查看更多>>摘要：? 2022 Elsevier B.V.A broadband terahertz waveguide type beam splitting scheme for multi-port outputs is proposed, which is realized by uniform mode coupling of the multiple subwavelength waveguides. The full-vectorial finite-difference beam propagation method (BPM) is adopted to numerically analyze the mechanism and splitting performance of the beam splitter, and the influence of the supporting structure between the waveguides and the outer dielectric ring on its transmission performance is studied. In particular, a 1 × 8 beam splitter is designed with a coupling zone length of 1.02 mm, which can couple the input THz wave into eight output ports uniformly. The insertion loss of the device is 9.3 dB at the center frequency of 0.54 THz and the bandwidth can reach 0.21 THz with insertion loss lower than 10 dB.
查看更多>>摘要：? 2022 Elsevier B.V.Recently, technologies such as artificial neural networks (ANNs) and asynchronous amplitude histograms (AAHs) are widely employed in the optical performance monitoring (OPM). In this paper, the number of layers and the optimization of the neural networks are investigated to complete a series of monitoring tasks in optical communication systems with a large range of chromatic dispersion (CD) and optical signal-to-noise ratio (OSNR). It is shown that the monitoring accuracy has been significantly improved with such implementations. Numerical simulations, which have been conducted for six different modulation formats, demonstrate a good monitoring performance, with an average OSNR monitoring error of 0.1064 dB (99.49% monitoring accuracy) for the OSNR range of 10–40 dB and an average CD monitoring error of 3.3324 ps/nm (99.45% monitoring accuracy) for the CD range of 170–1870 ps/nm, respectively. Furthermore, the CD monitoring and the modulation format identification (MFI) have also been investigated for the system with a large range of dispersion. An average CD error of 16.6832 ps/nm (99.45% monitoring accuracy) and a 100% MFI accuracy have been achieved for all six modulation formats. This scheme is also verified experimentally with OSNR errors of 0.41 dB and CD errors of 15.21 ps/nm, respectively. The identification accuracy of the three modulation formats in the experimental system is also 100%.
查看更多>>摘要：? 2022 Elsevier B.V.When using a metasurface to encode multi-channel color images, in addition to nanostructural shape on the metasurface, efficiently determining nanostructural size based on the coding information has become an urgent problem. Here, we take dual-channel color prints as the research object. By modulating the silicon nanofin's two arms’ length and the period of the meta-atom, the reflected colors are manipulated under different linearly polarized lights. According to any two color patches, to obtain the nanofin's appropriate parameters, a dual-task cascaded neural network (DT-CNN) is designed. And the model prediction is compared with the electromagnetic simulation. Then two colorful images are used as the model input. The model output shows that the DT-CNN is useful for dual-channel color prints. In addition, it also can be applied to optical anti-counterfeiting, information storage, color display, and encrypted communication.
查看更多>>摘要：? 2022 Elsevier B.V.We present a compact three-dimensional (3D) imaging system based on a virtual image array, which employs a single camera and Risley prisms. The original multi-perspective images are captured by the fixed camera with a dynamically deflected boresight. By applying planar homographies to original images, we obtain light field samples from a planar virtual image array. Moreover, a depth estimation framework on the epipolar-plane image (EPI) is adopted to generate 3D profiles of scenes. In the experiment, two-dimensional (2D) projections from a 5 × 5 virtual image array are obtained to exhibit the 3D imaging capability. The system has the potential to achieve stereovision in numerous applications such as industrial vision and visual navigation.
查看更多>>摘要：? 2022 Elsevier B.V.The demand for high-speed data transmission has increased rapidly over the past few years, leading to advanced optical communication techniques. However, most of machine learning based equalizers which are trained with offline dataset mainly focus on achieving low BER, neglecting the generalization ability when the properties of optical link change. In this paper, we propose Gaussian Mixture model (GMM) based low-complexity adaptive machine-learning equalizers. The proposed online training strategy can fine-tune the parameters in GMM with a small amount of training sequence by introducing Maximum a posteriori probability (MAP) algorithm and sliding window. The size of online training sequence can be reduced by utilizing the parameters trained from offline data as the priori information to reduce the size of online training sequence. The proposed adaptive GMM based equalizers can not only show an excellent capability of mitigating nonlinear distortions but also show the ability to update parameters automatically according to the channel conditions. In addition, experimental results show that by introducing MAP algorithm and sliding window, the convergence speed of the proposed equalizers have been accelerated by 2 times, compared with the case that the traditional Expectation Maximization (EM) algorithm is used for online parameter update. On the other hand, there is no significant increase in computational complexity during the online stage.
查看更多>>摘要：? 2022 Elsevier B.V.Efficient second harmonic generation (SHG) of ytterbium (Yb)-doped fiber-based amplified spontaneous emission (ASE) light with temporally low coherence is reported. A Yb-fiber ASE light with a tunable bandwidth was amplified in two power amplifier stages and was frequency-converted by a periodically-poled lithium tantalite waveguide. The second harmonic conversion efficiency of the ASE light achieved in the experiment was inversely proportional to the bandwidth, which was in very good agreement with theoretically calculated value using the self-convolution relationship of the power spectral density for nonlinear frequency conversion of low coherent light. The prospects for further improvement in nonlinear frequency conversion of the fiber ASE source will be discussed.
查看更多>>摘要：? 2022 Elsevier B.V.Depth of field (DoF) defines the depth range of a 3D scene that can be observed by viewers. In the conventional lens-based imaging process, the diffraction effect of lens limits the display depth range, which has an adverse effect on the display quality of lens-based light-field display systems and it is the major obstacle to the practical application of light-field display. Aiming to break through the inherent shallow DOF characteristics of lens and obtain better display effect, an extended depth of field method with a designed diffraction optical element (DOE) which combined multi-depth fusion and end-to-end optimization is proposed. By analyzing the effect of diffraction on optical imaging process, a DOE enables balance display quality over the whole depth range is designed, and a pre-filtering convolutional neural network (CNN) is constructed to complete end-to-end optimization framework and avoid image blur. The validity of the proposed method is confirmed by simulation experiments and the depth range is extended effectively without display sharpness declining severely.
查看更多>>摘要：? 2022 Elsevier B.V.Underwater dynamic positioning plays an important role in promoting the development of marine science and technology. We experimentally demonstrate a method to perform the underwater dynamic measurement based on the cross-sampling dual-comb. The scheme aims at achieving rapid measurements for the dynamic object, and obtaining the motion state to map the dynamic position in real time. From the interferometric cues, the cross-sampling gives an intriguing way to break the constraint of periodic ambiguity. The measuring rate is doubled in a set of interferograms. The experimental results demonstrate the method has good applicability to the underwater environment. Compared to the reference values, the difference in the dynamic distance is below 71μm at the range of 5 m, corresponding to the velocity deviation of 142 μm/s in our system.
查看更多>>摘要：? 2022 Elsevier B.V.We proposed a suspended sandwich chiral metamaterial with three-dimensional broken symmetry, in which the asymmetric transmission for linearly and circularly polarized light can be achieved due to the coupling between the plasmonic resonances excited in upper and lower metallic structures. The maximum asymmetric transmission parameter of linearly polarized light reached 0.77 at frequency 236.5 THz. In addition, a 90° optical rotation with the transmittance of 0.68 and the ellipticity of 0.197 was also obtained.
查看更多>>摘要：? 2022 Elsevier B.V.In order to reduce the influence of nonlinear factors such as gamma nonlinearity and color crosstalk on the phase retrieval accuracy with color fringe projection profilometry (CFPP) in rapid measurement, a single-shot high-precision backpropagation (BP) neural network based CFPP is proposed. Firstly, neural networks are trained from samples through a training process. The neural network inputs are the RGB components of composite color fringe patterns. The numerator and denominator of the arctangent function are used as network outputs. The monochromatic fringe patterns are adopted. The numerator and denominator of the arctangent function are calculated by the 15-step phase-shift method, which serves as the network training target. Applying this neural network to the prediction of the standard phase can reduce the influence of nonlinear factors on the phase accuracy. Then, the absolute phase was unwrapped by using geometric constraints. Finally, the 3D reconstruction with high precision can be realized by using only a single-shot color fringe. The theoretical model of the proposed method and the process of neural network training are introduced. The results of 3D reconstruction are compared with other algorithms in the same condition. The experimental results demonstrate that the RMS of the proposed algorithm can reach 0.0107 mm, compared with 0.0121 mm of the traditional 3-step phase-shift method. The proposed method is applied to the online quality inspection of relay components, and the results verify the high accuracy and high efficiency of measurement.
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