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Optica Publishing Group

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Accepted papers to appear in an upcoming issue

Optica Publishing Group posts prepublication articles as soon as they are accepted and cleared for production. See the FAQ for additional information.

Broadband Meta-hologram Enabled by a Double-Face Copper-Cladded Metasurface Based on Reflection–Transmission Amplitude Coding

Lei Zhu, Wenjuan Zhou, Liang Dong, Chunsheng Guan, Guanyu Shang, XUMIN DING, Shah Nawaz Burokur, and Qun Wu

DOI: 10.1364/OL.442464 Received 06 Sep 2021; Accepted 24 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: Here, we propose a broadband meta-hologram that can efficiently manipulate the amplitude of electromagnetic waves in both transmission and reflection spaces, depending on the polarization state of the incident electromagnetic wave. The proposed broadband meta-hologram is validated by encoding the transmission-reflection amplitude information of two independent images into a single metasurface. The holographic images obtained from measurements agree qualitatively with simulation results. The proposed metasurface presents a simplified design for a convenient integration in modern electromagnetic systems compared to multi-layer dielectric cascade structures and overcomes the limitations of narrow frequency band operation.

Ghost image restoring using random speckles created by a liquid crystal cell

Nikolay Davletshin, Denis Ikonnikov, Vitaly Sutormin, Nikolay Shestakov, Filipp Baron, and Andrey Vyunishev

DOI: 10.1364/OL.445684 Received 13 Oct 2021; Accepted 24 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: A liquid crystal cell is used for producing correlated light beams with speckle structures for implementation of pseudo-thermal ghost imaging. The liquid crystal cell makes it possible to provide random spatial intensity distributions, which are characterized by a low coefficient of mutual crosscorrelations. Ghost imaging of an object representing an amplitude mask is demonstrated. The quality of the reconstructed images was estimated by the method of structural similarity.

Variable focus convex microlens array on K9 glass substrate based on femtosecond laser processing and hot embossing lithography

Zhihao Chen, Hongbing Yuan, Peichao Wu, Wenwu Zhang, Saulius Juodkazis, huang haibo, and Xiao-wen Cao

DOI: 10.1364/OL.448344 Received 11 Nov 2021; Accepted 24 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: We propose a high precision method for fabrication of variable focus convex microlens array on K9 glass substrate by combining femtosecond laser direct writing and hot embossing lithography. A sapphire master mold with a blind cylindrical hole array was prepared first by femtosecond laser ablation. The profile control of microlenses dependent on the temperature and the diameter of the blind hole in sapphire mold was investigated. The curvature radius of the microlens decreased with temperature and increased with diameter. Uniform convex microlens arrays were fabricated with good imaging performance. Further, a variable focus convexmicrolens arrays were fabricated by changing the diameter of the blind hole in sapphire, which produced the image at variable z-planes. This method provides a highly-precise fabrication of convex microlens arrayand is well suited for batch production of micro-optical elements.

Low-loss fusion splicing between spacing-mismatched multicore fibers

Wei Ji, Ruowei Yu, Zihao Shen, Caoyuan Wang, Cong Xiong, and Limin Xiao

DOI: 10.1364/OL.447602 Received 03 Nov 2021; Accepted 23 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: Multicore fibers (MCFs) have offered a fascinating solution to increase the fiber density and thus meet the exponentially growing demand for capacity in optical communication networks. However, the overwhelming research on MCFs remains an unanswered challenge in a general fusion splicing scheme between dissimilar MCFs. Here we propose the tapering technique to reshape MCFs, including both the reverse tapering and the down-tapering schemes, which can be exploited to tailor the core-to-core spacing and modify the modal property of MCFs. By matching both the spacing and the mode field diameter, we have demonstrated a low loss (average 0.18 dB) and low crosstalk (average -68 dB) fusion splice between two spacing-mismatched MCFs with a spacing difference up to 26 μm. The proposed novel schemes are also suitable for the splice between MCFs with a slightly different spacing, and can provide a unique perspective in fabricating MCF device and boost various MCF applications.

Resolving plasmonic hotspots by label-free super-resolution microscopy

xue cheng, Congyue Liu, Guorui Zhang, Wei Liu, Ju wang, Yiqun Duan, Jianjun Chen, Hong Yang, and Shufeng Wang

DOI: 10.1364/OL.443571 Received 20 Sep 2021; Accepted 23 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: The plasmonic hotspot of metal nanostructures has small dimension far beyond optical diffraction limit. When trying to locate the hotspot using fluorescent probes, the localization is significantly distorted due to the coupling of emission and surface plasmon. Label-free technique can solve the problem, which uses hotspot emission as the native probe. We demonstrate a super-resolution microscopy investigation based on this idea. By modulating hotspot emission of crossed silver nanowires, which has a pair of plasmonic hotspots of ~100 nm apart at the intersection, we precise locate and separate them at nanometer precision. This label-free technique could be applied for analyzing hotspot distribution with high efficiency and precision.

Helical plasma filaments from the self-channeling of intense femtosecond laser pulses

Fabio Mangini, Mario Ferraro, Mario Zitelli, Alioune Niang, Tigran Mansuryan, Alessandro Tonello, Vincent COUDERC, Antonio De Luca, Sergey Babin, Fabrizio Frezza, and Stefan Wabnitz

DOI: 10.1364/OL.445321 Received 12 Oct 2021; Accepted 23 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: We experimentally and numerically study the ignitionof helical-shaped plasma filaments in standard optical fibers. Femtosecond pulses with megawatt peak power with proper off-axis and tilted coupling in the fiber core produce plasma skew rays. These last for distances as long as 1,000 wavelengths thanks to self-channeling effect. Peculiar is the case of graded-index multimode fibers: here the spatial self-imaging places constraints on the helix pitch. These results may find applications for fabricating fibers with helical-shaped core micro-structuration, as well as for designing laser components and 3D optical memories.

MXene-based ultra-sensitive optical fiber salinity sensor

Duo Yi, Wang Cong, Lingfeng Gao, yuzhi chen, Fei Liu, Youfu Geng, Han Zhang, and Xuejin LI

DOI: 10.1364/OL.446536 Received 21 Oct 2021; Accepted 22 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: This study proposes a Ti3CN MXene enabled ultra-sensitive optical fiber sensor, and salinity measurement is conducted to evaluate its sensing performance in a low-concentration target molecule detection environment. Owing to the abundance of hydrophilic functional groups, large specific surface and broad-spectrum absorption characteristics of the MXene layers, the sensing performance of the MXene-decorated sensor is greatly improved and an ultra-high salinity sensitivity of -5.34 nm/‰ is achieved (equivalent to refractive index sensitivity of -33429 nm/RIU). Such an excellent sensing performance is 137.33% higher than that of the bare fiber sensor and is significantly enhanced over the previously reported fiber sensors. Besides, the sensing performance of the sensor is improved without damaging the fiber structure, showing huge advantage when compared with the traditional fiber post-processing techniques. Finally, since the refractive index is commonly used to characterize the detection ability of biosensors, our contribution suggests the integration of MXene as a promising approach to develop high-performance optical fiber biosensors.

Efficient helicity control of four-wave mixing in gated graphene

Di Huang, Tao Jiang, Yangfan Yi, Yuwei Shan, yingguo Li, Zhihong Zhang, Kaihui Liu, Weitao Liu, and Shiwei Wu

DOI: 10.1364/OL.443600 Received 23 Sep 2021; Accepted 22 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: The gapless Dirac fermions in monolayer graphene give rise to an abundance of peculiar physical properties, including the exceptional broadband nonlinear optical responses. By tuning chemical potential, stacking order and photonic structures, the effective modulation of nonlinear optical phenomena in graphene have been demonstrated in recent years. Here, we demonstrate optical helicity as an extra tuning knob for four-wave mixing in gated graphene. Our results reveal the helicity selection rule of four-wave mixing in monolayer graphene, showing a nearly perfect circular polarization. The corresponding theoretical interpretations on the helicity selection rule are presented, which are applicable to other nonlinear optical processes and materials.

Suppression of gap plasmon resonance for high-responsivity MIM near-infrared hot-electron photodetectors

Xiao-Long Hu, Fen Li, Hai-Lin Wu, and Wen-Jie Liu

DOI: 10.1364/OL.444736 Received 06 Oct 2021; Accepted 22 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Binary metal layers composed of a grating and a thin film are designed for high-responsivity metal-insulator-metal (MIM) near-infrared hot-electron photodetectors (HEPDs). The binary metal grating structure HEPDs demonstrate a strong asymmetrical optical absorption and result in a high current responsivity. In our devices, the top and bottom absorption ratio is as high as 76:1, much higher than that in the traditional grating structure HEPDs. A maximum zero-biased responsivity is 0.585 mA/W at 1550 nm by employing a five-step electrical model, which is 3.42 times to that of the traditional silver grating structure devices. By simply changing the grating period, it achieves spectrally selective photodetection covering a wide range of 500 nm at the near-infrared band with net absorption higher than 0.95 and linewidths narrower than 0.7 meV.

Demonstration of in-depth analysis of Si-photonics circuits using OFDR: waveguides with grating couplers

Masatoshi Tokushima and Jun Ushida

DOI: 10.1364/OL.444876 Received 06 Oct 2021; Accepted 22 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Optical frequency domain reflectometry (OFDR) is a powerful technique to investigate backscatter in waveguides. However, its use on Si-photonics circuits has so far been limited to measuring the propagation loss and group index of a waveguide. We demonstrate that the transmittance (T) and reflectance (R) of elemental devices comprising a Si-photonics circuit can be determined from its OFDR data. An analysis of Si wire waveguides with grating couplers (GCs) is described in detail. The wavelength dependence of T and R of the GCs were determined by using a backscatter model incorporating time-equivalent multiple-reflection paths and were well reproduced by a numerical simulation.

Quantum key distribution integrating with ultra-high power classical optical communications based on ultra-low-loss fiber

Jiaqi Geng, Guan-Jie Fan-Yuan, Shuang Wang, QI-FA Zhang, Wei Chen, Zhen-Qiang Yin, De-Yong He, Guang-can Guo, and Zhen-fu Han

DOI: 10.1364/OL.446939 Received 27 Oct 2021; Accepted 22 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: The demand for the integration of quantum key distribution (QKD) and classical optical communication in the same optical fiber medium greatly increases as fiber resources and the flexibility of practical applications are taken into consideration. To satisfy the needs of mass deployment of ultra-high power required for classical optical networks integrating QKD, we implement the discrete variable quantum key distribution (DV-QKD) under up to 25 dBm launch power from classical channels over 75 km on ultra-low-loss fiber by combining a finite-key security analysis method with the noise model of classical signals. So far as we know this the highest power launched by classical signals on coexistence of DV-QKD and classical communication. The results exhibit the feasibility and tolerance of our QKD system for use in ultra-high power classical communications.

Broadband air clad LP02 mode convertor using tapered mode transition

Neethu Mariam Mathew, Lars Grüner-Nielsen, Zepeng Wang, Lars Rishoj, and Karsten Rottwitt

DOI: 10.1364/OL.446055 Received 14 Oct 2021; Accepted 21 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Higher order mode converters that work over a broad wavelength range are needed for various applications. A new simple and cost-effective LP02 mode converter is fabricated by tapering a bundle of single mode fibers.The device excites the LP02 mode in a four mode step index fiber with a mode purity higher than 10 dB. The polarization dependent crosstalk of the device is measured using the S2 technique. The LP02 mode selectivity of the device is measured over the entire C and L band, by selectively launching different modes into the device, using a spatial light modulator.

Plasmonic loss-mitigating broadband adiabatic polarizing beam splitter

Abdoulaye Ndao, Guang Yang, and Alexander Sergienko

DOI: 10.1364/OL.431887 Received 19 May 2021; Accepted 20 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: The intriguing analogy between quantum physics and optics has inspired the design of unconventional integrated photonics devices. In this paper, we numerically demonstrate a broadband integrated polarization beam splitter (PBS) by implementing the stimulated Raman adiabatic passage (STIRAP) technique in a three-waveguide plasmonic system. Our proposed PBS exhibits >250 nm TM bandwidth with <-40 dB extinction and >150 nm TE bandwidth with <-20 dB extinction, covering the entire S-, C- and L-band and partly E-band. Moreover, near-lossless light transfer is achieved in our system despite the incorporation of a plasmonic hybrid waveguide because of the unique loss mitigating feature of the STIRAP scheme. Through this approach, various broadband integrated devices that were previously impossible can be realized, which will allow innovation in integrated optics.

A wavelength tunable 1104-nm NALM laser based on polarization-maintaining double-cladding fiber

Kong Gao, Yizhou Liu, Wenchao Qiao, Yuzhi Song, Xian Zhao, Aimin Wang, and Tao Li

DOI: 10.1364/OL.445683 Received 27 Oct 2021; Accepted 20 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: A Ytterbium-doped stretched-pulse mode-locked fiber oscillator was fabricated by applying a nonlinear amplifier loop mirror (NALM). The fiber cavity was built using a large-mode area (LMA) polarization-maintaining (PM) double-cladding (DC) fiber. The central wavelength of the generated 24.7-MHz laser can be modified from 1034 to 1104 nm by tuning the intra-cavity loss. The output power of the laser with a wavelength of 1104 nm at the transmission and reflection ports is 7.61 and 0.33 mW, respectively. The corresponding compressed pulse durations are 192 and 187 fs, which are 1.54 and 1.02 times the transform limited pulse duration, respectively.

Ultrafast spatial phase unwrapping algorithm with accurately correcting transient phase error

Haitao Wu, Yiping Cao, Haihua An, Cai Xu, and Hongmei Li

DOI: 10.1364/OL.446022 Received 14 Oct 2021; Accepted 20 Nov 2021; Posted 24 Nov 2021  View: PDF

Abstract: In fringe projection profilometry, the wrapped phase is easily polluted by many factors such as noise, shadow, and so on. In this letter, we proposed an ultrafast bi-staggered spatial phase unwrapping (BSPU) method. By constructing another staggered phase, the fringe order jump (FOJ) and local transient phase error (LTPE) can be accurately and quickly located at the same time due to simple difference operation. For the first time, a pioneering threshold separation model is established to precisely distinguish FOJ and LTPE. Based on the continuity assumption, LTPE is effectively corrected by introducing the concept of "non-integer fringe order". Groundbreaking, the range of measurable discontinuity height is improved due to the distinction between real phase jump and random error in the spatial phase unwrapping. In addition, it is thousands of times faster than the traditional path-dependent algorithm and even has higher measurement accuracy. Experimental results show the effectiveness and robustness of the proposed method in various complex measurement environments.

All solid polarization maintaining silica fiber with birefringence induced by anisotropic metaglass

Alicja Anuszkiewicz, Monika Bouet, Damian Michalik, Grzegorz Stepniewski, Rafal Kasztelanic, Adam Filipkowski, Dariusz Pysz, Andy Cassez, Mariusz Klimczak, Geraud Bouwmans, Arnaud Mussot, and Ryszard Buczynski

DOI: 10.1364/OL.438622 Received 27 Jul 2021; Accepted 19 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: We report the development of silica glass single-mode polarization maintaining fiber with birefringence induced by artificial anisotropic glass in the circular core without any external stress zones or structured cladding. The fiber core is composed of silica and germanium-doped silica nanorods ordered in submicrometer interleaved layers. The fiber has a measured cut-off wavelength at 1113 nm, phase birefringence of 0.3×10-4, and an effective mode diameter of 10.5 µm at the wavelength of 1550 nm. Polarization extinction ratio in the fiber is 20 dB at 1550 nm. The fiber is compatible with the standard SMF-28 fiber and can be easily integrated using standard fusion splicing with losses of 0.1 dB.

An ultra-thin sputter-deposited infrared rugate mirror for enhancing solar to thermal energy conversion

Daniela De Luca, David Kortge, Emiliano Di Gennaro, Roberto Russo, and Peter Bermel

DOI: 10.1364/OL.442839 Received 09 Sep 2021; Accepted 19 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: A dielectric mirror with high IR reflection and high visible transmission, based on an easily fabricated Stepped Index Rugate (SIR) filter structure, is presented. Its fabrication involves sputtering depositions, using only two targets, to make five different material compositions. The ultra-wide reflection band is tuneable in both position and width, adapting the thickness of the layers and eventually introducing chirped layers. When applied to evacuated solar thermal devices, efficiency improvements of up to 30% can be achieved, making this mirror an attractive solution for reducing radiative losses through the cold side photon recycling mechanism.

Scanning interferometric near-infrared spectroscopy (iNIRS) for three-dimensional imaging of adult human forehead blood flow dynamics

Oybek Kholiqov, Wenjun Zhou, Tingwei Zhang, Mingjun Zhao, SOROUSH GHANDIPARSI, and Vivek Srinivasan

DOI: 10.1364/OL.443533 Received 21 Sep 2021; Accepted 19 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: In diffuse optics, quantitative assessment of the human brain is confounded by the skull and scalp. To better understand these superficial tissues, we advance interferometric near-infrared spectroscopy (iNIRS) to form time-of-flight (TOF)-resolved images of human forehead blood flow index (BFI). We describe a polarization splitting approach to overlap the source (S) and collector (C), enabling a galvanometer pair to scan the source light and de-scan the returning light to the collector. This null S-C approach also eliminates unwanted backscattered light that would degrade the dynamic range. We acquire laterally-resolved iNIRS data from the human forehead for the first time. Along the two lateral dimensions, an order-of-magnitude heterogeneity in superficial dynamics implies an order-of-magnitude heterogeneity in brain specificity, depending on forehead location. Along the third TOF dimension, the autocorrelation decay rates support a 3-layer model with increasing BFI from the skull to the scalp to the brain. By more accurately characterizing superficial tissues, this three-dimensional imaging approach can help guide approaches to improve specificity for the human brain.

Recording point spread functions by wavefront modulation for interferenceless coded aperture correlation holography

xuelian yu, KangWei Wang, Jun Jun Xiao, xiufang li, YanQian Sun, and Hao Chen

DOI: 10.1364/OL.439615 Received 05 Aug 2021; Accepted 18 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: Coded aperture correlation holography (COACH) needs the point spread function (PSF) for image reconstruction. Utilizing a pinhole to generate point light source is the mostly adopted way for measuring PSF, which, however, has significant issues to resolve. One of the problems is that the resolution of the reconstructed result is limited by the cutoff frequency of the pinhole. The other one is that the far-field PSF is undetectable because the amount of light illuminance decreases with the distance. In this work, we present a method for recording PSF based on wavefront modulation. By modulating a plane wave with both the carrier spherical wave and the coded phase mask, we obtain a virtual point spread function (VPSF) which is used for image reconstruction. It is shown that the resolution of reconstructed results and the imaging distance are not limited by the pinhole. We experimentally demonstrate high-resolution reconstruction and far-field imaging by the VPSF.

Ultrafast multi-photon excitation of ScVO₄:Bi³+ for luminescence thermometry

David Escofet Martin, Anthony Ojo, and Brian Peterson

DOI: 10.1364/OL.445763 Received 12 Oct 2021; Accepted 17 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: We demonstrate a multi-photon excitation (MPE) scheme for luminescence thermometry using ScVO₄:Bi³+. MPE is performed using a 37 fs Ti:Sapphire laser pulse centred at 800 nm. Log-log plots of the phosphorescence intensity versus excitation power show that the 800 nm MPE of ScVO₄:Bi³+ involves a 2- and 3-photon absorption process in comparison to a single-photon excitation (SPE) process at 266 nm and 400 nm. Spectroscopic investigation shows that with the 800 nm MPE and 266 nm SPE schemes, the emission spectra of ScVO₄:Bi³+ are similar, and are characterized by emissions of the VO₃−⁴+ groups and Bi³+. The MPE is advantageous to suppress fluorescence which interfere with the phosphorescence signal. We demonstrate this aspect for a ScVO₄:Bi³+ coating applied to an Al₂O₃ substrate. The luminescence lifetime is calibrated with temperature over 294-334 K; the MPE scheme has an equally impressive temperature sensitivity (3.4-1.7 %/K) and precision (0.2-0.7K) compared to the SPE schemes. The MPE scheme can be applied to a variety of phosphors is valuable for precise temperature measurements even in applications where isolating interfering background emissions is challenging.

Deep learning based time-frequency domain signal recovery for fiber-connected radar networks

Fangzheng Zhang, Yuewen Zhou, and Shilong Pan

DOI: 10.1364/OL.446000 Received 14 Oct 2021; Accepted 17 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: A deep learning based time-frequency domain signal recovery method is proposed to deal with the signal distortion in fiber-connected radar networks. In this method, the deteriorated signal is converted to time-frequency domain, and a two-dimension convolutional neural network is used to conduct signal recovery before inverse conversion to time-domain. This method can achieve high-accuracy signal recovery by learning the complete features in both time and frequency domain. In the experiment, distorted linearly-frequency modulated (LFM) radar signals with a bandwidth of 2 GHz after 8-km fiber transmission are recovered with the noises effectively suppressed. The proposed signal recovery method works well under different input signal to noise ratios. Specially, the average peak to floor ratio (PFR) after radar pulse compression is improved by 25.5 dB in the experiment. In addition, the method is proved to be able to recover radar signals of multiple targets.

Self mode-locking in a high-power hybrid silicon nitride integrated laser

Yvan Klaver, Jörn Epping, Chris Roeloffzen, and David Marpaung

DOI: 10.1364/OL.440898 Received 18 Aug 2021; Accepted 17 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: Integrated mode-locked lasers are useful tools in microwave photonic applications as a local oscillator. In particular hybrid integrated lasers could easily be integrated with passive processing circuits. In this letter, we report on the self-mode-locking of a hybrid integrated laser comprising two indium phosphide gain sections and a silicon nitride feedback circuit that filters light using two ring resonators. The hybrid laser is shown to mode-lock and to have a mostly frequency modulated field in the cavity using a stepped-heterodyne optical complex spectrum analysis. A mostly frequency modulated field output is good for high powers per line due to a more continuous emission compared to mode-locked lasers using a saturable absorber, additionally the filter limits the bandwidth of the comb condensing the power to the fewer comb lines.

Single-exposure multi-wavelength diffraction imaging with a blazed grating

Yuanyuan Liu, Qingwen Liu, Shuangxiang Zhao, Wenchen Sun, Bingxin Xu, Zuyuan He, and Junyong Zhang

DOI: 10.1364/OL.440336 Received 12 Aug 2021; Accepted 17 Nov 2021; Posted 17 Nov 2021  View: PDF

Abstract: Multi-wavelength diffraction imaging is a lensless, high-resolution imaging technology. In order to avoid multiple exposures and enable high-speed data collection, here an innovative setup for the single-exposure multi-wavelength diffraction imaging based on a blazed grating is proposed. Since the blazed angle varies with the wavelength, their own diffraction pattern can be separated from each other and recorded in a single measurement at a time. A method of high-precision position alignment between different wavelengths patterns is proposed in our system to achieve good image quality and high resolution. The experiments on phase-only USAF resolution target and biological samples are carried out to verify the effectiveness of our proposed method. This proposed setup has the advantages such as a simpler structure, fast recording and algorithm robustness.

Demonstration of an Ultra-compact 8-channel sinusoidal silicon waveguide array for optical phased array

Xiaogen Yi, Yibo Zhang, Huiying Zeng, Sai Gao, Shuting Guo, and Ciyuan Qiu

DOI: 10.1364/OL.442028 Received 31 Aug 2021; Accepted 17 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: Here we demonstrated an ultra-compact 8-channel sinusoidal silicon waveguide array for optical phased array. In our device, based on the sinusoidal bending, the crosstalk between waveguides can be efficiently reduced with a waveguide pitch of only 695 nm. For the TE mode, the simulation results show that the insertion loss (IL) of the device is −0.1 dB and the crosstalk between all waveguides is lower than −25 dB at 1550 nm. In the measurement, an IL < −1 dB and a crosstalk < −18 dB are obtained. Since the pitch is related to the beam-steering range and power consumption of the optical phased array, such an ultra-compact device potentially could be a good candidate to build the emitter for an energy-efficient optical phased array with a large field of view.

Bulk-edge correspondence and trapping at a non-Hermitian topological interface

Stefano Longhi

DOI: 10.1364/OL.445437 Received 11 Oct 2021; Accepted 17 Nov 2021; Posted 17 Nov 2021  View: PDF

Abstract: In Hermitian systems, according to the bulk-edge cor- respondence interfacing two topological optical media with different bulk topological numbers implies the ex- istence of edge states, which can trap light at the inter- face. However, such a general scenario can be violated when dealing with non-Hermitian systems. Here we show that interfacing two semi-infinite Hatano-Nelson chains with different bulk topological numbers can re- sult in the existence of infinitely many edge (interface) states, however light waves cannot be rather generally trapped at the interface.

Terahertz ptychography using a long-distance diffraction-free beam as the probe

Feidi Xiang, miao yu, Mengting Zhang, Meiyao Han, Qian Huang, Zhenggang Yang, Jinsong Liu, and Kejia Wang

DOI: 10.1364/OL.442964 Received 09 Sep 2021; Accepted 16 Nov 2021; Posted 23 Nov 2021  View: PDF

Abstract: We have implemented a terahertz (THz) ptychographic technique using a long-distance diffraction-free beam (DFB) instead of traditional low-energy pinhole-defined illumination as the probe. The DFB generating system containing of two lens-axicon doublets is very easily realized. Measured transvers intensities of such DFB display an Airy-pattern-like distribution. Based on well-developed extended ptychographic iterative engine, we simultaneously reconstruct a phase object and the DFB probe with both simulated and real data. Further calculation show that the DFB has abundant spatial high-frequency components that guarantee high coherence of the illuminating probe beam in our THz ptychographic system. In addition, we firmly believe that the proposed approach can be easily transplanted to the ptychography at other frequency band since both lens and axicon are very common optical elements.

Ceramic Yb:Lu2O3 thin-disk laser delivering an average power exceeding 1 kW in continuous-wave operation

Stefan Esser, Christian Röhrer, Xiaodong Xu, Jun Wang, Jian Zhang, Thomas Graf, and Marwan Abdou Ahmed

DOI: 10.1364/OL.445637 Received 11 Oct 2021; Accepted 16 Nov 2021; Posted 17 Nov 2021  View: PDF

Abstract: We report on continuous-wave (cw) laser experiments with a new generation of high-quality and large-size Yb:Lu2O3 polycrystalline transparent ceramics in a thin-disk laser configuration. An output power of up to 1190 W was achieved in multimode operation with an optical efficiency of 60.3 %. In fundamental-mode operation a cw output power of 409 W was extracted with an optical efficiency of 35.6 % and a beam propagation factor of M2 = 1.11.

Cerebrovascular imaging in vivo by non-contact photoacoustic microscopy based on photoacoustic remote sensing with a laser diode for interrogation

Siqi Liang, Jiasheng Zhou, Wenzhao Yang, and Sung-Liang Chen

DOI: 10.1364/OL.446787 Received 25 Oct 2021; Accepted 16 Nov 2021; Posted 17 Nov 2021  View: PDF

Abstract: Photoacoustic microscopy (PAM) is unique for biomedical applications because it can visualize optical absorption contrast in vivo. Recently, non-contact PAM based on non-interferometric photoacoustic remote sensing (PARS), termed PARS microscopy, has shown promise for selected imaging applications. So far a variety of superluminescent diodes (SLDs) have been employed in a PARS microscopy system as the interrogation light source. Here, we investigate the use of a low-cost laser diode (LD) as the interrogation light source in PARS microscopy, termed PARS-LD. Side-by-side comparison between PARS-LD and the PARS microscopy system using the SLD is conducted, showing comparable performance in resolution and signal-to-noise ratio. More importantly, in vivo PAM imaging of mouse brain vessels is conducted in a non-contact manner, for the first time to our knowledge, and the results show great performance of PARS-LD.

Ultrafast heterodyne mode imaging and refractive index mapping of a femtosecond laser written multimode waveguide

Shuang Wu, Zihe Gao, Tianwei Wu, Zhifeng Zhang, and Liang Feng

DOI: 10.1364/OL.444582 Received 28 Sep 2021; Accepted 16 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: We demonstrate imaging of individual modes in a femtosecond laser written multimode waveguide by spatial-heterodyne interferometry and decomposition in data post-processing. Despite the spatial and temporal overlap between multiple waveguide modes, we show the extraction of amplitude for each individual mode and their corresponding temporal dynamics. The mode imaging scheme is effective with the presence of intermodal interference and can be prospective for sensing of ultrafast phase and refractive index fluctuations. We also reconstruct the two-dimensional transverse refractive index map of the multimode waveguide leveraging all the imaged modes and substantiate the reconstructed index map by simulation.

Josephson oscillations of edge quasi-solitons in a photonic-topological coupler

Nataliia Bazhan, Aleksander Yakimenko, and Boris Malomed

DOI: 10.1364/OL.445298 Received 11 Oct 2021; Accepted 16 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: We introduce a scheme of a photonic coupler built of two parallel topological-insulator slab waveguides with the intrinsic Kerr nonlinearity, separated by a lattice spacing. Josephson oscillations (JO) of a single edge quasi-soliton (QS) created in one slab, and of a pair of quasi-solitons created in two slabs, are considered. The single QS jumping between the slabs is subject to quick radiative decay. On the other hand, the JO of the copropagating QS pair may be essentially more robust, as one QS can absorb dispersive waves emitted by the other. The most robust JO regime is featured by the pair of QSs with phase shift $\pi$ between them.

Multiplication sampling moiré method for full-field deformation measurement of composite materials

Qinghua Wang, Shien Ri, M.J. Mohammad Fikry, and Shinji Ogihara

DOI: 10.1364/OL.445854 Received 13 Oct 2021; Accepted 16 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: A multiplication sampling moiré (MSM) method was proposed for robust deformation distribution measurement by performing phase analysis of the 2nd harmonic (2nd-order frequency) of a single grating pattern. The MSM method has a very strong noise immunity because the 2nd harmonic spectrum is far from the low-frequency region of the background noise in the frequency domain. The phase analysis of an experimental grid image on a carbon fiber reinforced plastic (CFRP) specimen indicated that, the MSM method effectively solved the problem of non-negligible phase measurement errors of conventional methods that extracts the fundamental frequency of the grating, in the case of severe local noise. The displacement and strain distributions of CFRP in a tensile test were successfully measured. This method is suitable for deformation measurement of various composite materials.

Relaxation oscillations in a microcavity Brillouin laser

Yingchun Qin, Shulin Ding, Shujian Lei, Jie Liu, Yan Bai, Menghua Zhang, Yuhang Li, Jianming Wen, Xiaoshun Jiang, and Min Xiao

DOI: 10.1364/OL.440677 Received 16 Aug 2021; Accepted 15 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: We demonstrate a new kind of relaxation oscillation in a microcavity Brillouin laser, which induces a self-pulsation of the Brillouin laser. This specific oscillation is generated by the interplay between the Brillouin lasing and the thermo-optic effect in an optical microcavity. Intriguingly, the oscillatory behaviors are simultaneously present in both forward input pump and backward Brillouin lasing emission. By developing a coupled-mode theory, our numerical simulations display an excellent agreement with the experimental results.

Background-free single-beam coherent Raman spectroscopy assisted by air lasing

Fangbo Zhang, Hongqiang Xie, Luqi Yuan, Zhihao Zhang, Botao Fu, Shupeng Yu, Guihua Li, Ning Zhang, Xu Lu, Jinping Yao, Ya Cheng, and Zhizhan Xu

DOI: 10.1364/OL.441602 Received 26 Aug 2021; Accepted 15 Nov 2021; Posted 16 Nov 2021  View: PDF

Abstract: We develop a background-free single-beam coherent Raman scattering technique, enabling high-sensitivity detection of greenhouse gas. In this scheme, Raman coherence prepared by a femtosecond laser is interrogated by the self-generated narrowband air lasing, thus allowing for the single-beam measurement without complex pulse shaping. The unique spatio-temporal characteristics of air lasing are beneficial to improve signal-to-noise ratio and spectral resolution of Raman signals. With the method, the SF6 gas with a concentration of 0.38% was detected in the SF6-air mixture. This technique provides a simple and promising route for remote detection due to low divergence of Raman signals and availability of high-energy pump lasers, which might broaden potential applications of air lasing.

Above pile-up fluorescence microscopy with a 32 Mcps single-channel time-resolved SPAD system

Serena Farina, Ivan Labanca, Giulia Acconcia, Alberto Ghezzi, Andrea Farina, Cosimo D'Andrea, and Ivan Rech

DOI: 10.1364/OL.444815 Received 19 Oct 2021; Accepted 15 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: One of the major drawbacks of Time-Correlated Single Photon Counting (TCSPC) is generally represented by pile-up distortion, that strongly bounds the maximum acquisition speed to few percents of the laser excitation rate. Based on a previous theoretical analysis, recently we presented the first low-distortion and high-speed TCSPC system capable of overcoming the pile-up limitation by perfectly matching the Single-Photon Avalanche Diode (SPAD) dead time to the laser period. In this work, we validate the proposed system in a standard fluorescence measurement by comparing experimental data with the reference theoretical framework. As a result, a count rate of 32 Mcps was achieved with a single-channel system still observing a negligible lifetime distortion.

Maskless nanostructure photolithography by ultrahigh order modes of the symmetrical metal-cladding waveguide

Meng Zhang, Hailang Dai, Yuxi Shang, Zhuangqi Cao, and Xianfeng Chen

DOI: 10.1364/OL.446431 Received 21 Oct 2021; Accepted 15 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: In order to fabricate fine patterns beyond the diffraction limit, a nanostructure photolithography technique is promised. In this letter, we present a method that sub-100nm lines can be patterned photolithographically using ultrahigh order modes from the symmetrical metal-cladding waveguide (SMCW) in the near field excited by a continuous wave visible light without focusing. Meanwhile, the etching depth of the nano-pattern reaches more than 200 nm. Therein, the localized light intensity distribution can be used to map the photoresist exposure patterns, which agrees well with our theoretical model. This technique opens up the possibility of localizing light fields below the diffraction limit using maskless and lower power visible light.

Photonic bound states in the continuum in nanostructured transition metal dichalcogenides for strong photon–exciton coupling

Xueyang Zong, Lixia Li, and Yufang Liu

DOI: 10.1364/OL.446950 Received 28 Oct 2021; Accepted 14 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: Optical cavities, be they plasmonic, photonic, or both, can support the optical bound states in the continuum (BICs) with infinite radiative lifetime, which have in recent years been reported widely. Here, we investigate theoretically BICs with high radiative quality factors in a multilayer, nanostructured transition metal dichalcogenide (TMD) consisting of one-dimensional, nano-slotted TMD gratings on a dielectric substrate. Besides intrinsic exciton resonances, the photonic system supports two distinct groups of BICs: symmetry-protected and interference-based BICs, which are demonstrated by tuning the geometric dimensions. The dispersion of the BICs can be mediated by varying the grating period at normal incidence so that we explore the strong coupling between the photonic modes and the TMD exciton band in a same nanophotonic structure, evidenced by clear anti-crossings in extinction spectra. This work expands not only the library of traditional nanophotonic approaches, but also provides more possibilities for optoelectronic devices toward miniaturization.

Efficient soliton self-frequency shift in hydrogen-filled hollow-core fiber

Yi-Hao Chen, Pavel Sidorenko, Jose Antonio-Lopez, Rodrigo Amezcua Correa, and Frank Wise

DOI: 10.1364/OL.445499 Received 11 Oct 2021; Accepted 14 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: We report a study of soliton self-frequency shifting in hydrogen-filled hollow-core fiber. The combination of hydrogen and short 40-fs input pulses underlies clean and efficient generation of Raman solitons between 1080 and 1600 nm. With 240-nJ input pulses, the Raman soliton energy ranges from 110 to 20 nJ over that wavelength range, and the pulse duration is approximately 45 fs. In particular, 70-nJ and 42-fs pulses are generated at 1300 nm. Numerical simulations agree reasonably well with experiments and predict that microjoule-energy tunable pulses should be possible with higher-energy input pulses.

Correcting spectral baseline fluctuations in dual-comb interferometry

Mathieu Walsh, Alex Tourigny-Plante, Philippe Guay, Khaoula Fdil, and Jérôme Genest

DOI: 10.1364/OL.442748 Received 09 Sep 2021; Accepted 14 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: A method to measure and correct for spectral baseline fluctuations in dual-comb interferometry is presented. Fluctuations can be measured from the amplitude of beat notes between combs and a continuous wave laser or on a separate measurement of the combs repetition rates, filtered around the spectral region of interest. Amplitude-dependent spectral variations are characterized using low resolution Fourier-transforms around several interferograms' center burst and a non-stationary filter is applied to properly account for the combs variations during the measurement. This allows removing this source of statistical as well as systematic errors.

ultra-broadband, high-resolution microdroplet spectrometers for the Near Infrared

Marialuisa Capezzuto, Davide D'Ambrosio, Saverio Avino, Antonio Giorgini, Gianluca Gagliardi, and Pietro Malara

DOI: 10.1364/OL.444687 Received 14 Oct 2021; Accepted 14 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: By stabilizing the evaporation dynamics of a microliter fluorocarbon droplet, we demonstrate a fast-scan optofluidic Fourier Transform spectrometer on the tip of an optical fiber operating in the 1000-2000 nm window with resolution of 3.5 cm-1 (i.e. <1nm @ 1560nm). Compared to the other FT NIR small-scale spectrometers reported in literature, the fluorocarbon droplet spectrometer shows the largest wavelength span and span/resolution ratio, allowing to easily perform spectral analysis of broadband/narrowband radiation. Our results open the way for the practical application of droplet spectrometers as advanced optofluidic NIR analyzers with small size, low cost, and capable to operate even in harsh environments or in absence of electrical power sources.

All-PM Divided Pulse Fiber Oscillator Mode-locked with the Optical Kerr-effect

Marvin Edelmann, Yi Hua, Gabor Kulcsar, and Franz Kaertner

DOI: 10.1364/OL.445410 Received 07 Oct 2021; Accepted 13 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: In this letter, we investigate a Yb-doped mode-locked fiber oscillator that uses coherent pulse division and recombination to avoid excessive nonlinear phase shifts. The mode-locking mechanism of the laser is based on the accumulation of a differential nonlinear phase between orthogonal polarization modes in the polarization-maintaining fiber segment. The inserted coherent pulse divider, based on YVO4-crystals rotated successively by 45°, enables stable and undistorted mode-locked steady-states. The output pulse energy is increased from 89 pJ in the non-divided operation by ~6.5 dB to more than 400 pJ with three divisions. Measurement of amplitude-fluctuations reveals a simultaneous broadband noise suppression of up to ~9 dB in the frequency range from 10 kHz to 2MHz.

High-resolution on-chip Fourier transform spectrometer based on cascaded optical switches

Du Junjie, Hongyi Zhang, Xinyi Wang, Weihan Xu, Liangjun Lu, Jianping Chen, and Linjie Zhou

DOI: 10.1364/OL.437867 Received 16 Jul 2021; Accepted 13 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: Chip-level spectrometers provide a stable and cost-effective solution for spectral analysis in various applications. Here we present a silicon on-chip digital Fourier transform spectrometer consisting of eight cascaded optical switches connected by delay waveguides. By configuring the states of the optical switches, this chip can realize 127 Mach-Zehnder interferometers with linearly increased optical path differences. A machine learning regularization method is utilized to reconstruct the spectrum. Experimental results show that our chip can retrieve both sparse and broadband optical spectra with negligible reconstruction errors. The spectral resolution can be further improved by cascading more stages of optical switches. Our method has the advantages of compact size, high scalability, and high signal-to-noise ratio, which is a promising candidate for miniaturized spectrometers.

Broadband angular dispersion compensation for digital micromirror devices

Dihan Chen, Bingxu Chen, Qi Shao, and Shih-Chi CHEN

DOI: 10.1364/OL.436368 Received 08 Jul 2021; Accepted 12 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: In this Letter, we present a compact broadband angular dispersion compensation method for digital micromirror devices (DMDs) and ultrashort pulse lasers, which effectively extends the conventional single-wavelength compensation design to a wide wavelength range of 300 nm. This enables many wavelength-tunable DMD applications, e.g., two-photon microscopy and ultrafast spectroscopy etc. First, a parametric model is developed for the dispersion compensation unit, which consists of a transmission grating and a 4-f telescope sub-unit, to guide the selection of components and parameter optimization for broadband applications. In the experiments, we designed a single slit-based metrology system to measure and quantify the compensated angular dispersion of a Ti:Sapphire femtosecond laser with a pulse width of 75 fs. The results indicate that our method can reduce the angular dispersion to 0.04°, i.e., pulse widening less than 20 fs, over a wavelength range from 750 nm – 1050 nm. For demonstration, the DMD system is used as a multi-wavelength beam shaper to reconstruct a wavefront that contains the “CUHK” pattern. The results confirm the capability of broadband dispersion compensation. This means the DMD can be used in different applications that employ a broadband light source, e.g., wavelength tunable femtosecond laser, attosecond laser, supercontinuum laser, and multi-color LED etc.

Eliminating the effect of additional sidebands to sub-μGal in an atom gravimeter with phase modulated Raman laser

Qin Luo, Hang Zhou, Lele Chen, Xiaochun Duan, Minkang Zhou, and Zhongkun HU

DOI: 10.1364/OL.443629 Received 27 Sep 2021; Accepted 12 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: The additional sidebands (ASB) in Raman laser will cause significant impact on the performance of atom gravimeters (AGs) based on phase modulated Raman laser. We propose a method of modulating the sideband to carrier ratio in Raman laser to determine the magic time intervals where the phase shift induced by the ASB effect is minimized, and this method is demonstrated by experiments. Among these magic time intervals, some noise-immunity points are predicted. Based on the prediction and the result of ASB effect changing with the interval time T between adjacent Raman pulses, optimal magic time interval is selected. Therefore, the uncertainty to gravity measurement induced by the ASB effect when the AG works at themagic time interval is reduced to 0.5 μGal. Furthermore, the ASB effect and its zero phase points in four-pulses atom interferometers are also discussed. This work provides a clear way to eliminate the phase shift induced by the ASB effect in high precision AGs employing phase modulated Raman laser.

Extending the range of distributed fiber optic

Daniel Sweeney and Christian Petrie

DOI: 10.1364/OL.445245 Received 05 Oct 2021; Accepted 12 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: Optical frequency domain reflectometry (OFDR) is a spectral measurement technique in which shifts in the local Rayleigh backscatter spectra can be used to perform distributed temperature or strain measurements relative to a reference measurement using ordinary singlemode optical fibers. This work demonstrates a data processing methodology for improving the resolvable range of temperature and strain by adaptively varying the reference measurement position by position, based on the time evolution of the local optical intensities and the correlation between the reference and active measurements. These methods nearly double the resolvable range of temperature and strain compared to that achieved using the traditional static reference approach.

High-speed photonic reservoir computer based on a delayed Fano laser under electrical modulation

YU HUANG, Pei Zhou, Yigong Yang, and Nianqiang Li

DOI: 10.1364/OL.445278 Received 06 Oct 2021; Accepted 11 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: We propose and numerically demonstrate a high-speed photonic reservoir computing (RC) system using a compact Fano laser (FL) with optical feedback under electrical modulation. Benefiting from its insensitivity to external feedback, an FL has a wider dynamic steady-state region compared with a conventional Fabry-Perot laser, which significantly extends the ranges of desirable RC implementation. Interestingly, we observe two separate regions of good RC performances corresponding to two scenarios of the dynamic steady-state of FL, respectively. Moreover, the robust RC performance versus the feedback phase can be achieved in one of the steady-state regions, where the laser is not destabilized for lower external reflectivity. Owing to the ultra-short photon lifetime in the FL, the information processing rate of our proposed RC system may reach 10 Gbps. More importantly, as a specific type of microscopic laser, the FL offers potential applications to RC-based integrated neuromorphic photonic systems.

Cylindrical wave-based off-axis digital holography with long field of view

Gui-Xiang Chen, Hong-Yi Huang, Qing-Yang Yue, Qian Zhao, and Cheng-Shan Guo

DOI: 10.1364/OL.436298 Received 08 Jul 2021; Accepted 11 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: We present a new type of off-axis digital holographic imaging methods with long field of view (FOV). In the method, the pre-magnification recording geometry is realized by a cylindrical lens (CL) or cylindrical illumination beam, instead of a conventional objective or spherical illumination beam in traditional off-axis digital holography (DH). At the same time, the off-axis reference beam is also replaced by a divergent cylindrical beam in the new method. Theoretical analysis and experimental results have justified that, in off-axis DHs, the adoption of the cylindrical beams can realize a 1D pre-magnification of the object beam only in the off-axis direction to satisfy the bandwidth constraint of off-axis holographic recording, and at the same time the FOV of the recorded and reconstructed image in the orthogonal direction can remain unaffected. In comparison with existing off-axis DHs, this cylindrical wave-based DH (CWDH) method has a distinct advantage in expanding the FOV of the reconstructed image. The FOV feature of the CWDH makes it especially suitable for applications that require a long FOV such as imaging samples in microfluidic channels or in capillary tubes.

Adjustable and continuous eye-box replication for holographic Maxwellian near-eye display

ZHANG SHIJIE, Zhiqi Zhang, and Juan Liu

DOI: 10.1364/OL.438855 Received 03 Aug 2021; Accepted 11 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: The Maxwellian display presents always-focused images to the viewer, alleviating the vergence-accommodation conflict (VAC) in near-eye displays (NEDs). Recently, in order to improve its limited eyebox, many methods have been proposed, among which viewpoint replication has attracted a lot of attention. However, double image, blind area and image shift always happen in typical eye-box replication Maxwellian NED when the eye moves between the replicated viewpoints, which prevents it from wider applications. In this letter, we propose a method to design a holographic Maxwellian NED system with adjustable and continuous eye-box replication. In our system, the interval of the viewpoints can be easily adjusted by changing the projection angle of the reconstructed image. Thus, holograms corresponding to the positions of different viewpoints are calculated to match the interval of the replicated viewpoints with the human pupil diameter, making it possible to eliminate or alleviate double image or blind area effects. Besides, seamless viewpoint conversion in eyebox has been achieved through aligning the images of adjacent viewpoints on the retina by hologram pre-processing. These effects have been verified successfully by optical experiments, which has the potential to be applied in near eye 3D displays without VAC.

Accurate OSNR Monitoring based on Data Augmentation-assisted DNN with a Small-scale Dataset

Weiwei Zhao, Zheng Yang, Meng Xiang, Ming Tang, Yuwen Qin, and Songnian Fu

DOI: 10.1364/OL.445345 Received 06 Oct 2021; Accepted 11 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: The deep neural network (DNN) has been recently demonstrated with great success for accurate optical signal-to-noise ratio (OSNR) monitoring. However, the performance of OSNR monitoring substantially degrades, when the mega dataset is inaccessible. Here, we demonstrate an accurate OSNR monitoring scheme based on the data augmentation (DA)-assisted DNN with a small-scale dataset. When the 20-GBaud QPSK signal is transmitted over 400 to 2600-km standard single-mode fiber (SSMF) with an OSNR range from 8 to 14-dB, we experimentally evaluate the minimum size of the dataset that secures the mean absolute error (MAE) of OSNR monitoring to be less than 1-dB. The DA-assisted scheme only requires 50% of the raw data, in comparison with the traditional DNN scheme. Thus, the DA-assisted DNN scheme is promising for field-trial accurate OSNR monitoring, especially when the collection of mega datasets is inconvenient.

Programmable fabrication of a miniaturized photodetector with thermal stability via femtosecond laser direct writing

shuyu liang and Hong Xia

DOI: 10.1364/OL.446556 Received 22 Oct 2021; Accepted 11 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: With the increased integration of electronic devices, the problem of heat accumulation has become increasing more serious. Here, a miniaturized photodetector with thermal stability was fabricated by combining the excellent characteristics of femtosecond laser direct writing (FsLDW) and silicon (Si). The sensing part of the photodetector is a Si microwire composed of Si nanoparticles and the sensing area is only 300 μm2. As a result, the photodetector can work stably at a high temperature of 100 °C and the response speed of the photodetector becomes notably faster at high temperatures. Furthermore, an image sensor was successfully fabricated by integrating 16 photodetectors and the image sensor can also work stably at high temperatures. This work demonstrates the application potential in harsh conditions of photodetectors based on Si microwires prepared by FsLDW .

Strong coupling between monolayer quantum emitter WS2 and degenerate/non-degenerate surface lattice resonances

Fanzhou lv, Zhi-Hang Wang, YuDie Huang, Jiaxu Chen, Junqiao La, Dongda Wu, Zhicheng Guo, Yujun liu, Yufeng Zhang, Yi Wang, and Wenxin Wang

DOI: 10.1364/OL.444100 Received 30 Sep 2021; Accepted 11 Nov 2021; Posted 19 Nov 2021  View: PDF

Abstract: Strong light-matter coupling manifested of Rabi split-ting draws considerable interest due to its funda-mental significance of impressive interaction en-hancement in the fields of ultrafast active plasmonic devices and quantum information. In this paper, we investigated the coherent optical properties of a plasmonic system consisting of periodic metal nano-particle arrays covered by a WS2 thin film in atomic layer thickness. The coupling factor, energy splitting, and temporal dynamics of this coherent coupling phenomenon are quantitatively revealed by finite-difference time-domain (FDTD) simulation and a full quantum mechanical model proves the exciton be-havior of fermionic quantum emitter WS2 is carefully modulated by bosonic SLRs. This work could pave the road for coherent modulation of polariton and plasmon devices and can potentially open up diverse exciting possibilities like single-molecule sensing, nanoscale light sources, single-photon emitters, and all-optical transistors.

Laser frequency measurement at the short-wavelength region using an intermediate laser and a frequency noise cancellation method

Yuki Kojima, Kohei Ikeda, Yuto Tanabe, Daisuke Akamatsu, and Feng-Lei Hong

DOI: 10.1364/OL.446366 Received 20 Oct 2021; Accepted 10 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: Optical frequency combs have a crucial supporting role for optical frequency standards and cover a wide range of wavelengths over octaves. However, broadening the comb spectrum to the short-wavelength visible region (λ < 500 nm), where GaN-based blue diode lasers are available, is not an easy task. In this study, we propose a method for measuring the laser frequency in the short-wavelength region using an intermediate laser and a noise-cancelling scheme. We demonstrate this method by measuring the frequency of a GaN-based laser at 399 nm, confirming that the frequency measurement is not affected by the frequency noise of the intermediate laser.

Dual-microcomb generation in a synchronously-driven waveguide ring resonator

Yiqing Xu, Miro Erkintalo, Yi Lin, Stephane Coen, Huilian Ma, and Stuart Murdoch

DOI: 10.1364/OL.443153 Received 13 Sep 2021; Accepted 10 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: Microcombs -- optical frequency combs generated in coherently-driven nonlinear microresonators -- have attracted significant attention over the last decade. The ability to generate two such combs in a single resonator device has in particular enabled a host of applications from spectroscopy to imaging. Concurrently, novel comb generation techniques such as synchronous pulsed driving have been developed to enhance the efficiency and flexibility of microcomb generation. Here we report on the first experimental demonstration of dual-microcomb generation via synchronous pulsed pumping of a single microresonator. Specifically, we use two electro-optically generated pulse trains derived from a common continuous wave laser to simultaneously drive two orthogonal polarization modes of an integrated silica ring resonator, observing the generation of coherent dissipative Kerr cavity soliton combs on both polarization axes. Thanks to the resonator birefringence, the two soliton combs are associated with different repetition rates, thus realizing a dual-microcomb source. To illustrate the source's application potential, we demonstrate proof-of-concept spectroscopic measurements.

Toward Generalized Forked Gratings Via Deep Learning

Yue Zhao, Enliang Wang, Fulin Cao, and Changqing Xie

DOI: 10.1364/OL.444012 Received 24 Sep 2021; Accepted 10 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: We extend the concept of forked gratings to include the ability of high order diffraction suppression of a single pair of vortex beams. The main idea is to appropriately distribute rectangular holes over each open space of a conventional forked grating. We further introduce the deep convolutional neural network algorithm to assist us in reconstructing and obtaining the optimal parameters of generalized forked grating. The recovery rate of our neural network is 92.3%. Theoretical analysis and simulations reveal that distribution of diffraction intensity and suppression of high order diffraction are determined by both the length-to-period ratio and the deviation-to-period ratio. The 3rd order diffracted light intensity can be as low as 0.067% of the desired (1st) order diffracted light intensity. The verification experiment results are also presented, confirming the helical phase structures with multi-topological charges. The high order diffraction suppression properties of the generalized forked gratings hold promise for broad applications, such as imaging, microscopy and fundamental physics observation.

Photon detection at 1 ns time intervals using 16-element SNSPD array with SFQ multiplexer

Shigehito Miki, Shigeyuki Miyajima, Fumihiro China, Masahiro Yabuno, and H Terai

DOI: 10.1364/OL.438416 Received 28 Jul 2021; Accepted 10 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: We demonstrate the high-speed operation of a 16-element superconducting nanostrip single-photon detector (SNSPD) array with a single flux quantum (SFQ) multiplexer. The SFQ multiplexer can reshape the output signals from 16-element SNSPD into pulses with durations shorter than 1 ns and bundle these pulses into one output line, which is advantageous for high-speed operation of the SNSPD array system. We confirmed the correct operation of the 16-element SNSPD system with a system detection efficiency of 80% at a wavelength of 1550 nm, timing jitter of 45 ps, and successful observation of photons at 1 ns time intervals as distinguishable output pulses. The reduction in detection efficiency could also be suppressed to ~0.93 during the dead time of ~10 ns for each SNSPD pixel when the incident photon flux was relatively low at 0.1 photon/pulse.

Experimental Demonstration of Mode Selection in Bridge-Coupled Metallo-Dielectric Nanolasers

Sizhu Jiang, Dmitrii Belogolovskii, Suruj Deka, Si Hui Athena Pan, and Yeshaiahu Fainman

DOI: 10.1364/OL.443991 Received 22 Sep 2021; Accepted 10 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: We experimentally demonstrate bridge-coupled metallo-dielectric nanolasers that can operate in the in-phase or out-of-phase locking modes at room temperature. By varying the length of the bridge, we show that the coupling coefficients can be realized in support of the stable operation of any of these two modes. Both coupled nanolaser designs have been fabricated and characterized for experimental validation. Their lasing behavior has been confirmed by the spectral evolution, light-in light-out characterizations and emission linewidth narrowing. The operating mode is identified from the near-field and far-field emission pattern measurements. This is the first demonstration of mode selection in bridge-coupled metallo-dielectric nanolasers, that can serve as building blocks in nanolaser arrays for applications in imaging, virtual reality devices, and lidars.

Optical square-wave generation in a semiconductor laser with optoelectronic feedback

Md. Shariful Islam, Anton Kovalev, Evgeny Viktorov, David Citrin, and Alexandre Locquet

DOI: 10.1364/OL.444055 Received 23 Sep 2021; Accepted 10 Nov 2021; Posted 11 Nov 2021  View: PDF

Abstract: We report self-sustained optical square-wave (SW) generation in a semiconductor laser diode subjected to delayed optoelectronic feedback on its injection current ($J$). This optoelectronic oscillator relies on nonlinear effects present in both the laser diode and in the optoelectronic feedback loop through amplifier saturation. The repetition rate of the SW is an integer multiple of the inverse of the loop delay, while the duty cycle can be tuned with $J$. We present a rate-equation model that reproduces semi-quantitatively the observed SWs and attributes their origin to a delay-driven SW oscillation between two unstable equilibrium points.

Multicore Fibers with 10 and 16 Single-Mode Cores for the Visible Spectrum

Saeed Sharif Azadeh, Andrei Stalmashonak, Kevin Bennett, Fu-Der Chen, Wesley Sacher, and Joyce Poon

DOI: 10.1364/OL.446161 Received 17 Oct 2021; Accepted 09 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: We report multicore fibers (MCFs) with 10 and 16 linearly distributed cores with single-mode operation in the visible spectrum. The average propagation loss of the cores is 0.06 dB/m at λ= 445 nm and < 0.03 dB/m at wavelengths longer than 488 nm. The low inter-core crosstalk and nearly identical performance of the cores make these MCFs suitable for spatial division multiplexing in the visible spectrum. As a proof-of-concept application, one of the MCFs was coupled to an implantable neural probe to spatially address light emitting gratings on the probe.

Luminous efficiency improvement of polymer light-emitting diodes with platinum nanolayer at PEDOT:PSS–ITO interface

Sy-Hann Chen and Pei-Ju Hsu

DOI: 10.1364/OL.441131 Received 23 Aug 2021; Accepted 09 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: A platinum (Pt) nanolayer is successfully deposited on indium titanium oxide (ITO) as a buffer layer for polymer light-emitting diodes (PLEDs), using a rapid and low-cost sputtering system at room temperature. With a 5-s Pt-dispersed ITO as the anode window substrate of the PLED, a maximum current efficiency of 4.00 cd/A is realized, which is notably higher better than that of a typical PLED (1.13 cd/A). It is determined that the average current efficiency and electroluminescence intensity of the proposed PLED are enhanced by 2.5 times and 290%, respectively, compared to a typical PLED.

Ultrafast Saturable Absorption of BiOI Nanosheets Prepared by CVT

haixia zhu, chang wang, xingcheng xiao, zhihui chen, Yingwei Wang, Si Xiao, Yejun Li, and jun He

DOI: 10.1364/OL.444504 Received 30 Sep 2021; Accepted 09 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: The saturable absorption properties of BiOI nanosheets with exposed {110} facets prepared by the chemical vapor transport were investigated by Z-scan with femtosecond pulse laser. The nonlinear absorption coefficient at 400 nm is stronger and more sensitive to photoexcitation than its nonlinear response at 800 nm. The small saturation intensity could have been achieved, which is one order of magnitude smaller than that of black phosphorus (BP) nanosheets, while the Imχ(3) are determined to be -4.35ⅹ10¹² esu close to theoretical prediction. According to time-resolved photoluminescence (TRPL) spectrum results, this strong saturated absorption at 400 nm may be attributed to the interband recombination process, whose lifetime was 0 ps.

Coaxial double-hole PEDOT: PSS electrodes achieving tunable terahertz zoomable convergence

Shuai Li, Hao Tian, Peng Tan, guanchao wang, Wenpeng Guo, Jing Wang, Yao Zhang, Chengpeng Hu, Xiangda Meng, and Zhongxiang Zhou

DOI: 10.1364/OL.439379 Received 02 Aug 2021; Accepted 08 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: The local wavefront modulation technique in the terahertz band is an important basis for the development of terahertz quasi-optical devices. Here, an electrically controlled convergent tunable device based on patterned transparent electrode PEDOT: PSS is realized to locally tune the terahertz wavefront. The device consists of two substrates with circular-hole electrodes and the liquid crystal sandwiched between them. The refractive index gradient of liquid crystal in the device can be generated by the coaxial double-hole electrodes, which realize the continuous control of the significant focusing of terahertz wave. The test results show that the focal length can be modulated in the range of 3-12cm with varied external voltage and when it varies from 3V to 8V, the 1/e2 radius of the spot decreases to 1.3mm, 0.27 times the initial state, and the spot central intensity magnification increases gradually with the change, up to 3.31 times. In addition, increasing the external voltage, changing the position and the diameter of the incident beam can obtain significant beam deflection and spot center extinction respectively. The acquisition of the large tunable focal length range of the continuous terahertz zoom device shows that the construction of the gradient refractive index is an important method to regulate terahertz wavefront by optical means, which greatly promotes the research of terahertz imaging devices.

Double perovskite microcrystals based white light-emitting diodes without reabsorption of multiphase phosphors

Gang Yang, Xueguo Li, Jinshu Huang, Xiumei Xu, Xiaoxu Ji, Aihua Wang, Jinbing Cheng, Gencai Pan, and Yongsheng Zhu

DOI: 10.1364/OL.446314 Received 21 Oct 2021; Accepted 08 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: Lead-free halide double perovskite variants with self-trapped excitons emission have been synthesized and shown good application potential in white lighting due to nontoxicity, high stability, and unique optoelectronic properties. However, their spectral tenability as the essential influencing factor of white light quality is poor. Here, Tb3+ ions are incorporated into Cs2Ag0.6Na0.4InCl6:Bi double perovskite microcrystals via re-crystallization method. Tb3+ ions doping not only makes the white light spectrum adjustable, but also maintains the high photoluminescence quantum yield (PLQY). The optimal values of color rendering index (CRI) and PLQY are 94.3 and 95%, respectively. These are comparable to the current highest values. Noteworthy is that, intrinsic emission of Tb3+ ions is attributed to the effective energy transfer from the trapped exciton state of the double perovskite host to Tb3+ ions. Finally, mixing 30% Tb3+ alloyed Cs2Ag0.6Na0.4InCl6:Bi and Cs2NaInCl6:10%Sb DP phosphors, a series of double perovskite based white light-emitting diodes (WLEDs) are prepared. The color coordinates of the best WLEDs are (0.34, 0.32), the lumen efficiency is 42 lm/W, and the CRI is 94.3. It is worth mentioning here that there is no blue light loss caused by energy reabsorption between the two phosphors, because the excitation wavelengths of the two phosphors are concentrated in the ultraviolet band. This work provides a new strategy for preparing high-performance WLED.

Supermode noise suppression with polarization-multiplexed dual-loop for active mode-locking optoelectronic oscillator

Yan Li, Muguang Wang, jing zhang, Hongqian Mu, Chun can Wang, and Fengping Yan

DOI: 10.1364/OL.440663 Received 17 Aug 2021; Accepted 08 Nov 2021; Posted 15 Nov 2021  View: PDF

Abstract: The active mode-locking (AML) technique has been widely used in Erbium-doped fiber lasers to generate picosecond pulse trains. Here we propose a novel active mode-locking dual-loop optoelectronic oscillator (AML-DL-OEO), which can generate microwave frequency comb (MFC) signals with adjustable comb spacings. Based on this scheme, the order of harmonic mode-locking is dramatically decreased for a certain AML driving frequency comparing to a single-loop AML-OEO. Thus, the supermode noise caused by harmonic mode-locking can be efficiently suppressed. In addition, the sidemodes are well suppressed by the dual-loop architecture. An experiment is performed. MFC signals with different comb spacing are generated under fundamental or harmonic mode-locking state. AML-DL-OEO systems with different length differences between two loops are implemented to evaluate supermode noise suppression capability. The performance of the generated MFC signals is recorded and analyzed.

180 Gbit/s Si3N4-waveguide coupled germanium photodetector with improved quantum efficiency

Xiao Hu, dingyi Wu, Daigao Chen, Lei Wang, Xi Xiao, and Shaohua Yu

DOI: 10.1364/OL.438962 Received 28 Jul 2021; Accepted 07 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: A high quantum efficiency (QE) and high-speed silicon nitride (Si3N4) waveguide coupled germanium-on-silicon photodetector (Ge-on-Si PD) is presented. The proposed device is fabricated in a commercial 90 nm silicon photonics process platform. By decreasing the height between tapered Si3N4 waveguide and bottom silicon (Si) to 200 nm and Si3N4 thickness to 300 nm, the QE is significantly improved. Although the theoretical responsivity can reach to 0.92 A/W at 1550 nm, the measured value is calculated to be approximately 0.61 A/W. The maximum experimental responsivity is about 0.9 A/W at 1485 nm. The 3 dB optoelectrical (OE) bandwidth of up to 54 GHz is demonstrated at -3.3 V bias. Additionally, the 80, 90, 100, and 105 Gbit/s non-return-to-zero (NRZ) on-off-keying (OOK) and 150, 160, 170, and 180 Gbit/s four-level pulse amplitude (PAM-4) modulation clear openings of the electrical eye diagrams are attained. Overall, the Si3N4-waveguide coupled Ge-on-Si PD in this work possesses higher QE and operates at highest data-rates reported so far

Frequency stabilized Brillouin random fiber laser enabled by self-inscribed transient population grating

Liang Zhang, Zenghuan Qiu, Zhelan Xiao, Jilin Zhang, Fufei Pang, Tingyun Wang, and Xiaoyi Bao

DOI: 10.1364/OL.445264 Received 06 Oct 2021; Accepted 06 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: A frequency-stabilized Brillouin random fiber laser (BRFL) realized by a self-inscribed transient population grating (TPG) is proposed and demonstrated for the first time. The TPG is formed via the redistribution of the population in erbium doped fibers (EDFs) by bidirectionally injected phonon-controlled random laser beams. Long-lifetime meta-stable ion states in EDFs basically prolonged time dynamics of SBS laser up to milliseconds. Consequently, significant random modes are suppressed with low relative intensity noise due to reduced mode hoping in Stokes random laser, hence one dominating lasing mode at milliseconds of lifetime is established from numerous random modes competition, which is proved theoretical and experimentally via the TPG.

A plasmonic tapered-fiber interference sensor for simultaneously detecting refractive index and temperature

Xinghong Chen, Xuejin LI, Duo Yi, Xue Ming Hong, and yuzhi chen

DOI: 10.1364/OL.444981 Received 01 Oct 2021; Accepted 05 Nov 2021; Posted 05 Nov 2021  View: PDF

Abstract: A single-optic-fiber sensor is proposed to simultaneously detect the refractive index (RI) and temperature (T) at a single wavelength band. This sensor is based on the mixed effects of Mach-Zehnder interference (MZI) and surface plasmon resonance (SPR), where MZI is excited by a tapered-fiber structure and SPR is stimulated by a 45-nm gold film on the tapered-fiber surface. The detection signal of an SPR spectrum superimposed on interference stripes was obtained. After fast Fourier transform and filter processing, the MZI and SPR signals were separated. Experimental results indicate that our sensor can improve the RI sensitivity to 2054.95nm/RIU (22-fold greater than that of the original tapered-fiber MZI sensor) and eliminate the influence of T. Additionally, this highly integrated sensor simplifies the detection system, with potential applications in portable biochemical sensing.

Anisotropic ZnS:Cr2+ crystal: novel material for compact and efficient Mid-IR tunable lasers

Sergey Kurashkin, Olga Martynova, Dmitry Savin, Vladimir Ikonnikov, Evgeny Gavrishchuk, and Alexander Savikin

DOI: 10.1364/OL.437886 Received 23 Aug 2021; Accepted 05 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: Laser-quality anisotropic ZnS:Cr2+ element was obtained using prolonged hot isostatic pressing at high temperature. Lasing centered at a wavelength of 2.45 μm was obtained with longitudinal pumping at a wavelength of 1.94 μm. The short-cavity laser slope efficiency with respect to the absorbed power was about 78%. The lasing wavelength was continuously tuned in the range of 2.35–2.52 µm by rotating the Brewster active element around the normal to its surface.

Photonics-assisted joint high-speed communication and high-resolution radar detection system

yanyi wang, Ze Dong, Junjie Ding, li ping, mingxue wang, Feng Zhao, and Jianjun Yu

DOI: 10.1364/OL.444252 Received 30 Sep 2021; Accepted 04 Nov 2021; Posted 10 Nov 2021  View: PDF

Abstract: A photonics-aided dual-functional system that can simultaneously implement high-speed communication and high-resolution radar detection is proposed. In the proposed system, high-speed orthogonal frequency division multiplexing (OFDM) signals and broadband linear frequency modulated (LFM) signals are simultaneously realized based on photonic heterodyning. Based on this system, a net rate of 69.81 Gbit/s and a radar with a 15 GHz bandwidth in the W-band were achieved.

Enhanced sensitivity of optical fiber vibration sensor based on a radiofrequency Michelson interferometer

Ming Deng, Tao Zhu, xinhao nan, Yangxu tang, Danqi Feng, Nan Guo, Shudan Deng, and TIANHENG ZHANG

DOI: 10.1364/OL.445425 Received 08 Oct 2021; Accepted 03 Nov 2021; Posted 12 Nov 2021  View: PDF

Abstract: We propose and demonstrate a new scheme for enhancing the sensitivity of optical fiber vibration sensor based on microwave interferometry, which is realized by an incoherent optical Michelson interferometer (MI). The sensing arm of the MI is sensitive to the environmental vibration, which will cause changes in the phase of the reflection spectra in the microwave domain. The phase sensitivity can be improved by adjusting the power ratio of the two beams in the interferometer and the driving frequency of the modulator. The proposed scheme has merits of simplicity and compact configuration, which may provide a new way of high-precision fiber sensors for measuring the vibration, temperature, strain and so on.

Time-frequency analysis of two-photon absorption effect during optical rectification in a ZnTe crystal pumped at 1.024 µm

Léo Guiramand, xavier ropagnol, and Francois Blanchard

DOI: 10.1364/OL.441231 Received 23 Aug 2021; Accepted 03 Nov 2021; Posted 03 Nov 2021  View: PDF

Abstract: Optical rectification in nonlinear crystals is a well-established method for generating terahertz (THz) waves from ultra-short optical pulses. To achieve high conversion efficiency, the phase matching conditions between the pump pulse and the generated THz wave within the nonlinear medium must be satisfied. For ytterbium laser operating at 1.024 µm, a severe phase mismatch occurs in the zinc telluride (ZnTe) crystal, preventing the efficient generation of broadband THz pulses. Using time-frequency analysis, we show that the ultrafast charge carrier dynamic, mainly induced by two-photon absorption, generated in the nonlinear medium during optical rectification processes in ZnTe, plays a crucial role in the filtering of the out-of-phase components of the THz signal, thus enabling the recovery of broadband THz pulse generations.

Deep learning multi-shot 3D localization microscopy using hybrid optical-electronic computing

Hayato Ikoma, Takamasa Kudo, Yifan Peng, Michael Broxton, and Gordon Wetzstein

DOI: 10.1364/OL.441743 Received 30 Aug 2021; Accepted 03 Nov 2021; Posted 04 Nov 2021  View: PDF

Abstract: Current 3D localization microscopy approaches are fundamentally limited in their ability to image thick, densely labeled specimens. Here, we introduce a hybrid optical-electronic computing approach that jointly optimizes an optical encoder (a set of multiple, simultaneously imaged 3D point spread functions) and an electronic decoder (a neural network–based localization algorithm) to optimize 3D localization performance under these conditions. With extensive simulations and biological experiments, we demonstrate that our deep learning--based microscope achieves significantly higher 3D localization accuracy than existing approaches, especially in challenging scenarios with high molecular density over large depth ranges.

Achieving Extreme Light Confinement in Low-Index-Dielectric Resonators Through Quasi-Bound States in the Continuum

Wei Wang and Xuedan Ma

DOI: 10.1364/OL.445411 Received 07 Oct 2021; Accepted 02 Nov 2021; Posted 18 Nov 2021  View: PDF

Abstract: Obtaining large field enhancement in low-refractive index dielectric materials is highly relevant to many photonic and quantum optics applications. However, confining light in these materials is challenging due to light leakage through coupling to continuum modes in the surrounding environment. We investigate the possibility of achieving high quality factors in low-index dielectric resonators through the bound states in the continuum (BIC). Our simulations demonstrate that destructive interference between leaky modes can be achieved by tuning the geometrical parameters of the resonator arrays, which leads to the emergence of quasi-BIC in resonators that have a small index contrast to the underlying substrates. The resultant large field enhancement gives rise to giant quality factors and Purcell effects. By introducing vertical mirror symmetry, the quasi-BIC can be tuned into an ideal BIC. Additionally, the quasi-BIC can modify the emission patterns of the coupled emitters, rendering highly directional and focused far-field emission. These findings may provide a path for the practical implementation of photonic and quantum devices based on low-index dielectric materials.

Octave-spanning Supercontinuum Generation from Off-axis Raman Oscillation in Monolithic KTP

MingHsiung Wu, Yan-Jou Lin, Fredrik Laurell, and Yen-Chieh Huang

DOI: 10.1364/OL.441665 Received 01 Sep 2021; Accepted 01 Nov 2021; Posted 02 Nov 2021  View: PDF

Abstract: We report generation of visible and near-infrared supercontinuum from a high-gain, ultra-broadband, and mirrorless Raman oscillator in a monolithic KTP crystal. In the plane transverse to the pump axis, it resonates and traps off-axis Stokes waves and their frequency-up-converted components bouncing between two crystal surfaces via total internal reflection. The Raman gain is maximized with the Stokes polarization perpendicular to the plane of reflections. When pumped by a Q-switched Nd:YAG laser, the monolithic oscillator generates quasi-mode-locked Stokes pulses with octave-spanning spectral groups across the visible and near-infrared spectra between 540 and 1800 nm.

Violating Bell inequality using weak coherent states

Seyed Rafsanjani and Moslem Mahdavifar

DOI: 10.1364/OL.441499 Received 26 Aug 2021; Accepted 31 Oct 2021; Posted 01 Nov 2021  View: PDF

Abstract: We present an experimental investigation of two-photon interference using a continuous-wave laser. We demonstrate the violation of the CHSH inequality using the phase randomized weak coherent states from a continuous wave laser. Our implementation serves as an approach to reveal the quantum nature of a state that is considered to be a classical state.

Detachable head-mounted photoacoustic microscope in freely moving mice

Guo Heng, Qian Chen, Wei Qin, Weizhi Qi, and Lei Xi

DOI: 10.1364/OL.444226 Received 30 Sep 2021; Accepted 29 Oct 2021; Posted 04 Nov 2021  View: PDF

Abstract: Optical resolution photoacoustic microscopy (ORPAM) is a promising tool for investigating anatomical and functional dynamics in cerebral cortex. However, observation in freely moving mice has been a longstanding challenge for ORPAM. In this report, we extended ORPAM from anesthetized, head-restrained to awake, freely moving mice by using a detachable head-mounted ORPAM probe. We used a micro-electro-mechanical-system (MEMS) scanner and a miniaturized piezoelectric ultrasonic detector to scan the excitation laser beam and detect generated photoacoustic (PA) signals, respectively. The probe weights 1.8 grams and has a large field of view (FOV) of ~3×3 mm². We evaluated the performance of the probe by carrying out phantom experiments and the imaging of vascular networks in mouse cerebral cortex. The results suggest that the ORPAM probe is capable of providing stable and high-quality ORPAM images in freely moving mice.

A new scheme for optimizing direct current component in photonics-assisted OFDM MMW system

Li Zhao, Bohan Sang, Wen Zhou, yi wei, and Jianjun Yu

DOI: 10.1364/OL.442033 Received 01 Sep 2021; Accepted 28 Sep 2021; Posted 10 Nov 2021  View: PDF

Abstract: This letter proposes a new scheme for optimizing direct current (DC) component, by controlling the IQ modulator to adjust the DC component to accurately estimate the frequency offset and improve system performance. Compared with the traditional method, a large DC component is used as the pilot frequency for frequency offset estimation. When a small DC component is used, the system performance is better. 3Gbaud 16QAM OFDM signal is delivered over 70 m wireless, enabling a 3.5dB sensitivity increase at the bit error rate threshold of 1x10-2 in the E-band.

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