Abstract:An ultra-broadband optical filter was proposed and demonstrated based on leaky mode coupling in a coated chirped long-period fiber grating (CLPFG). The CLPFG was coated with a material whose refractive index (RI) was higher than that of the fiber cladding, enabling the coupling of the core mode to leaky modes, to achieve a desired coupling efficiency. Complex coupled-mode theory was used to investigate the power evolution of the core mode that resulted from the coupling. From this, the conditions in which the core mode power attenuates the most rapidly were identified. In addition, phase matching turning point (PMTP) was used in the design, to overcome the conflict between the range of grating period change and the grating length in the CLPFG. Finally, an optimized CLPFG-based filter with a length of 3.5 cm was obtained, which has a symmetrical attenuation band with an operating bandwidth over 300 nm. Within the operating bandwidth, the flatness is less than 2.5 dB and the transmittance is lower than 0.1%.

Abstract:Electro-optic (EO) modulator plays a very critical role in the optical communication systems. Here, we report a stimulated thin-film lithium niobate (LN) modulator with a half-wave voltage-length product of 1.8 V.cm, which can successfully achieve modulation and demodulation of a 1 GHz sinusoidal signal with an amplitude of 5 V in experiment. The optical loss caused by metal electrodes is reduced by optimizing the waveguide structure and depositing silica onto the waveguide, and group-velocity matching and characteristic impedance matching are achieved by optimizing the buffer silica layer and the electrode structure for larger bandwidth, which simultaneously improves the modulation efficiency and bandwidth performance. Our work demonstrated here paves a foundation for integrated photonics.

Abstract:A multi-channel laser interferometer (MCLI) is proposed to improve the coordinate measurement accuracy. A 780 nm external cavity laser is locked on the D2 line of 87Rb atom by polarization spectroscopy, and a high frequency stabilized laser source is obtained with a linewidth of 385.8 kHz at root mean square (RMS). The interferometers share the stabilized source and individually install on 4 axes of a coordinate measuring system. As a result, the measurement uncertainty is reduced from 1.2 µm to 0.2 µm within the dynamic measurement range of 1.0 m. The MCLI is adept at integrate and flexible installation, which caters to various applications on precision measurement.

Abstract:Near infrared-II (NIR-II, 1 000—1 700 nm) imaging with high penetration tissue depth and signal-noise ratio has attracted wide interest in biomedicine. As a two-dimensional (2D) material with narrow band gap, the band structure of layered black phosphorus, as an important characteristic of electronic structure, determines the electronic transport and infrared optical properties, which show great potential in NIIR-II imaging. Here, the electronic structure and NIR-II optical properties of black phosphorus have been investigated in detail by employing the generalized gradient approximation + U (GGA+U) correction based on density functional theory (DFT). First, we performed the band structure and density of states for different layers of black phosphorus. From the electronic structures, the location of valence band maximum didn’t shift obviously, and the position of conduction band minimum shifted downward gradually, inducing the band gaps decreased gradually with the increasing layer number. While the layer number increased to 5, the behaviour of electronic structure was very similar to that of the bulk black phosphorus. Then, we calculated the NIR-II optical properties, and found the optical band gap of black phosphorus also showed layer dependent properties. From a single layer to 5 layers, the optical band gap changed from 1.71 eV to 0.92 eV. It is noting that black phosphorus also showed the significant optical absorption in NIR-IIa (1 300—1 400 nm) and NIR-IIb (1 500—1 700 nm) windows. Especially, the NIR-II optical absorption can be enhanced with increasing the layer number to 5, indicating promising photoresponse materials in NIR-II imaging.

Abstract:We theoretically investigate the evolution of dark screening spatial solitons originating from the quadratic electro-optic effect in biased centrosymmetric photorefractive (CP) crystal by using beam propagation method (BPM). The results indicate that the multiple dark solitons sequence can be obtained in the CP crystal with the odd or even initial conditions. If the initial width of the dark notch is smaller, only a fundamental soliton or a Y-junction soliton pair is generated. When the initial width of the dark notch and the bias electric field are increased, the multiple dark spatial solitons sequence is formed, which realized a progressive transition from a low-order soliton to a higher-order solitons sequence in both odd and even conditions. The solitons characteristic is similar to that of screening solitons in the non-centrosymmetric photorefractive (NCP) crystals.

Abstract:In this work, the symbol error rate (SER) performance of a relay-assisted underwater wireless optical communication (UWOC) system has been investigated over the composite exponential-generalized gamma (EGG) distribution with the beam spread function (BSF) under two hard decision schemes of fixed decision threshold (FDT) and dynamic decision threshold (DDT). Specifically, the oceanic turbulence is assumed to follow the EGG distribution, and the impacts of absorption, scattering and misalignment loss are characterized by BSF. The cumulative distribution function (CDF) of this UWOC system is derived with the max-min criterion as the best path selection scheme. And with the help of Gauss-Laguerre quadrature function, the analytical SER expressions for these two threshold schemes are then achieved and validated by Monte Carlo (MC) simulation. Moreover, the SER performance is further studied under different temperature gradients, bubble levels (BLs) and water salinity over three water types, as well as the system structure parameters. Results show that the UWOC system with DDT scheme can efficiently overcome the error floor induced by FDT scheme and demonstrates better SER performance. Furthermore, the SER performance would be improved with lower BL, temperature gradients and water salinity as well as the concentration of dissolved particles. This work will benefit the design and research of relay-assisted UWOC system.

Abstract:For particle image velocimetry (PIV) technique, the two-dimensional (2D) PIV by one camera can only obtain 2D velocity field, while three-dimensional (3D) PIV based on tomography by three or four cameras is always complex and expensive. In this work, a binocular-PIV technology based on two cameras was proposed to reconstruct the 3D velocity field of gas-liquid two-phase flow, which is a combination of the binocular stereo vision and cross-correlation based on fast Fourier transform (CC-FFT). The depth of particle was calculated by binocular stereo vision on space scale, and the plane displacement of particles was acquired by CC-FFT on time scale. Experimental results have proved the effectiveness of the proposed method in 3D reconstruction of velocity field for gas-liquid two-phase flow.

Abstract:Aiming at the noise disturbance of unwrapping phases of control points in the camera-projector calibration system, a random sample consensus (RANSAC) based plane fitting method is proposed to filter the phase noise in this paper. Different from the classical least squares method using all data, the points with noise will not be used to fit the plane in the proposed RANSAC method, which improves the accuracy of plane fitting. The proposed method suits for any two-dimensional (2D) calibration patterns, such as checkerboard or black dots with white background, which improves the flexibility of camera-projector system calibration.

Abstract:Surface electromyography (EMG) is a bioelectrical signal that recognizes speech contents in a non-acoustic form. Activity detection is an important research direction in EMG research. However, in the low signal-to-noise ratio (SNR) environment, it is difficult for traditional methods to obtain accurate active signals. This paper proposes a new energy-based spectral subtraction backtracking (E-SSB) method to segment EMG active signal in the low SNR environment. Compared with traditional energy detection, the algorithm in this paper adds spectral subtraction (SS) to filter out the clutter, and raises a retrospective idea to improve the classification performance. The experiment results show the proposed activity detection method is more effective than other methods in the low SNR environment.

Abstract:Recently, the generative adversarial network (GAN) has been extensively applied to the cross-modality conversion of medical images and has shown outstanding performance than other image conversion algorithms. Hence, we propose a novel GAN-based multi-domain registration method named multiscale diffeomorphic jointed network of registration and synthesis (MDJRS-Net). The deviation of the generator of the GAN-based approach affects the alignment phase, so a joint training strategy is introduced to improve the performance of the generator, which feedbacks the structural loss contained in the deformation field. Meanwhile, the nature of diffeomorphism can enable the network to generate deformation fields with more anatomical properties. The average dice score (Dice) is improved by 1.95% for the computer tomography venous (CTV) to magnetic resonance imaging (MRI) registration task and by 1.92% for the CTV to computer tomography plain (CTP) task compared with the other methods.

Abstract:We study the dynamics of short optical pulses in a real system with periodically distributed dispersion and the nonlinearity is governed by the higher order nonlinear Schrödinger equation (HNLSE) with linear and nonlinear gain (loss). Under specific parametric circumstances, where the dark and bright solitary waves are combined, a set of entirely new types of solitary waves with nonlinear chirp have emerged. For a properly intense optical pulse in the combined solitary waves, the binding of the bright and dark solitary waves is very strong. It is seen that by numerical simulation, these pooled types of solitary-like solutions show a high degree of stability while propagating over an extremely long distance in the considering system, even in the presence of a high degree of perturbation of the amplitude and white noise. All constraint relations on the physical parameters are explicitly shown to be related to the development and to the dynamical study of the chirped solitary like solution in the considering medium.