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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.

Higher-order topological states in two-dimensional Stampfli-Triangle photonic crystals

Yuchen Peng, Exian Liu, Bei Yan, Jianlan Xie, Aoqian Shi, Peng Peng, Hang Li, and Jianjun Liu

DOI: 10.1364/OL.457058 Received 28 Feb 2022; Accepted 23 May 2022; Posted 23 May 2022  View: PDF

Abstract: Herein, the higher-order topological state (HOTS) and its mechanism in the two-dimensional Stampfli-Triangle (2D S-T) photonic crystals (PhCs) are explored. The topological corner states (TCSs) in the 2D S-T PhCs are based on two physical mechanisms: one is caused by the photonic quantum spin Hall effect and the other is caused by the topological interface state. While the former leads to the spin-direction locked effect which can change the distribution of the TCS, the latter is conducive to the emergence of the multiband TCS in the same structure due to the characteristics of plentiful PBG and broadband in the 2D S-T PhCs. These findings allow one to gain new insight into the HOTS, having a guiding significance to the design of photonic microcavities, high-quality factor lasers and other related integrated multiband photonic devices.

High-speed and high-power germanium photodetector based on trapezoidal absorber

Yupeng Zhu, zhi Liu, Chaoqun Niu, Yaqing Pang, Diandian Zhang, Xiangquan Liu, Jun Zheng, Yuhua Zuo, HAIYUN XUE, and Buwen Cheng

DOI: 10.1364/OL.461673 Received 25 Apr 2022; Accepted 23 May 2022; Posted 23 May 2022  View: PDF

Abstract: A compact high-power germanium photodetector (Ge PD) is experimentally demonstrated by re-engineering light distribution in the absorber. Compared with a conventional Ge PD, the proposed structure shows a DC saturation photocurrent improved by 28.9% and 3-dB bandwidth as high as 49.5 GHz. Under the same photocurrent of 10.5 mA, the proposed Ge PD shows a 3-dB bandwidth of 11.1 GHz, which is almost double the conventional Ge PD (5.6 GHz). 25 Gb/s eye-diagram measurement verifies the improved power handling capability. The compact size and manufacturing simplicity of this structure will enable the new applications for integrated silicon photonics.

Diode-pumped single-frequency Ti:sapphire laser

Alexander Lagatsky, Gerald Bonner, Peter Schlosser, David Stothard, and Loyd McKnight

DOI: 10.1364/OL.460367 Received 08 Apr 2022; Accepted 20 May 2022; Posted 20 May 2022  View: PDF

Abstract: We report on the first demonstration of a single-frequency Ti:sapphire laser under direct diode pumping. Single-longitudinal-mode operation is realized from a compact plane-parallel resonator using a volume Bragg grating as an output coupler. InGaN laser diodes operating at around 470 nm and 490 nm with a combined power of 6.7 W are used as an optical pump. A maximum output power of 700 mW is generated during single-frequency operation at 813.4 nm. A laser linewidth of 2.4 MHz is measured during free-running operation, and it was reduced to about 170 kHz when the laser is locked to an external reference cavity.

Synergistic Effect of Indium Doping and Chlorine Surface Passivation on CsPbI3 perovskite quantum dots for Deep-Red Light-Emitting Devices

Xin Zhou, Xuan Tong, Xiaolong Li, Yonggang Min, and Yannan Qian

DOI: 10.1364/OL.458017 Received 16 Mar 2022; Accepted 20 May 2022; Posted 20 May 2022  View: PDF

Abstract: Deep-red CsPbI3 perovskite quantum dots (PeQDs) are essential for high efficiency perovskite light-emitting diodes (PeLEDs) because of their high color purity and high photoluminescence quantum yield (PLQY). The synergetic strategy of indium doping and chlorine surface passivation not only partial replaced Pb2+ ions by the smaller ionic In3+ but also filled I- vacancies by Cl- on the surface, maintaining the humidity stability more than 24 days and yielding excellent PLQY. Benefiting from this synergetic strategy, deep-red (approximately at 683 nm) CsPbI3 PeLEDs showed a maximum luminance and EQE of 311 cd m-2 and 8.32%, respectively.

Non-Hermitian topological mobility edges and transport in photonic quantum walks

Stefano Longhi

DOI: 10.1364/OL.460484 Received 06 Apr 2022; Accepted 20 May 2022; Posted 20 May 2022  View: PDF

Abstract: In non-Hermitian quasicrystals, mobility edges (ME) separating localized and extended wave functions in complex energy plane can arise as a result of non-Hermitian terms in the Hamiltonian. Such ME are of topological nature, in the sense that the energies of localizedand extended states exhibit distinct topological structures in the complex energy plane. However, depending on the origin of non-Hermiticity, i.e. asymmetry of hopping amplitudes or complexification of the incommensurate potential phase, different winding numbers are introduced, corresponding to different transport features in the lattice: while ballistic transport is allowed in the former case, pseudo dynamical localization is observed in the latter case. The results are illustrated by considering non-Hermitian photonic quantum walks in synthetic mesh lattices.

Non-critical phase-matching fourth- and fifth-harmonic generation of 1077 nm laser using KDP-family crystals

Zijian Cui, Lu Han, chao wang, Mingying Sun, Dean Liu, and Jianqiang Zhu

DOI: 10.1364/OL.458952 Received 22 Mar 2022; Accepted 19 May 2022; Posted 19 May 2022  View: PDF

Abstract: We systematically demonstrated the angular and temperature characteristics of non-critical phase-matching (NCPM) fourth- and fifth-harmonic generation (FHG and FiHG) of 1077 nm laser in NH4H2PO4 (ADP), KH2PO4 (KDP), and KD2PO4 (DKDP) crystals. In this work, a new frequency laser with a wavelength of 1077 nm was generated by optical parametric amplification, in which the pump light (526.3 nm) was the frequency doubling of Nd:YLF laser (1052.7 nm), and the signal light was the Yb:YAG laser (1029.5 nm). Subsequently, 1077 nm laser was used as the fundamental wave for the FHG and FiHG to obtain deep-ultraviolet laser. For ADP and DKDP crystals, NCPM FHG of 1077 nm laser was realized at 74.0◦C and 132.5◦C, respectively, and large angular acceptances of 59.8 and 61.6 mrad were measured. For the FiHG, NCPM was realized in KDP crystal at 48.5◦C with an angular acceptance of 56.4 mrad. The results pave the way for the high-energy and power deep-ultraviolet laser generation using KDP-family crystals under non-cryogenic condition.

Beamshaping for InfraRed hyperspectral imaging: asequential optimisation for infrared source coupling

Mohammad Azizian Kalkhoran, Gianfelice Cinque, and A. Douglas Winter

DOI: 10.1364/OL.456049 Received 14 Feb 2022; Accepted 19 May 2022; Posted 20 May 2022  View: PDF

Abstract: Full field IR microspectroscopy via FPA detector provides chemical sensitivity and rapid spectroscopy imaging on large field of view. High signal to noise ratio per pixel has been experimentally proven by Synchrotron IR imaging, but at the cost of complex and highly customized beamline optical schemes. Here an unique implementation of adaptive optics to IR is presented as efficient solution for SR source when coupling an FPA detector with Schwarzschild objectives in a microscope. The major contributions are: i) a novel design of dual adaptive optics for IR beam shaping into a microscope; ii) benchmarking the Adam algorithm in optimisation with sound performance by several metrics; iii) exploring optimum metrics apt to sequentially achieve beam shaping and homogenous light intensity distribution. A diagnostic system is validated by optically conjugating an IR microbolometer camera to the entrance pupil of the microscope or the exit of the second deformable mirror. Further monitoring the beam at focal spot of a schwarzschild-type condenser from a conventional IR source, we demonstrate 150% gain in the coupling efficiency and 100% improvement in spatial resolution, confirming that efficient beam transport and use of the double adaptive optics in view of applications

Low-loss and alignment-tolerant optical coupling with tapered self-written waveguide between silicon photonics chip and standard single-mode fiber

Yohei Saito, Kota Shikama, Tai Tsuchizawa, and Norio Sato

DOI: 10.1364/OL.456396 Received 07 Mar 2022; Accepted 19 May 2022; Posted 20 May 2022  View: PDF

Abstract: An optical coupling method with high alignment tolerance by self-written waveguide (SWW) formation is a promising candidate for co-packaged optics by silicon photonics (SiPh). However, conventional SWWs cannot be used with Si waveguides because visible light for SWW formation cannot radiate from the waveguide facet. Here, we devised a new optical circuit with SiOxNy waveguides for SWW formation from a SiPh chip. With our circuit, we achieved optical coupling between a SiPh chip and an SSMF with a tapered SWW. The lowest excess coupling loss compared to butt coupling with a high-numerical aperture (NA) fiber is approximately 0.6 dB over the C-band with the tapered SWW. In addition, our coupling method has higher alignment tolerances than butt coupling with a high-NA fiber.


Mustafa Ordu and Ahmet Akosman

DOI: 10.1364/OL.455118 Received 31 Jan 2022; Accepted 19 May 2022; Posted 20 May 2022  View: PDF

Abstract: Nested negative curvature hollow-core fibers mark the state-of-art optical guidance in the near infrared region. In this letter, we propose a unique design approach for these type of fibers in order to further improve optical transmission via the optimal positioning of the nested elements. The nested elements in the proposed design are located at the center of the cladding tubes, and supported with bar-type structures. The topological optimization for the nested elements results in improved light guidance by two orders of magnitude, as low as numerical confinement losses of 0.003 dB/km, within the targeted wavelength range of 1450 nm to 1600 nm. This bar-supported design features strong single-mode operation and low bending sensitivity in a wide range of bending radii.

Simple continuous-variable quantum key distribution scheme using a Sagnac-based Gaussian modulator

Huanxi Zhao, Huasheng Li, Yuehan Xu, Peng Huang, Tao Wang, and Guihua Zeng

DOI: 10.1364/OL.458443 Received 18 Mar 2022; Accepted 18 May 2022; Posted 19 May 2022  View: PDF

Abstract: Continuous-variable quantum key distribution (CV-QKD) is a protocol that uses quantum mechanics to ensure the distribution of the encryption key is secure even in the presence of eavesdroppers. The wide application of CV-QKD requires low cost, system simplicity, and system stability. However, due to the particularity of Gaussian modulation in CV-QKD, an amplitude modulator (AM) and a bias controller are required, making the system structure complex and unstable. In this letter, we achieve two-dimensional Gaussian modulation with only one phase modulator (PM) and Sagnac ring structure, which significantly reduces the complexity of the system. We test the Gaussian modulation stability for 10 hours, and the result shows that the expected secure key rate can be maintained at 80 kbps under the transmission distance of 50 km. This scheme opens up new possibilities for a new generation of highly stable and simple CV-QKD systems.

A Quantitative X-ray Fluorescence Imaging System for Non-destructive 3D Tumor Histology

shaozhou pu, yunwen huang, ning zhao, hui liu, yang zuo, hangcheng zhou, lei zhu, and Yidong Yang

DOI: 10.1364/OL.458905 Received 21 Mar 2022; Accepted 18 May 2022; Posted 20 May 2022  View: PDF

Abstract: An in-house dual-modality X-ray fluorescence tomography (XFT) and X-ray computed tomography (XCT) system was developed to quantify iodine contrast distribution through the whole tumor volume ex vivo. The quantitative XFT was calibrated with water phantoms containing iodine solutions of various concentrations (0.0175–1.4 wt.%). The vasculature distribution was reflected by the iodine perfusion, which was validated with histology. This technique may open a new way for non-destructive three-dimensional imaging based histological analysis of tumor samples.

Optical control of light polarization in heliconical cholesteric liquid crystals

Giovanni Nava, Raouf Barboza, Francesco Simoni, Olena Iadlovska, Oleg Lavrentovich, and Liana Lucchetti

DOI: 10.1364/OL.454431 Received 01 Feb 2022; Accepted 17 May 2022; Posted 19 May 2022  View: PDF

Abstract: We show here that light polarization of a beam propagating through a heliconical cholesteric cell can be controlled by tuning the Bragg resonance of the structure. We demonstrate that this control is achieved by varying either the low frequency electric field or the intensity of a pump beam impinging on the sample. The study confirms the recently reported phenomenon of optical tuning of the heliconical cholesterics and opens the door for the development of simple and efficient polarization modulators controlled electrically or optically.

Extended L-band Few-Mode Er/Yb Co-doped Fiber Amplifier with a Cladding-Pumped Pseudo-two-stage Configuration

Qiang Qiu, Le He, Zhimu Gu, yang chen, Yang lou, Xinyue Zhao, Jinggang Peng, Haiqing Li, yingbin Xing, Yingbo Chu, Nengli Dai, and Jinyan Li

DOI: 10.1364/OL.457955 Received 08 Mar 2022; Accepted 17 May 2022; Posted 17 May 2022  View: PDF

Abstract: Spatial division multiplexing (SDM) is one of the important technologies to solve the future capacity crisis. However, to date, SDM optical amplification is still a challenge for its application. Herein, we numerically and experimentally demonstrated a few-mode Er/Yb co-doped fiber amplifier (FM-EYDFA) for extended L-band operation. A double cladding Er/Yb co-doped fiber was fabricated to expand the L-band bandwidth and a novel cladding-pumped pseudo-two-stage amplification configuration was proposed to enhance the L-band gain. With an initial signal power of -16.8 dBm and an injected pump power of 8.8 W at 940 nm, the 20-dB gain range was covered to 1620 nm for two-mode groups of LP01 and LP11. Importantly, the average gain of 25 dB and average differential modal gain (DMG) of <1 dB were obtained in the wavelength range of 1570 nm-1620 nm for all modes. Our results suggest that the cladding-pumped pseudo-two-stage amplifier based on Er/Yb co-doped fiber providing low DMG, and broad bandwidth has a great potential for increasing the future SDM capacity.

Monolithic coherent LABS Lidar based on integrated transceiver array

Chao Li, Kan Wu, Xianyi Cao, Guangjin Zhang, Tianyi Li, Zheng Deng, Mingchao Chang, Yanan Wang, Xinwan Li, and Jianping Chen

DOI: 10.1364/OL.453201 Received 07 Jan 2022; Accepted 16 May 2022; Posted 16 May 2022  View: PDF

Abstract: We demonstrate a monolithic frequency-modulated continuous-wave (FMCW) Lidar chip with integrated transceiver array based on lens-assisted beam steering (LABS) technology. It enables beam emitting, steering, receiving and coherent detecting on a single chip with simultaneous distance and velocity detection. Integrated transceiver is designed with a composite structure of a Bragg grating in the middle and a U-shaped photodetector (PD) surrounding it. For proof-of-concept demonstration, a chip with 2×2 switchable transceiver array was fabricated. A monolithic coherent LABS Lidar system with a scanning angle of 2.86 °, and a scanning speed of 5.3 μs is implemented for 5-m ranging and 0.45-m/s velocity detection.

Realization of a low-crosstalk half-wavelength pitch waveguide array on SOI platform

Cheng Chen, Zhao Xianmeng, shengjie tang, Xiaoping Liu, and Haibin Lü

DOI: 10.1364/OL.455977 Received 10 Feb 2022; Accepted 16 May 2022; Posted 17 May 2022  View: PDF

Abstract: Realizing a densely packed waveguide antenna array is of great importance in light detection and ranging (LiDAR) due to its suppressed grating lobes. In this work, a low crosstalk half-wavelength pitch silicon waveguide array is proposed and experimentally demonstrated. It has a periodic arrangement of silicon strip nanophotonic waveguides, between which deep-subwavelength silicon strips are placed. Our experimental results show this array’s crosstalk suppression is nearly 20 dB and has a broad bandwidth covering a wavelength range from 1500 nm to 1560 nm. Our realization of a half-wavelength pitch waveguide array may offer a promising platform for studying integrated optical phased arrays for solid-state LiDAR with a very low grating lobe and thus potentially a large field of view.

All-pass phase shifting enabled by symmetric topological unidirectional guided resonances

zixuan zhang, Feifan Wang, Haoran Wang, Yuefeng Hu, Xuefan Yin, Weiwei Hu, and Chao Peng

DOI: 10.1364/OL.460435 Received 07 Apr 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: All-pass phase shifting (APS), which is a wave propagating in a constant, unitary amplitude but with pure phase variation, is extremely desired in many optoelectronic applications. In this work, we propose a method of realizing APS from out-of-plane exciting a topological enabled unidirectional guided resonance (UGR), that resides in a photonic crystal slab with P or C2z symmetries. Briefly, the symmetries and unidirectional features deduce the number of ports to one that simultaneously adds or drops energy. As a result, the phase independently shifts during frequency varying but the amplitude remains as unitary under plane-wave excitation. Theory and simulations confirmed our findings. A paradox that the background contribution deviated from Fabry-Perot resonance was clarified from a multi-resonances picture.

C-band 67 GHz silicon photonic microring modulator for dispersion uncompensated 100 Gbaud PAM-4

David Chan, Xiong Wu, Zunyue Zhang, Chao Lu, Alan Pak Tao Lau, and Hon Tsang

DOI: 10.1364/OL.460602 Received 07 Apr 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: A very high bandwidth integrated silicon microring modulator (Si-MRM) designed on a commercial silicon photonic (SiP) platform for C-band operation is presented. The MRM has a 3-dB electro-optic (EO) bandwidth of over 67 GHz and features a small footprint of 24 µm x 70 µm. Using the MRM, we demonstrate intensity modulation-direct detection (IM-DD) transmission with PAM-4 signaling of over 100 Gbaud. By utilizing the optical peaking effect and negative chirp in the MRM, we extend the transmission distance which is limited by the fiber dispersion induced frequency fading. Using standard single mode fiber (SSMF) for transmission up to distances of 2 km, we measured data transmission of 100 Gbaud PAM-4 signals with bit error rate (BER) under the general 7% hard-decision forward-error correction (HD-FEC) threshold. The MRM enables an extended transmission distance for 100 Gbaud signaling in C-band without dispersion compensation.

A normal form for frequency combs and localized states in time-delayed Kerr-Gires-Tournois interferometers

Julien Javaloyes, Svetlana Gurevich, and Thomas Seidel

DOI: 10.1364/OL.457777 Received 08 Mar 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: We elucidate the mechanisms that underly the formation of temporal localized states and frequency combs in vertical external-cavity Kerr-Gires-Tournois interferometers. We reduce our first principle model based upon delay algebraic equations to a minimal pattern formation scenario. It consists in a real cubic Ginzburg-Landau equation modified by high-order effects such as third order dispersion and nonlinear drift. The latter are responsible for generating localized states via the locking of domain walls connecting the high and low intensity levels of the injected micro-cavity. We interpret the effective parameters of the normal form in relation with the configuration of the optical setup. Comparing the two models, we observe an excellent agreement close to the onset of bistability.

Spectral interference contrast based non-contact photoacoustic microscopy realized by SDOCT

Zhenhe Ma, Ning Ding, Zhen Li, Keliang Zhu, Ang Li, Zhanhong Lin, Yi Wang, Yuqian Zhao, Yao Yu, Jingmin Luan, Xin Zhu, and JIAN LIU

DOI: 10.1364/OL.458383 Received 15 Mar 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: We introduce a method to extract photoacoustic (PA) signal from contrast reduction of interference spectrum acquired by spectral domain optical coherence tomography (SDOCT). This all-optical detection achieved in a noncontact manner directly on the water surface covered on the sample by using its specular reflection. During SDOCT exposure, the phase of interference spectrum keeps shaking according to the water surface vibration induced by PA excitation. This results in interference contrast reduction which is quantified by FFT for PA imaging. Tungsten filament, asparagus fern leaf and mouse auricle were imaged to demonstrate the method.

Sign singularity solution in single-frame Speckle Interferometry with Vortex phase-assisted

Hong Zhao, Yijun Du, and Zixin Zhao

DOI: 10.1364/OL.458593 Received 16 Mar 2022; Accepted 15 May 2022; Posted 16 May 2022  View: PDF

Abstract: We propose a sign singularity solution in single-shot digital speckle interferometry based on vortex phase modulation. The vortex phase is introduced through the reference beam as variation information by a reflective liquid crystal spatial light modulator. The sign singularity is eliminated with the help of a single spiral speckle fringe pattern. To effectively obtain the deformation phase, the measurement process is specially designed on the consideration of the modulation principle of vortex phase, which has been discussed in terms of theoretical analysis and simulation results. Experiments on out-of-plane deformation measurement reveal the feasibility of the proposed method, and suggest the potential in actual dynamic measurement applications.

Low-cost micro-spectrometer based on nano-imprint and spectral-feature reconstruction algorithm

Qingquan Liu, Zhiyi Xuan, Zi Wang, Xinchao Zhao, Zhiqin Yin, Chenlu Li, gang chen, Shao-Wei Wang, and Wei Lu

DOI: 10.1364/OL.458469 Received 17 Mar 2022; Accepted 14 May 2022; Posted 16 May 2022  View: PDF

Abstract: Reconstructive micro-spectrometers have shown great potential in many fields such as medicine, agriculture and astronomy. However, the performance of these spectrometers is seriously limited by the spectral varieties of response pixels and anti-noise ability of reconstruction algorithms. In this work, we proposed a spectral-feature reconstruction (SR) algorithm, whose anti-noise ability is at least 4 times better than the current algorithms. A micro-spectrometer is realized by fabricating a large amount of Fabry-Perot (FP) micro-filters onto a cheap Complementary Metal Oxide Semiconductor (CMOS) chip for demonstration by using a very high-efficiency technology of nano-imprint. Nano-imprint technology can complete hundreds of spectral pixels with rich spectral features at one time and low cost. In cooperation with the SR algorithm, such a micro-spectrometer can have spectral resolution as high as 3 nm while with much lower angular sensitivity than photonic crystal-based micro-spectrometer. It can obtain the target's spectrum by only a single shot, which has wide applications in spectral analysis etc.

Wavelength-modulation dispersion spectroscopy with heterodyne phase-sensitive detection

Mengyuan HU and Wei Ren

DOI: 10.1364/OL.460042 Received 08 Apr 2022; Accepted 14 May 2022; Posted 16 May 2022  View: PDF

Abstract: Heterodyne phase-sensitive dispersion spectroscopy (HPSDS) provides an agile method for gas detection by measuring the phase of an amplitude modulation signal. However, the previous HPSDS gas sensors show limited sensitivity. In this work, we report a new dispersion spectroscopic technique, named wavelength-modulation heterodyne phase-sensitive dispersion spectroscopy (WM-HPSDS), to improve the detection sensitivity. As a proof-of-principle demonstration, a quantum cascade laser (QCL) at 5.26 µm is used to exploit the absorption line of nitric oxide (NO) in a 35 cm long hollow-core fiber. Besides modulating the injection current of the QCL at 1 GHz to generate the three-tone beam, a 10-kHz sinusoidal waveform is superimposed on the laser current to produce an additional wavelength modulation. We achieve a noise equivalent concentration of 40 ppb NO using WM-HPSDS at an integration time of 90 s, corresponding to a noise equivalent absorption (NEA) coefficient of 6.9×10-7 cm-1. Compared to the conventional HPSDS technique, the developed WM-HPSDS improves the sensitivity by a factor of 8.3.

Smart Vector-inspired Optical Vision Guiding (VIOVG) method for Autonomous Underwater Vehicle Docking and Formation

Yue Zhang, Xinwei Wang, PINGSHUN LEI, Shaomeng Wang, Yuqing Yang, Liang Sun, and zhou yan

DOI: 10.1364/OL.456544 Received 21 Feb 2022; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: A smart vector-inspired optical vision guiding (VIOVG) method for autonomous underwater vehicle (AUV) docking and formation is proposed. Unlike traditional optical guiding methods based on LED arrays, the method is inspired by vector, and uses four laser diodes to form a wing-light pattern to realize smart optical guiding for AUVs. Due to the light scattering effect from water, the four laser diodes can emit four bright and slim laser beams in water, which can be grasped by underwater camera. The intersections of the laser beams form wing-lights as markers for AUV distance and pose estimation. The wing-light pattern is easily tuned and extended by changing the propagation direction and the power of laser beams beyond the limitation of the physical size of AUV or docking station. The simulations and experiments show that the proposed method can achieve high precision positioning. Relative distance error of 3.35% is achieved in a positioning experiment at ~10m. It has great potential for AUV docking and formation, especially for small AUVs.

Universal Dimension Reduced Phase Compensation Algorithm for Optical Phased Array

Yanwei Huang, Kangzhe Wang, Qianying Yan, Yifan Wang, Xiangru Wang, Haipeng Liu, Jijun Feng, Fuling Zhang, and Zhihua Feng

DOI: 10.1364/OL.460324 Received 05 Apr 2022; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: Optical phased array (OPA) can achieve non-mechanical beam deflection. Many kinds of OPAs face the problem of low deflection efficiency, due to the phase distortion caused by mutual coupling between adjacent elements. In this letter, a universal optimization algorithm is proposed to compensate the OPA’s structural phase distortion, while the adjacent sampling principal component analysis (AS-PCA) method is introduced to reduce the dimension of solution space. Simulation and experiment results on different classes of OPAs both verified that, this algorithm can considerably optimize the deflection beam with a rapid convergence speed, irrespective of the number of OPA’s elements, and maintain the universal feature, laying the foundation for large-scale, high-density OPA in-line optimization. We envision it to become a general method for different kinds of OPAs.

Performance analysis of serial relay orbital satellite optical communication over turbulent channels

ALI REFAAI, FATMA NEWAGY, Mohamed Fathy Abo Sree, Hadia Elhennawy, Mostafa Hussein Aly, and Mohamed Abaza

DOI: 10.1364/OL.461652 Received 25 Apr 2022; Accepted 13 May 2022; Posted 16 May 2022  View: PDF

Abstract: Optical space communication has been proven to be areliable relay satellite transmission system. The difficulty that occurs in RF satellite communication (SatCom)can be alleviated by using Free-Space Optical (FSO) orlaser SatCom. In this work, we analyse a proposed laserdownlink relay SatCom model with existing channelturbulence employing intensity modulation direct detection (IM/DD), amplification, and forwarding (AF) technology compared to that of the optical direct link SatCom. Accounting for atmospheric attenuations and turbulence, the effect of model parameters such as zenithangle, receiver aperture radius, best number of opticalground stations (OGSs), and end-to-end operating wavelength on system performance has been investigated fordifferent OGS height scenarios. We provide exact closedform expressions for the proposed model and optimisesystem performance by selecting the best OGS using aselective diversity technique that can boost system SNRby up to 20 dB (25 %).

Boundary configured chiral edge states in valley topological photonic crystal

Guochao Wei, Zhenzhen Liu, Huizhou Wu, Licheng Wang, and Jun Jun Xiao

DOI: 10.1364/OL.462005 Received 25 Apr 2022; Accepted 12 May 2022; Posted 16 May 2022  View: PDF

Abstract: Topological edge states in valley photonic crystals (VPCs) are of two types, valley edge state (VES) and chiral edge state (CES). The former is widely studied at the interface between structures with distinct valley Chern number, while the latter exists at the external boundary (usually in contact with air) of VPC. In contrast to VES, the dispersion of CES is highly correlated to the on-site boundary potentials of VPC, making it more controllable and tunable. In this work, we show that by configuring the external boundary structure of VPC, CESs for the same valley can propagate towards the same direction at the two parallel external boundaries, termed anti-helical chiral edge states. Furthermore, we design a meta-structure with four types of air-contacting external boundaries and a VES enabled internal interface channel. The CES, while excited at a given port, can be exclusively coupled to the other three ports, depending on the operating frequency. Our work provides an alternative way in designing compact topological devices for optical waveguide and wave splitter.

Inverse design of ultracompact multi-focal optical devices by diffractive neural networks

Yuyao Chen, Yilin Zhu, Wesley Britton, and Luca Dal Negro

DOI: 10.1364/OL.460186 Received 07 Apr 2022; Accepted 12 May 2022; Posted 12 May 2022  View: PDF

Abstract: We propose an efficient inverse design approach for multifunctional optical elements based on adaptive deep diffractive neural networks (a-D²NNs). Specifically, we introduce a-D²NNs and design two-layer diffractive devices that can selectively focus incident radiation over two well-separated spectral bands at desired distances. We investigate focusing efficiencies at two wavelengths and achieve targeted spectral lineshapes and spatial point-spread functions (PSFs) with optimal focusing efficiency. In particular, we demonstrate control of the spectral bandwidths at separate focal positions beyond the theoretical limit of single-lens devices with the same aperture size. Finally, we demonstrate devices that produce super-oscillatory focal spots at desired wavelengths. The proposed method is compatible with current diffractive optics and doublet metasurface technology for ultracompact multispectral imaging and lensless microscopy applications.

Compact generation of robust Airy beam pattern with spatial coherence engineering

Yahong Chen, Yimeng Zhu, Zhen Dong, Fei Wang, and Yangjian Cai

DOI: 10.1364/OL.460191 Received 05 Apr 2022; Accepted 12 May 2022; Posted 12 May 2022  View: PDF

Abstract: We present a class of partially coherent light source having Airy-type amplitude and Airy-correlated spatial coherence. We show that the light beam generated by such source can preserve the Airy pattern well during its propagation from source to far field. We demonstrate the robust feature of the Airy pattern by introducing a hard aperture to largely block the beam source. We find the coherence-induced Airy pattern can still be well reconstructed during propagation. We successfully synthesize such partially coherent source with complex random modes decomposition principle by using a single phase-only spatial light modulator. The proposed robust Airy pattern may find applications in information transmission through complex media.

Coherence–Orbital Angular Momentum matrix of Schell-model sources

Fei Wang, haohui yang, Yahong Chen, Olga Korotkova, and Yangjian Cai

DOI: 10.1364/OL.460911 Received 11 Apr 2022; Accepted 12 May 2022; Posted 12 May 2022  View: PDF

Abstract: Coherence – Orbital Angular Momentum (COAM) matrix characterizes the second-order field correlations in stationary sources or fields, at a pair of spiral modes with the same or different topological charges, say l and m, and at a pair of radial positions. In this Letter we reveal the general properties of the COAM matrix for the broad class of the Schell-model sources with circularly symmetric spectral densities. Our results imply that the structure of the COAM matrix is intimately related to the symmetries of the degree of coherence (DOC). In particular, the COAM matrix is diagonal only if the DOC is real-valued and rotationally symmetric; otherwise, it may acquire non-zero off-diagonal elements. In particular, if the real part of the DOC has Cartesian symmetry, the COAM matrix’ elements with the even/odd index difference |l-m| contain information about the real/imaginary part of the DOC. A potential application of our results is envisioned for extracting the rotation angle of the DOC of light (or an object transparency) through measuring of the off-axis COAM matrix elements.

OFDM-NOMA combined with LFM signal for W-band communication and radar detection simultaneously

Jing He and Rang Song

DOI: 10.1364/OL.460188 Received 05 Apr 2022; Accepted 11 May 2022; Posted 16 May 2022  View: PDF

Abstract: In this paper, an effective integrated waveform generation method that orthogonal frequency division multiplexing (OFDM)-based non-orthogonal multiple access (NOMA) communication signal combined with linear frequency modulation (LFM) radar signal is proposed and experimentally demonstrated in W-band communication and radar detection system. By using the proposed method, it can generate communication and radar signals simultaneously. And multiple users' communication and multi-target radar detection can be realized. In addition, for the OFDM-NOMA signal, discrete Fourier transform (DFT) precoding is used to keep the optimal power ratio consistent on different subcarriers, thus, it can improve the communication performance effectively. The experimental results show that the maximum distance estimation error is 1.2 cm for radar signals. Meanwhile, after transmission over 0.8m free space, the total aggregate rate for two users with the OFDM-NOMA signal is 3.125 Gb/s under the bit error rate of 8.5 × 10−4.

Index-leveling for forced-flow turbulent face-cooling of laser amplifiers

Denis Marion, Philippe Balcou, Christophe Feral, Antoine Rohm, and Jérôme Lhermite

DOI: 10.1364/OL.455616 Received 08 Feb 2022; Accepted 11 May 2022; Posted 11 May 2022  View: PDF

Abstract: Direct laser slab face-cooling by a fluid crossing the main and pump laser beams is an important method to reach high average laser powers. However, the flow regime is usually maintained at low Reynolds numbers, to prevent the onset of turbulence features in the flow, that would degrade wavefront quality. We show here how bringing the fluid temperature to the thermo-optical null point, close to the water/ice transition in the case of water, allows one to mitigate the optical consequences of hydrodynamic instabilities, by bleaching optically the temperature inhomogeneities within the flow. This optical process, dubbed index-leveling, opens the door to a highly efficient forced-flow, weakly turbulent face-cooling regime that should be instrumental to boost the kilowatt capabilities of next generation high-power lasers.

Terahertz tightly focused Bessel beam generation and point-to-point focusing based on nonlocal diffraction engineering

Jie -Rong Cheng, Yang Yang, Fei Fan, Xianghui Wang, and Shengjiang Chang

DOI: 10.1364/OL.460747 Received 12 Apr 2022; Accepted 10 May 2022; Posted 11 May 2022  View: PDF

Abstract: Metasurfaces transform the wavefront by spatially varying the amplitude or phase of the incoming beam. Instead of encoding such variation by subwavelength unit cells, it is achievable over diffraction engineering of supercell structures, which outperforms the unit-cell method when the spatial gradient is large. Besides of tight focusing, here we apply this method to achieve plane wave-to-Bessel beam transformation and point-to-point focusing at terahertz frequencies. The generated Bessel beam has both small beam waist (0.57λ) and long depth of focus (9.1λ) for subwavelength-resolution imaging over a long distance. The point-to-point focusing is successful due to the consideration of different incident angles among the supercells during the diffraction engineering, which changes the divergence angle from 16° to 70°. Both devices are validated by numerical simulations and experimental results with good agreement.

Photonic crystal resonators for inverse-designed multi-dimensional optical interconnects

Chinmay Shirpurkar, Jizhao Zang, Kiyoul Yang, David Carlson, Su-Peng Yu, Erwan Lucas, Srinivas Varma Pericherla, Joshua Yang, Melissa Guidry, Daniil Lukin, Lawrence Trask, Firooz Aflatouni, Jelena Vuckovic, Scott Papp, Peter Delfyett, Geun Ho Ahn, and Jesse Lu

DOI: 10.1364/OL.461272 Received 21 Apr 2022; Accepted 10 May 2022; Posted 11 May 2022  View: PDF

Abstract: We experimentally demonstrate a 400 Gbps optical communication link utilizing wavelength-division multiplexing and mode-division multiplexing for a total of 40 channels. This link utilizes a novel 400 GHz frequency comb source based on a chip-scale photonic crystal resonator. Silicon-on-insulator photonic inversedesigned 4×4 mode-division multiplexer structures enablea fourfold increase in data capacity. We show less than −10 dBm of optical receiver power for error-free data transmission using a PRBS31 pattern.

10 μJ few-cycle 12 μm source based on difference-frequency generation driven by a 1 kHz mid-wave infrared OPCPA

Martin Duda, Lorenz von Grafenstein, Martin Bock, Dennis Ueberschaer, Pia Fürtjes, Lukas Roskot, Martin Smrz, Ondrej Novak, and Uwe Griebner

DOI: 10.1364/OL.456971 Received 11 Mar 2022; Accepted 10 May 2022; Posted 11 May 2022  View: PDF

Abstract: We report on high-energy, few-cycle pulse generation in the long-wave infrared spectral region via difference frequency generation (DFG) in GaSe and AgGaSe2 nonlinear crystals. The DFG is driven by the signal at 3.5 μm and idler at 5 μm of a high-power mid-wave infrared optical parametric chirped pulse amplification system operating at a 1 kHz repetition rate. The DFG pulses contain up to 17 μJ energy and cover a spectrum from 8.5 to 14.5 μm. They are generated with a conversion efficiency of 2.1 %. Compression results in 10.2 μJ pulses with sub-150 fs duration, corresponding to less than 4 optical cycles. The achieved pulse energies are the highest reported for femtosecond DFG or optical parametric amplifier sources beyond 10 μm so far.

Better magneto-optical filters with cascaded vapor cells in the Faraday-Faraday and Faraday-Voigt geometries.

Fraser Logue, Jack Briscoe, Danielle Boddy, Ifan Hughes, and Steven Wrathmall

DOI: 10.1364/OL.459291 Received 28 Mar 2022; Accepted 10 May 2022; Posted 16 May 2022  View: PDF

Abstract: Single-cell magneto-optical Faraday filters find great utility and are realized with either ‘wing’ or ‘line center’ spectral profiles. We show that cascading a second cell with independent axial (Faraday) or transverse (Voigt) magnetic field leads to improved performance in terms of figure of merit (FOM) and spectral profile. The first cell optically rotates the plane of polarization of light creating the high transmission window; the second cell selectively absorbs the light eliminating unwanted transmission. Using naturally-abundant Rb vapor cells, we realize a Faraday-Faraday wing filter and the first recorded Faraday-Voigt line center filter which show excellent agreement with theory. The two filters have FOM values of 0.86 and 1.63~GHz$^{-1}$ respectively, the latter of which is the largest FOM atomic line filter recorded.

Quantum terahertz Cherenkov radiation: theory and experimental feasibility

Hesham Fares

DOI: 10.1364/OL.456465 Received 18 Feb 2022; Accepted 10 May 2022; Posted 16 May 2022  View: PDF

Abstract: A quantum approach is developed for describing the behavior of Cherenkov Radiation (CR). Then we propose a scheme of terahertz CR operating in a quantum regime at which the discreteness of momentum exchange in the free electrons-light interaction is significant. In this quantum regime, the spectrum of the emitted radiation appears as discrete spectral lines with a significantly narrow linewidth. The resonant wavelength, wavelength spacing, and linewidth of the emission lines can be tuned by adjusting the parameters of electron beam and waveguide. It is verified that the criteria and constraints of the requirements for essential experimental parameters are practically feasible. The theoretical analysis and main findings of this paper are applicable to various light sources based on the Cherenkov effect (e.g., Smith-Purcell radiation and surface plasmon polaritons). This work has implications for future studies of the quantum interaction of free electrons with light as well as visualization of quantum light applications in sensing, imaging, and spectroscopy.

Optical Kerr nonlinearity of dielectric nanohole array metasurface at anapole state

Andrey Panov

DOI: 10.1364/OL.459989 Received 07 Apr 2022; Accepted 10 May 2022; Posted 10 May 2022  View: PDF

Abstract: Nowadays, metasurfaces have attracted a great deal of attention from researchers due to prominent optical properties.In particular, the metasurfaces may consist of structures possessing optical anapole resonances with strong field confinement and substantially suppressed scattering. As a result, such nanostructures display enhanced nonlinear optical properties.In this paper by means of three-dimensional finite-difference time-domain simulations, the ability of anapole modes in high-index dielectric metasurfaces with circular nanopores is shown. In the vicinity of the anapole state, the effective optical Kerr nonlinearity increases by orders of magnitude. Simultaneously, the optical transmission of the metasurface can reach high values up to unity.

Generation of Accelerating Waves in Smith-Purcell Radiation

Liqiao Jing, dashuang liao, Jie Tao, Hongsheng Chen, and zuojia wang

DOI: 10.1364/OL.460106 Received 08 Apr 2022; Accepted 10 May 2022; Posted 10 May 2022  View: PDF

Abstract: Metasurface has recently emerged as a powerful platform to engineer wave packets of free electron radiation at the mesoscale. Here, we propose that accelerating waves can be generated when moving electrons interact with the bianisotropic metasurface. By changing the intrinsic coupling strength, full amplitude coverage and 0-to-π phase switching of Smith-Purcell radiation can be realized from the meta-atoms. This unusual property shifts the wave front of assembled Airy beam towards a parabolic trajectory. Experimental implementation displays that evanescent fields bounded at slotted waveguides can be coupled into accelerating waves via Smith-Purcell radiation from a designed bianisotropic metasurface. Our method and design strategy offer an alternative route towards free electron lasers with diffraction-free, self-accelerating, and self-healing beam properties.

Kerr-lens mode-locked Yb:(Y,Gd)AlO3 laser

Weidong Chen, Zhanglang Lin, Wenze Xue, huangjun zeng, Zhang Ge, Peixiong Zhang, Zhenqiang Chen, Zhen Li, Haifeng Lin, Pavel Loiko, Xavier Mateos, Valentin Petrov, and Li Wang

DOI: 10.1364/OL.460701 Received 25 Apr 2022; Accepted 10 May 2022; Posted 10 May 2022  View: PDF

Abstract: We report on a Kerr-lens mode-locked Yb:(Y,Gd)AlO3 laser delivering soliton pulses as short as 33 fs at 1057 nm with an average output power of 305 mW and a pulse repetition rate of ~83.2 MHz. Power scaling operation was also achieved with a maximum average output power of 2.07 W at the expense of longer pulse duration of 72 fs, which corresponds to an optical efficiency of 43.9% and a peak power of 302.8 kW. To the best of our knowledge, this is the first demonstration of Kerr-lens mode-locking in Yb3+-doped orthorhombic perovskite aluminate crystal.

Spectrum sampling optimization for quantitative phase imaging based on Kramers-Kronig relations

Yutong Li, Xiu Wen, Ming Sun, xuyang Zhou, Yu Ji, Guancheng Huang, Keya Zhou, Shutian Liu, and Zhengjun Liu

DOI: 10.1364/OL.460084 Received 05 Apr 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: Annular-illumination quantitative phase imaging based on space-domain Kramers-Kronig relations (AIKK) is a newly developed technique, in which the reconstruction is non-iterative and object-independent. Only capturing four low-resolution images, the AIKK system gains a resolution enhancement of nearly twofold. Under the extreme condition that the illumination wave vector matches the aperture of the pupil function precisely, we set a spectrum sampling criterion and establish a spectrum effective utilization model for the specific annular structure to search for the optimal solution of frequency domain distribution.On the basis of spectrum square structure, a diagonal expanded sampling based AIKK method (DES-AIKK) is presented to get rid of the pixel aliasing problem. It is worth noting that space–bandwidth–time product (SBP-T) further increased to 439.51 megapixels (1.8× of AIKK).Our work provides the guidelines and insights for designing the most suitable AIKK platform for high-throughput microscopic applications in pathology and real-time dynamic observation.

Multi-Timescale Infrared Quantum Cascade Laser Ellipsometry

Andreas Furchner, Christoph Kratz, Jörg Rappich, and Karsten Hinrichs

DOI: 10.1364/OL.457688 Received 11 Mar 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: We recently introduced a novel infrared (IR) laser ellipsometer for sub-decisecond spectroscopy [Opt. Lett. 44, 4387 (2019)] and 0.03 mm² spot-sized hyperspectral imaging [Opt. Lett. 44, 4893 (2019)]. Here we report on the next device generation for thin-film sensitive simultaneous single-shot amplitude and phase measurements. The multi-timescale ellipsometer achieves 10 µs time resolution and long-term stability over hours at high spectral resolution (0.2 cm⁻¹). We investigate the temporal stages (from min to ms) of fatty-acid thin-film formation upon solvent evaporation from acetone-diluted µl droplets. Optical thickness variations, structure modifications, and molecular interactions are probed during the liquid-to-solid phase transition. Multi-timescale ellipsometry could greatly impact fields like in situ biosensing, microfluidics and polymer analytics, but also operando applications in membrane research, catalysis, and studies of interface processes and surface reactions.

GaN-Based Green Resonant-Cavity Light-Emitting Diodes with Al mirror and copper plate

shuai yang, Xu Huan, Long Hao, Leiying Ying, Ronghuang Luo, Mengjie Zhong, Wenrui Lu, Xiang Hou, Yang Mei, and Baoping Zhang

DOI: 10.1364/OL.458088 Received 10 Mar 2022; Accepted 09 May 2022; Posted 10 May 2022  View: PDF

Abstract: In this work, the GaN-based green vertical resonant-cavity light-emitting diodes (RCLED) with a low-cost aluminum (Al) metal bottom mirror, a dielectric top mirror, as well as a copper (Cu) supporting plate were fabricated. The green emitting epitaxial wafer was grown on a patterned sapphire substrate (PSS) to ensure high crystal quality. Laser lift-off (LLO) of the PSS and electrical plating of a Cu supporting plate were then carried out to realize the vertical device structure. The emission wavelength and full width at half maximum (FWHM) of the main emission peak of the device are ~518 nm and 14 nm, respectively. Under the current density of 50 A/cm2, a relatively high light output power (LOP) of 11.1 mW can be obtained from the green vertical RCLED. Moreover, when the current injection is 20 mA (8 A/cm2), the corresponding forward bias voltage is as low as ~2.46 V. The reasons for the low operating voltage and high LOP can be attributed to the improvement of crystal quality, the release of residual compressive stress of GaN-based epilayer due to the removal of the PSS, and the better heat dissipation properties of Cu supporting plate.

Distributed fiber optic sensing with enhanced sensitivity based on microwave-photonic Vernier effect

Chen Zhu, Muhammad Roman, Yiyang Zhuang, and Jie Huang

DOI: 10.1364/OL.461307 Received 19 Apr 2022; Accepted 08 May 2022; Posted 10 May 2022  View: PDF

Abstract: The Vernier effect has been widely used in the field of measurement and instrumentation for sensitivity enhancement. Single-point optical fiber sensors based on the Vernier effect have been extensively reported in recent years. In this letter, for the first time, a distributed optical fiber sensor based on microwave photonics with improved sensitivity enabled by the Vernier effect is demonstrated. Distributed sensing is realized by interrogating a Fabry-Perot interferometer (FPI) array formed by cascaded reflectors along an optical fiber using an optical carrier-based microwave interferometry (OCMI) system. A reference FPI is also included in the system. The interferogram of each of the sensing FPIs can be unambiguously reconstructed and superimposed with the reconstructed interferogram of the reference FPI to generate the Vernier effect. By tracking the spectral shift of the envelope signals in the superimposed spectra, the measurement sensitivities of the sensing FPIs can be significantly improved. A simple direct modulation-based OCMI system is used in the proof-of-concept demonstration, showing sensitivity-enhanced distributed sensing capability. Moreover, the sensitivity amplification factor can be adjusted by varying the optical length difference of the sensing and reference FPIs, similar to that of Vernier effect-based single-point optical fiber sensors.

Optical isolation via direction-dependent soliton routing in birefringent soft matter

Gaetano Assanto, Enrique Calisto, and Noel Smyth

DOI: 10.1364/OL.459564 Received 29 Mar 2022; Accepted 07 May 2022; Posted 09 May 2022  View: PDF

Abstract: We introduce optical isolation based on reorientational solitary waves in non-uniformly oriented uniaxial soft matter, namely nematic liquid crystals. A longitudinally non-symmetric angular distribution of the optic axis provides the system with direction dependent routing, resulting in an all-optical diode owing to input side sensitive steering. Numerical experiments demonstrate the phenomenon and its effectiveness in realistic samples.

Internal reference method for the study of up-conversion luminescence of nanoparticle suspensions

Sergey Burikov, Anna Fedyanina, Kirill Laptinskiy, and Tatiana Dolenko

DOI: 10.1364/OL.456674 Received 23 Feb 2022; Accepted 07 May 2022; Posted 09 May 2022  View: PDF

Abstract: A new internal reference method has been developed for the study of up-conversion luminescence of nanoparticle suspensions, which provides correct analysis and comparison of luminescent signals obtained under different conditions. For excitation of the echo-signal of the samples, it is proposed to use the radiation of the optical parametric oscillator at two wavelengths: for simultaneous excitation of the up-conversion luminescence of particles and the Raman scattering signal of the medium in the Stokes region of the spectrum. Due to the linear dependence of the intensity of the Raman scattering of the medium on the excitation power, the normalization of the up-conversion luminescence signal of particles on the intensity of the Raman scattering of the medium makes it possible to eliminate the influence of the instability of the power of laser radiation, light scattering by the medium, inaccuracies of alignment, etc. on the luminescence signal.

Deep distributed optimization for blind diffuser-modulation ptychography

Xuyang Chang, Liheng Bian, Shaowei Jiang, Guoan Zheng, and Jun Zhang

DOI: 10.1364/OL.458434 Received 14 Mar 2022; Accepted 07 May 2022; Posted 10 May 2022  View: PDF

Abstract: Blind diffuser-modulation ptychography has emerged as a low-cost technique for micro-nano holographic imaging, which enables breaking the resolution limit of optical systems. However, the existing reconstruction method requires thousands of measurements to recover object and diffuser's profile simultaneously, which makes the data acquisition time-consuming and cumbersome. In this letter, we report a novel blind ptychography technique with deep distributed optimization, termed BPD²O. It decomposes the complicated optimization task into subproblems, then introduces ePIE and enhancing network solver to optimize each in a distributed strategy. In this way, BPD²O combines the advantages of both model-driven and data-driven strategies, realizing high-fidelity robust ptychography imaging. Extensive experiments validate that BPD²O can realize better resolution and reduce more than one order of magnitude in the number of measurements.

Predicting the dynamical behaviors for chaotic semiconductor lasers by reservoir computing

Xiao-Zhou Li, Bin Sheng, and Man Zhang

DOI: 10.1364/OL.459638 Received 06 Apr 2022; Accepted 06 May 2022; Posted 10 May 2022  View: PDF

Abstract: We demonstrate successful prediction of the continuous intensity time series and reproduction of the underlying dynamical behaviors for a chaotic semiconductor laser by reservoir computing. The laser subject to continuous-wave optical injection is considered using the rate-equation model. A reservoir network is constructed and trained using over 20000 data points sampled every 1.19 ps from the simulated chaotic intensity time series. Upon careful optimization of the reservoir parameters, the future evolution of the continuous intensity time series can be accurately predicted for a time duration of longer than 0.6 ns, which is 6 times the reciprocal of the relaxation resonance frequency of the laser. Moreover, we demonstrate for the first time that the predicted intensity time series allow for accurate reproduction of the chaotic dynamical behaviors, including the microwave power spectrum, probability density function, and the chaotic attractor. In general, the demonstrated approach offers a relatively high flexibility in the choice of reservoir parameters according to the simulation results, and it provides new insights into the learning and prediction of semiconductor laser dynamics based on measured intensity time series.

Chaotic optical communications of 12.5Gbaud OOK and 10Gbaud QPSK signals based on mutual injection of semiconductor lasers

Shunkai Xiang, Min Yang, and Jian Wang

DOI: 10.1364/OL.457625 Received 04 Mar 2022; Accepted 06 May 2022; Posted 09 May 2022  View: PDF

Abstract: Chaotic optical communications can provide a high level of security in data transmission. High-speed chaotic optical communications have little been undertaken so far limited by the bandwidth of chaotic signals and the difficulties of wideband chaos synchronization. Here, we experimentally demonstrate all-optical wideband chaos synchronization and communications based on mutual injection of semiconductor lasers. Both 12.5-Gbaud on-off keying (OOK) signals and 10-Gbaud quadrature phase shift keying (QPSK) signals are successfully encrypted and transmitted over 10-km and 2-km single-mode fiber (SMF) respectively.

Ultra-broadband wavelength-swept Ti:sapphire crystal fiber laser

Yu-Chan Lin, Teng I Yang, and Sheng-Lung Huang

DOI: 10.1364/OL.459072 Received 22 Mar 2022; Accepted 06 May 2022; Posted 09 May 2022  View: PDF

Abstract: Ultra-broadband wavelength-swept laser (WSL) was generated using glass-clad Ti:sapphire crystal fiber as the gain media. With the low signal propagation loss of the crystal fiber, the swept laser has a tuning bandwidth of 250 nm (i.e., 683 nm to 933 nm) at a repetition rate of 1200 Hz. The steady-state and pulsed dynamics of the WSL were analyzed. The 0.018-nm instantaneous linewidth corresponds to a 3-dB coherence roll-off of 7 mm. When using the laser for swept-source optical coherence tomography, an estimated axial resolution of 1.8 μm can be achieved.

Observation of topological Anderson phase in laser-written quasi-periodic waveguide arrays

Feng Chen, Weizhao Cheng, Weijie Liu, and Quancheng Liu

DOI: 10.1364/OL.461485 Received 20 Apr 2022; Accepted 06 May 2022; Posted 06 May 2022  View: PDF

Abstract: We report on the experimental observation of the topological Anderson phase in one-dimensional quasi-periodical waveguide arrays produced by femtosecond laser writing. The evanescently coupled waveguides are with alternating coupling constants, constructing photonic lattices analogous to the Su–Schrieffer–Heeger model. Dynamic tuning of interdimer hopping amplitudes of the waveguide array generates quasi-periodic disorders of coupling constants. As light propagates in the corresponding photonic waveguides, it exhibits different modes depending on the magnitude of the disorder. The topological Anderson phase is observed as the disorder is sufficiently strong, which corresponds to the zero-energy mode in its spectrum. The experimental results are consistent with the theoretical simulations, confirming the existence of the disorder-driven topological phase from a trivial band in the photonic lattice.

Spectrally separable photon-pair generation in dispersion engineered thin-film lithium niobate

CJ Xin, Jatadhari Mishra, Changchen Chen, Di Zhu, Amirhassan Shams-Ansari, Carsten Langrock, Neil Sinclair, Franco Wong, Martin Fejer, and Marko Loncar

DOI: 10.1364/OL.456873 Received 25 Feb 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: Existing nonlinear-optic implementations of pure, unfiltered heralded single-photon sources do not offer the scalability required for densely integrated quantum networks. Additionally, lithium niobate has hitherto been unsuitable for such use due to its material dispersion. We engineer the dispersion and quasi-phasematching conditions of a waveguide in the rapidly emerging thin-film lithium niobate platform to generated spectrally separable photons in the telecommunications band. Such photons pairs can be used as spectrally pure heralded single-photon sources in quantum networks. We estimate a heralded-state spectral purity of >94% based on joint spectral intensity measurements. Further, a joint spectral phase-sensitive measurement of the unheralded time-integrated second-order correlation function yields a heralded-state purity of (86 ± 5)%.

Weighted Sampling-adaptive Single-pixel Sensing

Xinrui Zhan, Liheng Bian, Chunli Zhu, and Jun Zhang

DOI: 10.1364/OL.458311 Received 11 Mar 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: The novel single-pixel sensing technique that uses an end-to-end neural network for joint optimization achieves high-level semantic sensing, which is effective but computation-consuming for varied sampling rates. In this letter, we report a weighted optimization technique for sampling-adaptive single-pixel sensing, which only needs to train the network for one time that is available for dynamic sampling rates. Specifically, we innovatively introduce a weighting scheme in the encoding process to characterize different patterns' modulation efficiencies, in which the modulation patterns and their corresponding weights are updated iteratively. The optimal pattern series with the highest weights are employed for light modulation in the experimental implementation, thus achieving highly-efficient sensing. Experiments validated that once the network is trained with a sampling rate of 1, the single-target classification accuracy reaches up to 95.00\% at a sampling rate of 0.03 on MNIST and 90.20\% at a sampling rate of 0.07 on CCPD for multi-target sensing.

Holographic point source for digital lensless holographic microscopy

MARIA LOPERA and Carlos Trujillo

DOI: 10.1364/OL.459146 Received 23 Mar 2022; Accepted 05 May 2022; Posted 06 May 2022  View: PDF

Abstract: A holographic point source (HPS) developed for digital lensless holographic microscopy (HPS-DLHM) is presented. The HPS is an off-axis phase transmission hologram of an experimental micrometer pinhole recorded on a photopolymer holographic film. An amplitude division interferometer, adjusted to operate at maximum diffraction efficiency, has been employed to record the hologram. The results of the HPS-DLHM have been contrasted to the results obtained via conventional DLHM, achieving similar measurements. Compared to conventional pinhole-based DLHM illumination, our cost-effective proposal provides increased mechanical stability, the possibility for wider spherical illumination cones, and shorter reconstruction distances. These superior features pave the way for applying this quantitative phase imaging (QPI) technique in biomedical and telemedicine applications. The imaging capabilities of our HPS-DLHM proposal have been demonstrated by using an intricate sample of a honey bee leg and a low-absorption sample of epithelial cheek cells.

Binary-Lens-Embedded Photonic Crystals

Chun Xia, EDGAR BUSTAMANTE, Stephen Kuebler, Raymond Rumpf, Jimmy touma, and Noel Martinez

DOI: 10.1364/OL.458854 Received 06 Apr 2022; Accepted 02 May 2022; Posted 17 May 2022  View: PDF

Abstract: A binary lens-embedded photonic crystal (B-LEPC) was designed for operation at 1550 nm and fabricated by multiphoton lithography. The lens is binary in the sense that the lens is formed within the lattice using just two distinct fill factors. The unit cells have a "rod-in-wall" structure that exhibits three-dimensional self-collimation. Simulations show that self-collimation forces light to move through the device without diffracting or focusing, even as the wavefront is reshaped by the lensing region. Upon exiting the device, the curved wavefront causes the light to focus. The thickness of a B-LEPC was reduced threefold by wrapping phase in the style of a Fresnel lens. Embedding a faster-varying phase profile enables tighter focusing and NA = 0.59 was demonstrated experimentally.

Spectral linewidth narrowing of broad-area blue diode bar in V-shape external Talbot cavity

Yehuda Braiman, Yehuda Nyaupane, and Patrick Likamwa

DOI: 10.1364/OL.456782 Received 23 Feb 2022; Accepted 02 May 2022; Posted 02 May 2022  View: PDF

Abstract: A 1-D linear array of high-power broad-area lasers diode (BALD) beams in blue spectral region (447 nm) were combined employing a V-shape external Talbot cavity in Littrow configuration. Surface grating provided optical feedback via self-imaged diffractive coupling to the diode bar and induced all the emitters to lase in a common central wavelength. The external cavity reduced the spectral linewidth of the free-running laser diode bar from several nm to (20-50) pm (FWHM) with the power level close to 13 W. The narrow spectrum of the external cavity locked laser can be tuned in the range of (3-4) nm by adjusting the tilt angle of the grating while the laser diode bar was operated in constant current mode at 20° C temperature.

Thickness-dependent Slow Light Gap Solitons in Three-Dimensional Coupled Photonic Crystal Waveguides

Christian Bohley, Vakhtang Jandieri, benjamin schwager, Ramaz Khomeriki, Dominik Schulz, Daniel Erni, Douglas Werner, and Jamal Berakdar

DOI: 10.1364/OL.457044 Received 25 Feb 2022; Accepted 02 May 2022; Posted 02 May 2022  View: PDF

Abstract: The thickness-dependent multimodal nature of three-dimensional (3D) coupled photonic crystal waveguides is investigated with the aim of realizing a medium for controlled optical gap soliton formation in the slow light regime. In the linear case, spectral properties of the modes (dispersion diagrams), location of the gap regions versus a thickness of the 3D photonic crystal and the near field distributions at the frequencies in the slow light region are analyzed using a full-wave electromagnetic solver. In the nonlinear regime (Kerr-type nonlinearity), we infer an existence of crystal-thickness-dependent temporal solitons with stable pulse envelope and use the solitonic pulses for driving quantum transitions in localized quantum systems within the photonic crystal waveguide. The results may be useful for applications in optical communications, multiplexing systems, nonlinear physics and ultrafast spectroscopy.

Underwater image restoration via Stokes decomposition

Xiaobo Li, Jianuo Xu, Liping Zhang, Haofeng Hu, and Shih-Chi CHEN

DOI: 10.1364/OL.457964 Received 09 Mar 2022; Accepted 30 Apr 2022; Posted 02 May 2022  View: PDF

Abstract: In this Letter, we present a Stokes imaging-based method to restore objects and enhance image contrast in turbid water. In the system, a light source illuminates the object with two orthometric polarization states; based on a new Stokes decomposition model, the recorded images are converted to Stokes maps and subsequently restored to a clear image, free of reflections and scattered lights. A mathematical model has been developed to explain the Stokes decomposition and how the undesired reflections and scattered lights are rejected. Imaging experiments have been devised and performed on different objects, e.g., metals and plastics, under different turbidities. The results demonstrate enhanced image quality and capability to distinguish polarization differences. The new method can be readily applied to practical underwater object detections and potentially realize clear visions in other scattering media.

Giant electro-optic coefficient in a graphene oxide film

Mohammadmahdi Jahanbakhshian, MOHAMAD ALI KIANI, Maqsoud Arshadi Pirlar, and Rouhollah Karimzadeh

DOI: 10.1364/OL.451503 Received 20 Dec 2021; Accepted 29 Apr 2022; Posted 02 May 2022  View: PDF

Abstract: The electro-optic effect is an important mechanism for actively tuning the refractive index of materials. This effect has various important applications in communication, switching, modulation, and nonlinear optics. This research measured the quadratic electro-optic coefficient for the graphene oxide (GO) film with ellipsometry spectroscopy. The results show that this coefficient is about three orders of magnitude greater than that of other materials. The GO film with its giant electro-optic Kerr coefficient can improve devices based on this effect. For example, it can decrease power consumption and the complexity of these devices due to the need for less electric field. In addition, birefringence is obtained about Δn=0.08 at 730 nm, which can have promising improvements in commercial devices, such as the reduction of working voltage to 10 volts.

2×2 kW near-single-mode bidirectional high power output from a single-cavity monolithic fiber laser

Peng Lin Zhong, Xiaolin Wang, Baolai Yang, Li Wang, hanwei zhang, Xiaoming Xi, and Wang Peng

DOI: 10.1364/OL.458581 Received 22 Mar 2022; Accepted 28 Apr 2022; Posted 02 May 2022  View: PDF

Abstract: Traditional monolithic fiber lasers can only achieve unidirectional high power laser output. In this letter, a novel high-power linear cavity fiber laser that can achieve bidirectional high power output is proposed and demonstrated. In an ordinary laser resonant cavity, we replace the high reflectivity fiber Bragg grating with a low reflectivity fiber Bragg grating to realize bidirectional laser output. In our experiment, the laser cavity was composed of two fiber Bragg gratings with reflectivity of about 10%. The pump power provided by 976 nm laser diodes was injected into double-cladding Yb-doped fiber with core/cladding diameter of 20/400 µm. At the maximum pump power, the bidirectional output power were 2025 W and 1948 W respectively, the output laser beam quality (M2 factor) at both ends was about 1.5. For the first time, the feasibility of bidirectional output laser to achieve double 2-kW level high power is verified. Compared with traditional unidirectional output laser, this type of bidirectional output laser can achieve double high power laser employing a laser resonant cavity. Thus the average cost and structure size can be further reduced in the massive production.

Secure OFDM transmission scheme based on chaotic encryption and noise-masking key distribution

Yibin Wan, Jianxin Ren, Bo Liu, Yaya Mao, shuaidong chen, xiangyu wu, Yongfeng Wu, Lilong Zhao, Tingting Sun, Rahat Ullah, and Ying Li

DOI: 10.1364/OL.460052 Received 01 Apr 2022; Accepted 16 Apr 2022; Posted 03 May 2022  View: PDF

Abstract: In this letter, we propose a secure orthogonal frequency division multiplexing (OFDM) transmission scheme based on chaotic encryption and noise-masking key distribution. With the implementation of a three-dimensional digital chaotic system, the security performance is effectively enhanced by scrambling the phase, symbol, and subcarrier frequency of the OFDM signal. The proposed noise-masking key distribution can mask the key information of the chaotic system into noise and transmit it with the chaotic encrypted signal simultaneously. By this mechanism, the legal receiver can realize uninterrupted authentication and decryption even if the key is constantly updated. Moreover, the proposed scheme has the advantage of low cost, compatible with current optical fiber networks, since the entire operation is implemented by digital signal processing. 62.2 Gb/s QPSK and 124.4 Gb/s 16QAM OFDM transmission over a 2-km 7-core fiber using the proposed scheme is experimentally demonstrated. The results show that the proposed scheme can realize security enhancement and cost-effective key distribution without significant BER performance degradation.

Tunable concentration-dependent upconversion and downconversion luminescence in NaYF4: Yb3+, Er3+@ NaYF4: Yb3+, Nd3+ core-shell nanocrystals for dual-mode anti-counterfeiting imaging application

Shaobo Cui, Li Tao, Wen Kiat Chan, Donglei Zhou, Zhongzheng Yu, and Wen Xu

DOI: 10.1364/OL.452089 Received 27 Dec 2021; Accepted 13 Apr 2022; Posted 06 May 2022  View: PDF

Abstract: Lanthanide-doped luminescent nanocrystals display both the upconversion and downconversion properties, which display the potential applications in near-infrared-Ⅱ (NIR-Ⅱ) images and biology sensors. Both upconversion and downconversion luminescence are sensitive to concentrations of activators. However, few works reveal the mechanism of the concentration-dependent upconversion and downconversion luminescence. Herein, we synthesis the core-shell upconversion nanocrystals (UCNCs) NaYF4: Yb3+(20%), Er3+ (2%)@NaYF4: Yb3+ (x%), Nd3+ (y%) with varied concentration of Nd and Yb ions. The upconversion and downconversion spectra are recorded under excitation of 980 and 808 nm lasers. The results indicate that the luminescence of core-shell UCNCs is influenced by the non-radiative rate between activators (Yb3+ and Nd3+) and the back energy transfer rate from Er3+ ions to activators. The upconversion luminescence tends to be obtained at a relatively low concentration of Yb3+ and Nd3+ ions (about 5%), whereas the NIR emisison tends to be obtained at a relatively high concentration of Yb3+ and Nd3+ ions (not higher than 20%). The dual-mode anti-counterfeiting imaging is successfully fabricated using the core-shell UCNCs, which can be detected and distinguished by the visible and infrared detectors. The visible vs infrared brightness of dual-mode anti-counterfeiting imaging can be tuned by varying the concentration of activators (Yb3+, Nd3+). Our work demonstrates the concentration-dependent upconversion and downconversion luminescence in the core-shell UCNCs, which provides reference to obtain the NIR emission at NIR-Ⅱ region and adds encrypted dimensions for the anti-counterfeiting pattern in file encryption field.