Volume: 1 Issue 1
7 Article(s)

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Deep learning spatial phase unwrapping: a comparative review | Article Video
Kaiqiang Wang, Qian Kemao, Jianglei Di, and Jianlin Zhao
Phase unwrapping is an indispensable step for many optical imaging and metrology techniques. The rapid development of deep learning has brought ideas to phase unwrapping. In the past four years, various phase dataset generation methods and deep-learning-involved spatial phase unwrapping methods have emerged quickly. However, these methods were proposed and analyzed individually, using different strategies, neural networks, and datasets, and applied to different scenarios. It is thus necessary to do a detailed comparison of these deep-learning-involved methods and the traditional methods in the same context. We first divide the phase dataset generation methods into random matrix enlargement, Gauss matrix superposition, and Zernike polynomials superposition, and then divide the deep-learning-involved phase unwrapping methods into deep-learning-performed regression, deep-learning-performed wrap count, and deep-learning-assisted denoising. For the phase dataset generation methods, the richness of the datasets and the generalization capabilities of the trained networks are compared in detail. In addition, the deep-learning-involved methods are analyzed and compared with the traditional methods in ideal, noisy, discontinuous, and aliasing cases. Finally, we give suggestions on the best methods for different situations and propose the potential development direction for the dataset generation method, neural network structure, generalization ability enhancement, and neural network training strategy for the deep-learning-involved spatial phase unwrapping methods.
Advanced Photonics Nexus
  • Publication Date: Aug. 03, 2022
  • Vol. 1, Issue 1, 014001 (2022)
Research Articles
Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate | Article Video
Xiaoyue Liu, Shengqian Gao, Chi Zhang, Ying Pan, Rui Ma, Xian Zhang, Lin Liu, Zhenda Xie, Shining Zhu, Siyuan Yu, and Xinlun Cai
Thin-film lithium niobate is a promising material platform for integrated nonlinear photonics, due to its high refractive index contrast with the excellent optical properties. However, the high refractive index contrast and correspondingly small mode field diameter limit the attainable coupling between the waveguide and fiber. In second harmonic generation processes, lack of efficient fiber-chip coupling schemes covering both the fundamental and second harmonic wavelengths has greatly limited the overall efficiency. We design and fabricate an ultra-broadband tri-layer edge coupler with a high coupling efficiency. The coupler allows efficient coupling of 1 dB / facet at 1550 nm and 3 dB / facet at 775 nm. This enables us to achieve an ultrahigh overall second harmonic generation normalized efficiency (fiber-to-fiber) of 1027 % W - 1 cm - 2 (on-chip second harmonic efficiency ∼3256 % W - 1 cm - 2) in a 5-mm-long periodically-poled lithium niobate waveguide, which is two to three orders of magnitude higher than that in state-of-the-art devices.
Advanced Photonics Nexus
  • Publication Date: Jun. 29, 2022
  • Vol. 1, Issue 1, 016001 (2022)
Deterministic generation of large-scale hyperentanglement in three degrees of freedom
Xutong Wang, Sheng Yu, Shengshuai Liu, Kai Zhang, Yanbo Lou, Wei Wang, and Jietai Jing
Entanglement serves as a fundamental resource for quantum information protocols, and hyperentanglement has received an increasing amount of attention for its high-capacity characteristic. Increasing the scale of hyperentanglement, i.e., the number of modes in a hyperentangled system, is crucial for enhancing its capability in quantum information processing. Here, we demonstrate the generation of large-scale continuous-variable (CV) hyperentanglement in three degrees of freedom (DOFs), including azimuthal and radial indices of Laguerre–Gaussian (LG) modes and frequency. In our experiment, 216 pairs of hyperentangled modes are deterministically generated from the four-wave mixing process in an atomic vapor. In addition, we show that the entanglement between coherent LG superposition modes denoted by both azimuthal and radial quantum numbers can also be generated from this system. Such large-scale CV hyperentanglement in three DOFs presents an efficient scheme to significantly increase the information capacity of the CV system. Our results provide a new platform for studying CV quantum information and open the avenue for constructing high-capacity parallel and multiple-DOF CV quantum information protocols.
Advanced Photonics Nexus
  • Publication Date: Jun. 29, 2022
  • Vol. 1, Issue 1, 016002 (2022)
Orbital angular momentum comb generation from azimuthal binary phases | Article Video , On the Cover
Shiyao Fu, Zijun Shang, Lan Hai, Lei Huang, Yanlai Lv, and Chunqing Gao
Since Allen et al. demonstrated 30 years ago that beams with helical wavefronts carry orbital angular momentum (OAM), the OAM of beams has attracted extensive attention and stimulated lots of applications in both classical and quantum physics. Akin to an optical frequency comb, a beam can carry multiple various OAM components simultaneously. A series of discrete, equally spaced, and equally weighted OAM modes comprise an OAM comb. Inspired by the spatially extended laser lattice, we demonstrate both theoretically and experimentally an approach to producing OAM combs through azimuthal binary phases. Our study shows that transition points in the azimuth determine the OAM distributions of diffracted beams. Multiple azimuthal transition points lead to a wide OAM spectrum. Moreover, an OAM comb with any mode spacing is achievable through reasonably setting the position and number of azimuthal transition points. The experimental results fit well with theory. This work presents a simple approach that opens new prospects for OAM spectrum manipulation and paves the way for many applications including OAM-based high-security encryption and optical data transmission, and other advanced applications.
Advanced Photonics Nexus
  • Publication Date: Jun. 29, 2022
  • Vol. 1, Issue 1, 016003 (2022)
Multiwavelength high-order optical vortex detection and demultiplexing coding using a metasurface
Dahai Yang, Jie Lin, Chen Chen, Chang Li, Junbo Hao, Baiying Lv, Keya Zhou, Yiqun Wang, and Peng Jin
Orbital angular momentum (OAM) of an optical vortex has attracted great interest from the scientific community due to its significant values in high-capacity optical communications such as mode or wavelength division multiplexer/demultiplexer. Although several configurations have been developed to demultiplex an optical vortex, the multiwavelength high-order optical vortex (HOOV) demultiplexer remains elusive due to lack of effective control technologies. In this study, we present the design, fabrication, and test of metasurface optical elements for multiwavelength HOOV demultiplexing based on optical gyrator transformation transformations in the visible light range. Its realization in a metasurface form enables the combined measurement of OAM, the radial index p, and wavelength using a single optical component. Each wavelength channel HOOV can be independently converted to a high-order Hermitian–Gaussian beam mode, and each of the OAM beams is demultiplexed at the converter output. Furthermore, we extend the scheme to realize encoding of the three-digit gray code by controlling the wavelength or polarization state. Experimental results obtained at three wavelengths in the visible band exhibit good agreement with the numerical modeling. With the merits of ultracompact device size, simple optical configuration, and HOOV recognition ability, our approach may provide great potential applications in photonic integrated devices and systems for high-capacity and demultiplex-channel OAM communication.
Advanced Photonics Nexus
  • Publication Date: Aug. 09, 2022
  • Vol. 1, Issue 1, 016005 (2022)