/handheld ORPAMs are bulky, heavy, and suffer from low imaging quality/speed. Here we present an ultracompact ORPAM probe, featuring miniature, light, and high imaging quality. The probe only weighs 20 grams, and has an outer size of 22×30×13 mm3, a high lateral resolution of 3.8 μm, and an effective imaging domain of 2×2 mm2. To show its advantages over existing ORPAMs, we apply this probe to image vasculatures of internal organs in rat abdominal cavity and inspect the entire human oral cavity.

Laser stabilisation with a frequency-to-voltage chip for narrow-line laser cooling

John Mc Ferran

Doc ID: 320047 Received 18 Jan 2018; Accepted 26 Feb 2018; Posted 27 Feb 2018  View: PDF

Abstract: We use integrated circuit based frequency-to-voltage conversion of a frequency comb beat signal as the means for laser frequency stabilisation that is suitable for narrow-line laser cooling. The method is compared to an atomic beam lock where the laser frequency instability for the new scheme shows an improvement of two orders of magnitude at sub-1s and grants a lock-capture range that is approximately thirty times greater. We employ the locking method on a 1111.6nm laser that is frequency doubled and used in a dual-wavelength magneto-optical trap for 171Yb atoms, producing atomic cloud temperatures of ~20μK.

Michelson mode selector for spectral range stabilization in a self-sweeping fiber laser

Alina Tkachenko, Anastasia Vladimirskaya, Ivan Lobach, and Sergey Kablukov

Doc ID: 320226 Received 23 Jan 2018; Accepted 26 Feb 2018; Posted 27 Feb 2018  View: PDF

Abstract: We report on spectral range stabilization in a self-sweeping laser by adding a narrowband fiber Bragg grating (FBG) to output mirror in Michelson configuration. The effects of FBG reflectivity and optical path difference in the Michelson interferometer on the laser spectral dynamics are investigated. Optimization of the interferometer allows us to demonstrate broadband (over 16 nm) self-sweeping operation and reduction of the start and stop wavelengths fluctuations by two and one order of magnitude (~100 and 15 times) for start and stop bounds respectively (down to several picometers). The proposed approaches improves significantly quality of the spectral dynamics and facilitates application of the self-sweeping lasers.

Watt-level dysprosium fiber laser at 3.15 μm with 73% slope efficiency

Robert Woodward, Matthew Majewski, Gayathri Bharathan, Darren Hudson, Alex Fuerbach, and Stuart Jackson

Doc ID: 320964 Received 30 Jan 2018; Accepted 26 Feb 2018; Posted 27 Feb 2018  View: PDF

Abstract: Rare-earth-doped fiber lasers are emerging as promising high-power mid-infrared sources for the 2.6–3.0 μm and 3.3–3.8 μm regions based on erbium and holmium ions. The intermediate wavelength range, however, remains vastly underserved, despite prospects for important manufacturing and defense applications. Here, we demonstrate the potential of dysprosium-doped fiber to solve this problem, with a simple in-band-pumped grating-stabilized linear cavity generating up to 1.06 W at 3.15 μm. A slope efficiency of 73% with respect to launched power (77% relative to absorbed power) is achieved—the highest value for any mid-infrared fiber laser to date. Opportunities for further power and efficiency scaling are also discussed.

Polarization-insensitive 4-channel CWDM (de)multiplexer based on Mach-Zehnder interferometers with bent directional couplers and polarization rotators

Yaocheng Shi, Hongnan Xu, and Liu Liu

Doc ID: 319220 Received 08 Jan 2018; Accepted 25 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: A polarization-insensitive 4-channel CWDM (de)multiplexer based on Mach-Zehnder interferometers is proposed and demonstrated. By utilizing the bent directional couplers and the polarization rotators, the CWDM (de)multiplexer could work for both TE and TM polarizations. The measurement results of the fabricated devices show that the 1-dB bandwidth is ~ 15 nm for dual polarizations, and the polarization dependent losses are < ~ 0.5 dB for all the 4 channels.

Digital CD pre-management enabled single lane 112 Gb/s PAM-4 signal transmission over 80 km SSMF

Songnian Fu, Chen Chen, Gao Fan, Xiaolei Li, Lei Deng, Ming Tang, and Deming Liu

Doc ID: 319292 Received 09 Jan 2018; Accepted 25 Feb 2018; Posted 28 Feb 2018  View: PDF

Abstract: We experimentally demonstrate single-lane 112 Gb/s four-level pulse-amplitude modulation (PAM-4) signal transmission over 80 km standard single-mode fiber (SSMF) by digital chromatic dispersion (CD) pre-management technique. By theoretically investigating the interplay between CD and self-phase modulation (SPM) during the SSMF transmission, we identify that the SPM induced impairment can be optically mitigated by the residual positive CD of SSMF link. Thanks to the digital CD pre-management for the purpose of both CD compensation and nonlinear impairment mitigation, simplified Volterra filter equalizer is good enough to achieve BER below the 7% HD-FEC threshold.

Extreme electro-optic tuning of Bragg mirrors integrated in lithium niobate nanowaveguides

Marc Reig, David Pohl, Anton Sergeyev, and Rachel Grange

Doc ID: 315838 Received 15 Dec 2017; Accepted 25 Feb 2018; Posted 27 Feb 2018  View: PDF

Abstract: Bragg reflectors are essential elements in optical telecommunication and sensing applications. Their miniaturization down to the sub-micron scale has been achieved in silicon-on-insulator chips. However, their tunability is limited only to thermal tuning. In order to achieve a faster and more practical tunability operation, here we report on electro-optically tunable Bragg reflectors with ~14 dB signal filtering on a lithium niobate-on-insulator platform while keeping sub-micron cross-sections. Due to the lithium niobate electro-optic properties and the chosen electrodes configuration, a Bragg tunability coefficient of .37±0.55 pm/V is achieved, which enhances ~33 times the tunability performance of state-of-the-art Bragg reflectors.

Design and demonstration of ultra-high Q silicon microring resonator based on multi-mode ridge waveguide

Yuguang Zhang, Xiao Hu, Daigao Chen, Lei Wang, miaofeng li, Peng Feng, Xi Xiao, and Shaohua Yu

Doc ID: 319485 Received 16 Jan 2018; Accepted 25 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: We present the design and experimental demonstration of the ultra-high Q factor silicon microring resonator based on multi-mode ridge waveguide. The multi-mode ridge waveguide is designed to decrease the propagation loss and to improve the Q factor. The ultra-high Q factor of 1.1×〖10〗^6 is experimentally demonstrated, with the free spectrum range (FSR) of 0.208 nm. The single mode ridge waveguide is used in the coupling region to reduce the dimension of the microring resonator, and the bend radius is only 20 μm. To precisely control the resonant wavelength, a small heater is implemented on the silicon microring resonator with the tuning efficiency of 7.1 pm/mW. The degenerate four-wave mixing (FWM) of the silicon microring resonator is investigated, and the conversion efficiency is measured to be -10.8 dB without optimizing the dispersion of the microring resonator and carriers extraction.

The influence of shielding effect on the formation of micro-texture on the cermet with nanosecond pulsed laser ablation

Jiandong Yuan, Liang Liang, Yelun Jiang, and Xin Liu

Doc ID: 321196 Received 02 Feb 2018; Accepted 24 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: The degree of laser pulse overlapping in laser scanning path has a significant impact on the ablation regime in laser machining of micro-texture. In this study a nanosecond pulsed laser is used to prepare the micro-scaled groove on WC-8Co cermet under different scanning speed. It is observed that as the scanning speed increases, the ablated trace morphology in the first scanning passes transits from a succession of intermittent deep dimples to the consecutive overlapped shallow pits. The test result also indicates that ablated trace morphology with respect to the low scanning speed stems from plume shielding effect. Moreover, the ablation regime considering the shielding effect in micro-groove formation process is clarified. The critical scanning speed that can circumvent the shielding effect is also summarized with respect to different laser power.

Formulating the design of two freeform lens surfaces for point-like light sources

Rengmao Wu, Chang Shengqian, Zhenrong Zheng, LIEFENG ZHAO, and Xu Liu

Doc ID: 320116 Received 18 Jan 2018; Accepted 24 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: Two freeform surfaces provide more degrees of freedom in designing illumination optics and can yield a better solution. The existing methods for point-like sources are mostly valid in designing one freeform surface. Designing two freeform surfaces for point-like sources still remains a challenging issue. In this letter we develop a general formulation of designing two freeform lens surfaces for point-like sources without paraxial approximation. The proposed method is very efficient and robust in designing freeform lenses with two elaborately designed surfaces. The examples clearly show that using two freeform surfaces yields better solutions to challenging illumination design problems with ultra-high energy efficiency.

Two-dimensional arbitraty nano-manipulation on plasmonic metasurface

min Jiang, Guanghui Wang, Wenhao Xu, Wenbin Ji, Ningmu Zou, Ho-Pui Ho, and Xuping Zhang

Doc ID: 319298 Received 11 Jan 2018; Accepted 23 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: We report a plasmonic nano-ellipses metasurface for the realization of trapping and two-dimensional (2D) arbitrary transport of nanoparticles through rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off adjacent hot spots and then to convey trapped target by rotating the incident polarization state. For the case of metasurface with unit cell of perpendicular orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and anti-clockwise rotation would guide the particle to different way, which make it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.

Critically phase-matched Ti:sapphire-laser-pumped deep-infrared femtosecond optical parametric oscillator based on CdSiP₂

Callum O'Donnell, Chaitanya Kumar Suddapalli, Kevin Zawilski, Peter Schunemann, and Majid Ebrahim-Zadeh

Doc ID: 320287 Received 24 Jan 2018; Accepted 22 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: We report a high-repetition-rate femtosecond optical parametric oscillator (OPO) for the deep-infrared based on type-I critical phase-matching in CdSiP₂ (CSP), pumped directly by a Ti:sapphire laser. Using angle-tuning in the CSP crystal, the OPO can be continuously tuned across 7306–8329 nm (1201–1369 cm-1) in the deep-infrared. It delivers up to 18 mW of idler average power at 7306 nm and >7 mW beyond 8000 nm at 80.5 MHz repetition rate, with the spectra exhibiting bandwidths of >150 nm across the tuning range. Moreover, the signal is tunable across 1128–1150 nm in the near-infrared, providing up to 35 mW of average power in ~266 fs pulses at 1150 nm. Both beams exhibit single-peak Gaussian distribution in TEM₀₀ spatial profile. With an equivalent spectral brightness of ~5.6×10²⁰ photons s-1 mm-2 sr-1 0.1% BW-1, this OPO represents a viable alternative to synchrotron and supercontinuum sources for deep-infrared applications in spectroscopy, metrology and medical diagnostics.

Spectral interpretation of Talbot self-healing effect, and application to optical arbitrary waveform generation

Hugues Guillet de Chatellus and Côme Schnebelin

Doc ID: 322617 Received 07 Feb 2018; Accepted 22 Feb 2018; Posted 27 Feb 2018  View: PDF

Abstract: Talbot effect has the intrinsic capability of mitigating the effects of grating imperfections. Here we investigate, in the frequency domain, the influence of an imperfect grating on the Talbot images. In particular, we show that for specific propagation distances, the spectra of the individual Talbot images are directly related to the envelope of the grating transmission function, mapped in the spectral domain. This novel space-to-frequency mapping property enables us to describe the capability of image restoration (or self-healing) of Talbot effect, in terms of spectral filtering. The transposition of this effect from the spatial to the temporal domain, offers promising possibilities for the spectral shaping of optical waveforms.

High power, fixed and tunable wavelength, Grating-free Cascaded Raman fiber Lasers

V Balaswamy, Arun S, Santosh Aparanji, vishal choudhury, and V R Supradeepa

Doc ID: 315611 Received 14 Dec 2017; Accepted 22 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: Cascaded Raman lasers enable high powers at various wavelength bands inaccessible with conventional rare-earth doped lasers. The input and output wavelengths of conventional implementations are fixed by the constituent fiber gratings necessary for cascaded Raman conversion. We demonstrate here, a simple architecture for high power, fixed and wavelength tunable, grating-free, cascaded Raman conversion between different wavelength bands. The architecture is based on the recently proposed distributed feedback Raman lasers. Here, we implement a module which converts the Ytterbium band to the eye-safe 1.5micron region. We demonstrate pump-limited output powers of over 30W in fixed and continuously wavelength tunable configurations.

Quadratic nonlinear optical properties of the organic N-benzyl-2-methyl-4-nitroaniline (BNA) biaxial crystal

cyril bernerd, Patricia Segonds, Jérôme Debray, Takashi Notake, Mio Koyama, Hiroaki Minamide, Hiromasa Ito, and Benoit Boulanger

Doc ID: 320101 Received 18 Jan 2018; Accepted 22 Feb 2018; Posted 15 Mar 2018  View: PDF

Abstract: We performed the direct measurement of second harmonic generation and sum frequency generation phase-matching directions in the organic N-benzyl-2-methyl-4-nitroaniline (BNA) crystal over its visible and near-infrared transparency range. The fit of these data allowed us to refine the Sellmeier equations of the three principal refractive indices in this range. With these equations, we improved the calculated tuning curves of THz emission from a phase-matched difference frequency process. We also determined the absolute magnitude of the d24 nonlinear coefficient.

Mechanism of large optical nonlinearity in gold nanoparticle films

Inam Mirza, David McCloskey, Werner Blau, and James Lunney

Doc ID: 315638 Received 15 Dec 2017; Accepted 21 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: The Z-scan technique, using femtosecond (fs) laser pulses at 1480 nm laser pulses, was used to measure the nonlinear optical properties of gold (Au) nanoparticle (NP) films made by both nanosecond (ns) and femtosecond (fs) pulsed laser deposition (PLD) in vacuum. At irradiance levels of 10¹² W m-² the ns-PLD films displayed induced absorption with β = 4×10-² m W-¹, and a negative lensing effect with n₂ = -4.7×10-¹¹ m² W-¹, with somewhat smaller values for the fs-PLD films. These values of n₂ imply an unphysically large change in the real part of the refractive index, and the need to take account of nonlinear changes of the Fresnel coefficients and multiple beam interference in Z-scan measurements on nanoscale films. Following this approach, the Z-scan observations were analysed to determine the effective complex refractive index of the NP film at high irradiance. It appears that at high irradiance the NP film behaves as a metal, while at low irradiance it behaves as a low-loss dielectric. Thus it is conjectured that, for the high irradiance near the waist of the Z-scan laser beam, laser driven electron tunnelling between NPs gives rise to metal-like optical behaviour.

Resource-theoretic approach to vectorial coherence

Alfredo Luis, Gustavo Martin Bosyk, and Guido Bellomo

Doc ID: 319124 Received 05 Jan 2018; Accepted 21 Feb 2018; Posted 22 Feb 2018  View: PDF

Abstract: We propose a formal resource theoretic approach to asses the coherence between partially polarized electromagnetic fields. We show that naturally defined incoherent operations endow partial coherence with a preorder relation that must be respected by all coherence measures. We examine most previously introduced coherence measures from this perspective.

Is twisting an ``inherently Gaussian' business?

Riccardo Borghi

Doc ID: 319764 Received 15 Jan 2018; Accepted 21 Feb 2018; Posted 05 Mar 2018  View: PDF

Abstract: Twisted Gaussian Schell-model beams were introduced twenty-five years ago as a celebrated example of``genuinely two-dimensional' partially coherent wavefield. Today, a definite answer about the effect a twist phase should produce on an \emph{arbitrary} cross-spectral densityhas not yet been reached. In the present Letter, the necessary and sufficient condition for a typical Schell-model partially coherent CSD endowed with axial symmetry to be successfully mapped onto abonafide \emph{twisted} CSD is addressed. In particular, it is proved that any shift-invariantdegree of coherence of the form $\mu(|\bfr_1-\bfr_2|)$ is ``twistable' if and only if the zeroth-orderHankel transform of the radial function $\mu(r)\exp(ur^2/2)$ (with $u$ begin the twist strength)turns out to be a well defined, non-negative function. Testing this condition over some examplesof partially coherent sources seems to corroborate the idea that twisting is connected toGaussian optics more than expected.

High-efficiency 1064-nm nonplanar ring oscillator Nd:YAG laser with diode pumping at 885 nm

weiping deng, tao yang, cao jianping, ErJun Zang, li liufeng, Lisheng Chen, and Zhanjun Fang

Doc ID: 319105 Received 22 Jan 2018; Accepted 19 Feb 2018; Posted 01 Mar 2018  View: PDF

Abstract: We experimentally demonstrate a monolithic single-frequency nonplanar ring oscillator (NPRO) laser pumped by diode lasers from thermally excited ground level directly into metastable level of Nd:YAG gain medium. Continuous-wave output power of 4.54 W is obtained at 1064 nm with 7.6 W input at 885 nm and the slope efficiency is 76.9% at incident power above 2 W. Compared with the 59.7% slope efficiency obtained using 808 nm pumping for the same NPRO, this direct pumping shows potential advantage in improving the energy efficiency. The observation is in consistent with an analysis of the heat generation in both direct and traditional pumping schemes. The power fluctuation is measured to be within 0.8% at 3-W output during a 10-hour period.

Line-scan spectrum-encoded imaging by dual-comb interferometry

chao wang, Zejiang Deng, CHENGLIN Gu, Yang Liu, Daping Luo, Zhiwei Zhu, Wenxue Li, and Heping Zeng

Doc ID: 320189 Received 22 Jan 2018; Accepted 18 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: Herein, the method of spectrum-encoded dual-comb interferometry is introduced to measure a three-dimensional (3-D) profile with absolute distance information. By combining spectral encoding for wavelength-to-space mapping, dual-comb interferometry for decoding and optical reference for calibration, this system can obtain 3-D profile of an object at a stand-off distance of 114 mm with a depth precision of 12 μm. With the help of the reference arm, the absolute distance, reflectivity distribution and depth information are simultaneously measured at a 5-kHz line-scan rate with free-running carrier-envelop offsets frequencies (CEOs). To verify the concept, experiments are conducted with multiple objects, including a resolution test chart, a three-stair structure and a designed “ECNU” letter chain. The results show a horizontal resolution of ~22 μm and a measurement range of 1.93 mm.

Flying Particle Microlaser and Temperature Sensor in Hollow-Core Photonic Crystal Fiber

Richard Zeltner, Riccardo Pennetta, Shangran Xie, and Philip Russell

Doc ID: 308716 Received 06 Oct 2017; Accepted 17 Feb 2018; Posted 20 Feb 2018  View: PDF

Abstract: Whispering-gallery mode (WGM) resonators combine small optical mode volumes with narrow resonance linewidths, making them exciting platforms for a variety of applications. Here we report a flying WGM microlaser, realized by optically trapping a dye-doped microparticle within a liquid-filled hollow-core photonic crystal fiber (HC-PCF) using a CW laser and then pumping it with a pulsed excitation laser whose wavelength matches the absorption band of the dye. The laser emits into core-guided modes that can be detected at the endfaces of the HC-PCF. Using radiation forces, the microlaser can be freely propelled along the HC-PCF over multi-cm distances—orders of magnitude further than in previous experiments where tweezers and fiber traps were used. The system can be used to measure temperature with high spatial resolution, by exploiting the temperature-dependent frequency shift of the lasing modes, and also for precise delivery of light to remote locations.

Local Selective Activation of Surface and Hybrid Acoustic Waves in Optical Microwires

Desmond Chow, Jean Charles Beugnot, Adrien Godet, Kien Phan Huy, Marcelo Soto, and Luc Thevenaz

Doc ID: 313347 Received 13 Nov 2017; Accepted 13 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: Elastic vibrations in sub-wavelength structures have gained importance recently in fundamental light-matter studies and various opto-acoustic applications. Existing techniques have revealed the presence of distinct acoustic resonances inside silica microwires yet remain unable to individually localize them. Here, we locally and selectively activate distinct classes of acoustic resonances inside a tapered fiber using a phase-correlation distributed Brillouin method. Experimental results verify the presence of surface and hybrid acoustic waves at distinct fiber locations and demonstrate, to the best of our knowledge, the first distributed surface acoustic wave measurement. This technique is important for understanding fundamental opto-acoustic interactions and enabling designs of novel optomechanical devices.

Curving h-BN thin films can create extra phonon polariton modes

Yu Zhou, HaoRan Qi, yueke wang, Dong-Xiang Qi, and Qing Hu

Doc ID: 318781 Received 03 Jan 2018; Accepted 09 Feb 2018; Posted 26 Feb 2018  View: PDF

Abstract: Hexagonal boron nitride (h-BN) thin films support volume-confined phonon polariton modes within the bulk material as well as surface-confined modes at the edges of thin films. In this paper, we theoretically investigate the phonon polaritons in curved hexagonal boron nitride thin films. One-dimensional guided phonon polariton modes are found, which are caused by the curved geometry and do not exist in extended flat films. These modes are guided along specific direction with relatively low propagation losses. So far, one-dimensional guided phonon polariton modes have only been proposed in nanowire and nanoribbon structures. Our study offers an another way with the advantage of keeping the h-BN film intact, which can avoid huge scattering losses due to the structural defects. These investigations may offer an easy and robust approach towards phonon-polariton-based nanophotonic circuitry.

Optimized inhibited-coupling Kagome fibers at Yb-Nd:Yag (8.5 dB/km) and Ti:Saph (30 dB/km) ranges

Martin Maurel, Matthieu Chafer, Abhilash Amsanpally, Muhammad Adnan, Foued Amrani, Benoît Debord, Luca Vincetti, Frédéric Gérôme, and Fetah Benabid

Doc ID: 315178 Received 07 Dec 2017; Accepted 08 Feb 2018; Posted 15 Feb 2018  View: PDF

Abstract: We report on the development of hypocycloid core-contour inhibited coupling Kagome HC-PCFs with record transmission loss and spectral coverage which include the common industrial laser wavelengths. Using the scaling of the confinement loss with the core-contour negative curvature and the silica strut thickness, we fabricated IC Kagome HC-PCF for Yb and Nd:Yag laser guidance with record loss level of 8.5 dB/km associated with a 225 nm wide 3-dB bandwidth. A second HC-PCF is fabricated with reduced silica strut thickness whilst keeping the hypocycloid core-contour. It exhibits a fundamental transmission window spanning down to the Ti:Sa spectral range, and with a loss figure of 30 dB/km at 750 nm. The fibers modal properties (S2 and PER) and bending sensitivity show these HC-PCFs to be ideal for ultra-low loss, flexible and robust laser beam delivery.

21 spatial modes erbium doped fiber amplifier for mode division multiplexing transmission

Zhenzhen Zhang, Cheng Guo, Liang Cui, Qi Mo, Ningbo Zhao, Cheng Du, Xiaoying Li, and Guifang Li

Doc ID: 314438 Received 28 Nov 2017; Accepted 24 Jan 2018; Posted 25 Jan 2018  View: PDF

Abstract: We experimentally demonstrate a 12 mode groups (21 spatial modes) cladding pumped few mode erbium doped fiber amplifier. The differential modal gain is dramatically reduced by using the double cladding erbium doped fiber (DC-EDF) with a refractive index trench structure, which helps to tightly confine all the 21 spatial modes in the core and to mitigate the bending loss. Our experimental results show the differential modal gain is about 3 dB when average signal modal gain for all 12 mode groups is up to 15 dB across the C band. Our method of using DC-EDF with a properly designed trench structure can be used to develop gain equalized few mode amplifiers supporting more spatial modes.

Efficient Continuous Wave and ReS₂ Passively Q-switched Er:SrF₂ Laser at ~3 µm

Mingqi Fan, Tao Li, Jia Zhao, Shengzhi Zhao, Guiqiu Li, Kejian Yang, Liangbi Su, Houyi Ma, and Christian Kraenkel

Doc ID: 314716 Received 30 Nov 2017; Accepted 14 Jan 2018; Posted 08 Mar 2018  View: PDF

Abstract: We report on an efficient Er:SrF₂ laser at 2.79 μm. A continuous wave output power of 1.06 W was obtained with a slope efficiency of 41%, significantly exceeding the Stokes efficiency of 35%. Stable Q-switched laser operation was realized by using ReS₂ saturable absorber, generating an average output power of 0.58 W with a pulse duration of 508 ns at a repetition rate of 49 kHz, corresponding to a pulse energy of 12.1 μJ.

Select as filters

    Select Topics Cancel
    © Copyright 2018 | The Optical Society. All Rights Reserved