查看更多>>摘要：? 2021 Elsevier B.V.A cavity length demodulation method, which is dedicated to solving the frequency spectrum leakage and barrier effect of FFT analysis, is proposed. The signal interception is adopted to improve the demodulation effect, and the curve fitting method is applied to find a more accurate characteristic frequency of the spectrum. Compared with the pre-setting additional phase method, the demodulation performance is not influenced by the change of the additional phase, and the lower limit of cavity length can be extended to 50μm. The simulation and experiment verification are carried out to evaluate the performance of the proposed method. The strain stability experiment indicates that the proposed method has a cavity length demodulation resolution of 1.21 nm at the length of about 131μm. The proposed method is promising for the application in the situation of the short cavity and indeterminate additional phase difference.
查看更多>>摘要：? 2021 Elsevier B.V.Micro-displacement is a classic application of laser self-mixing interference (SMI) measurement system, but most of the current researches are focused on single-channel measurement. In this paper, a dual-channel displacement-sensing system is proposed and demonstrated based on the SMI effect and intracavity spectroscopy. The 90.14-pm/μm sensitivity for the displacement sensing can be achieved when the intracavity spectroscopy is employed to monitoring the output spectra. For the SMI operation mode, the variational mode decomposition (VMD) is used to decompose and reduce the noise of the SMI signal, and the interference fringe counting method is used to reconstruct the displacement of the noise-reduced SMI signal to realize the displacement measurement of the target object. It is demonstrated that the displacement-reconstruction error is 110 nm when the frequency is 30 Hz and the peak-to-peak amplitude is 4 μm.
查看更多>>摘要：? 2021 Elsevier B.V.This paper studies the amplitude variation phenomenon in optical physical-layer network coding (OPNC) with intensity modulation and direct detection (IM/DD). Although PNC has been studied thoroughly in wireless communications, the application of PNC to optical communication networks is still challenging. In OPNC, two optical signals are transmitted simultaneously, and the received signals are overlapped in the optical field and then detected by a photo-detector (PD). Previous works assumed the OPNC signal and its electrical signal converted by a PD are linearly related. We find that the relationship is not the case. To fill this gap, we theoretically analyze OPNC and discover that an interference signal is generated when an OPNC signal is converted to its corresponding electrical PNC signal in most conditions. Besides, we implement the simulation and experiment to validate the theoretical analysis. The results coincide with our analysis and indicate that the PNC system does not work when the signal is severely damaged by the interference signal. However, we can recover the PNC signal by filtering the interference signal when their frequency spectrums are not aliasing. We further evaluate the BER performance in the simulation and experiment and found that when the frequency spectrum of the PNC signal is not aliased with the interference signal, using a low-pass filter can improve the BER performance by approximately 12 dB.
查看更多>>摘要：? 2021 Elsevier B.V.Manipulation of the refractive index has been of growing interest lately. We consider parameters and possibilities of enhancing the absolute-value limit of the linear index in coherent atomic systems. Starting with a review of how two-level transitions – without and with added coherence effects – do not realistically allow for a significantly enhanced index at fixed atomic density, we discuss a possible way around this using wave mixing. This confirms that the only parameters, besides the medium optical depth/density, that can effectively change the value of the attainable index are the frequencies of the involved transitions.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, the optical forces of particles in optical tweezers under non-ideal conditions are investigated. The calculation of optical forces are carried out by combining three dimensional finite-difference time-domain (FDTD) method and Maxwell stress Tensor (MST). Numerical results show the changes of trapping stiffness and equilibrium position induced by several common seen non-ideal conditions in experiment. This study is expected to estimate the error of optical force in real situation and that is of great use to further optimize the performance of optical tweezers.
查看更多>>摘要：? 2021 Elsevier B.V.We numerically demonstrate robust terahertz (THz) transmissions in a topological photonic crystal comprised of dielectric rod structures. Topological edge states (i.e., the vortex states) in the structure are excited via a zig-zag domain boundary at the interface of two nontrivial valley photonic crystals (VPCs). A comprehensive study of the effect of asymmetry (defined as δd=d1?d2) on transmission loss and bandwidth is performed for three different kinds of domain boundaries: straight, Z-shape, and Ω-shape. An increase in the bandwidth of the topological THz transmission is achieved with the increasing asymmetry. The study further demonstrates blue shifting of the topological transmission bandwidth with the increase in asymmetry. An optimized value of the asymmetry parameter is obtained based on the bandwidth as well as the transmission loss. The most robust propagation in terms of the widest bandwidth and low loss transmission is achieved with asymmetry ≥0.25a. The robust THz transmission is attributed to the strong edge modes confinement at the domain boundary of the VPC due to intervalley scattering suppression at this optimal value. Our observations open the path for a strong electromagnetic wave flow in the THz regime with a high transmission, which could be useful in photonic broadband communication and other on-chip applications at THz frequencies, particularly for 6G communications.
查看更多>>摘要：? 2021 The Author(s)This work studies the use of machine learning and, in particular, a Convolutional Neural Network (CNN) to generate digital holograms and how such a network compares to state-of-the-art iterative methods, both in terms of reconstruction quality and computation time. Since CNNs only require a single pass through the network by a target image to generate a result, and not tens or hundreds of expensive iterations as in the iterative methods, they may be able to accomplish real-time hologram generation; an ability that could open the technology to proper commercial use. In this work, a CNN built on the UNet architecture, capable of hologram generation, is presented. The network is trained on 4096 images of varying spatial frequencies, both user-generated and from the DIV2K dataset. It is compared to the most common iterative method for hologram generation, namely the Gerchberg–Saxton (GS) algorithm and its modern and improved implementations. In reconstruction quality, the neural network outperforms the original implementation of GS when evaluating Mean Square Error (MSE), geometric error (GE), Structural Similarity Index Measurement (SSIM), and Peak Signal-Noise Ratio (PSNR) of 64 unseen test images. However, on the same test images, the network lacks behind the modern, optimized GS implementations in all error and accuracy measurements. The network does, however, achieve these results at a rate 70–280 times faster than the iterative methods, depending on the particular implementation of the GS algorithm, which corresponds to a possible generation rate of the network of 32 FPS on average.
查看更多>>摘要：? 2021 Elsevier B.V.In this paper, we derive a general expression of the quantum Fisher information of an SU(1,1) interferometer with an arbitrary state and a Fock state as inputs by the phase-averaging method. Our results show that the same quantum Fisher information can be obtained regardless of the specific form of the arbitrary state. Then, we analytically prove that the parity measurement can saturate the quantum Cramer–Rao bound when the estimated phase sits at the optimal working point. For practical reasons, we investigate the phase sensitivity when the arbitrary state is a coherent or thermal state. We further show that a Fock state can indeed enhance the phase sensitivity within a constraint on the total mean photon number inside the interferometer.
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