Research On The Decoherence Effect And Its Suppression Technology In Active Optical Heterodyne Detection

Optical heterodyne detection is a high-precision holographic detection technology,which is widely used in various fields owing to its noise-reduction capability and high spectral resolution,especially in the long-range and weak signal detection.In heterodyne detection,the matching state between the local oscillator(LO)beam and signal beam determines the sensitivity of the heterodyne system.However,optically rough surfaces whose surface roughness is large compared to the laser wavelength is inevitable in many engineering applications.When a laser beam is reflected from an optically rough target surface,the beam wavefront is modulated by the surface will produce speckle and cause the distribution mismatching of the LO beam and signal beam on the detector surface.In a heterodyne system,the intermediate frequency(IF)signals generated by different parts of the photodetector cancel each other out due to the random phase distribution of the speckle field,possibly diminishing the system sensitivity,this phenomenon is called the decoherence effect.In applications of active detection,the decoherence effect caused by the rough surface of the target is still the major obstacle restricting the practical application of the optical heterodyne detection.This dissertation is mainly focused on the problem of the decoherence effect in active optical heterodyne detection and its suppression technology.The research is as follows:1.Based on the conventional optical heterodyne theory and speckle statistical theory,a detailed theoretical analysis of the decoherence effect caused by rough targets in the active optical heterodyne detection is carried out.We use the Monte Carlo method to develop a target speckle field module received by the heterodyne system.An expression is derived by substituting the speckle module into the heterodyne efficiency for analyzing the quantitative influence of speckle phase statistical parameters on heterodyne detection.Based on the simulation results and our theoretical works,the influence of the Rayleigh speckle field on heterodyne detection is discussed,and a criterion on Rayleigh speckle statistics in heterodyne detection is given.2.The three existing methods to correct the mismatches between the LO beam and the signal beam in heterodyne detection are summarized.Consequently,the research direction in this paper of solving the decoherence effect based on the array detector method is determined.We present a method for modeling the heterodyne efficiency of array detector systems in the presence of target speckle.The heterodyne efficiency is calculated by accounting for equidistantly divided speckle fields and using an additional parameter to simulate coherent summation processing.Based on the simulation results,it is proved that using an array detector method can effectively improve the signal-to-noise ratio(SNR)of heterodyne systems.Under low SNR conditions,the heterodyne efficiency of N×N array detector systems increases linearly with N.3.To solve the key problem of the determination of the phase adjustment of the array signal in the array detector method,the optimization theory is introduced into the array detector method.We consider the heterodyne frequency component of the total output of the array detector as the fitness function.In the proposed array detector method,the intelligent optimization algorithm is used to compute the quantities of phase adjustment of each array element.Based on the numerical results,our proposed array detector model exhibits efficient performance in compensation for the spatial speckle phase in optical coherent detection.4.To evaluate the effectiveness of array detectors for the decoherence effect suppression in applications of active heterodyne detection,a high-speed camera heterodyne system was developed.In the experiment,two acoustic-optic modulators on optical paths conferred an available heterodyne frequency range to satisfy the camera frame rate.The heterodyne images received by the high-speed camera were equidistantly divided into a set of array signals.The sum of the frequency components within the frequency range of the heterodyne signal was applied as the fitness function.Moreover,the phase adjustment quantities of each array element were determined using the APSO algorithm.An array grouping method is additionally proposed to overcome the problem of insufficient computing power due to an excessive number of array elements.The experimental results demonstrate that,when encountering the decoherence effect caused by target speckle,the proposed array detector significantly enhances the heterodyne system performance.In summary,the theoretical analysis and simulation modeling work of the decoherence effect caused by the rough targets in the active optical heterodyne detection is carried out.An array detector heterodyne system was developed using a high-speed camera to examine the efficacy of array detectors for target speckle correction.The experimental results demonstrate that the proposed array detector method corrects phase mismatches caused by a rough target,thereby improving the heterodyne system sensitivity and thus benefiting active detection applications.