Fengli Gao

Object reconstitution using pseudo-inverse for ghost imaging

We propose a novel method for object reconstruction of ghost imaging based on Pseudo-Inverse, where the original objects are reconstructed by computing the pseudo-inverse of the matrix constituted by the row vectors of each speckle field. We conduct reconstructions for binary images and gray-scale images. With equal number of measurements, our method presents a satisfying performance on enhancing Peak Signal to Noise Ratio (PSNR) and reducing computing time. Being compared with the other existing methods, its PSNR distinctly exceeds that of the traditional Ghost Imaging (GI) and Differential Ghost Imaging (DGI). In comparison with the Compressive-sensing Ghost Imaging (CGI), the computing time is substantially shortened, and in regard to PSNR our method exceeds CGI on grayscale images and performs as well as CGI visually on binary images. The influence of both the detection noise and the accuracy of measurement matrix on PSNR are also presented.

Scalar-matrix-structured ghost imaging

The features of the characteristic matrix used in linear intensity correlation reconstruction methods are directly related to the quality of ghost imaging. In order to suppress the noise caused by the off-diagonal elements in the characteristic matrix, we propose a reconstruction method for ghost imaging called scalar-matrix-structured ghost imaging (SMGI). The characteristic matrix is made to approximate a scalar matrix by modifying the measurement matrix. Experimental results show that SMGI improves the peak signal-to-noise ratio of the object reconstruction significantly compared with differential ghost imaging, even in the case of a nonzero two-arm longitudinal difference, which is a promising result for practical applications.

Omnidirectional beam steering using aperiodic optical phased array with high error margin

We propose a pattern-search-like algorithm to design an aperiodic optical phased array for extensive applications in light detection and ranging and free-space communication. The designed phased array with 128 isotropic elements achieves a scan range, peak side-lobe level, minimum beam width, and mean pitch of ± 82°, -14.34 dB, 0.062°, and 9.75 μm, respectively. To our knowledge, it has the widest steering range, narrowest divergence, and largest mean pitch for the same waveguide number. The minimum pitch can be greater than 2.67λ to avoid cross-coupling. The calculated relationship between the machine error and side-lobe level indicates that the designed structure has a higher error tolerance than its uniformly spaced counterpart.

The influence of 1/ f noise on the electrical derivative initial peak of high-power semiconductor laser diodes

We report a close connection between the fluctuation characteristics of the electrical derivative (ED) initial peaks and the 1/f noise intensities of different samples we found during the investigation of the 1/f noise origins of InGaAs quantum well high-power semiconductor laser diodes (LDs). We conduct contrast measurements on over fifty samples, where the current 1/f noise is measured under different bias currents, expressed by power spectrum density (PSD) and the EDs are computed from the current-voltage (I-V) measurement results. Then the influence of 1/f noise on the ED initial peaks is presented by comparing these parameters of different samples. The results show a clear pattern between the noise intensity and the ED initial peak fluctuation, and distinct differences between functional and aged LD devices, showing that ED initial peak can also be a non-destructive testing method for high power LD cavity damage and surface defects.

Detection and analysis of 1/f noise correlation in semiconductor laser diodes

In this paper, firstly we established the theoretical model of the relationship between correlation and wavelet vanishing moment of 1/f noise. Then we designed and built the 1/f noise measurement system for semiconductor laser diodes (LDs). Based on this, we introduced the measurement method of low-frequency electrical noise in semiconductor LDs and the extraction process of parameters associated. The wavelet transform decorrelation calculation is applied to the time-domain of the low-frequency noise signals, and the calculation and analysis of the relation between wavelet coefficients variances and scales are completed. The experimental results show that the noise wavelet coefficient correlation E and the vanishing moment N of wavelet function satisfy the decorrelation theoretical model at some scales, which implies that the low-frequency noise signals measured from the LDs belong to 1/f noise. Besides, the wavelet coefficient variance calculation results indicate that the noise measured in the experiments is 1/f noise from another aspect. Finally we theoretically proposed a new idea of semiconductor laser 1/f noise judging based on the methods above.

Denoising of electrical derivative data of semiconductor lasers based on nonlinear diffusion equation

The method of screening semiconductor lasers by using electrical derivative technique is described in detail. The nonlinear diffusion equation is applied to denoising of electrical derivative data according to its denoising theory in signal processing. The denoising experiments of electrical derivative data for several dozens semiconductor lasers indicate that the denoising method can effectively reduce the noise in electrical derivative data and the errors of the measured parameters. The farther experiments indicate that the accurate estimate ratio of the devices can be effectively increased by using the measured parameters which have been denoised, to estimate the quality and reliability of semiconductor lasers.