Study On The Technology Of Nonlinear Reconstruction Of Weak Optical Signal Via Stochastic Resonance

Optical signal processing has significant application in laser radar,optical space communication,and remote optical sensing.In practice,the low-level signal images are often merged by the scattering noise or fluctuation,leading to the dramatic decrease of signal-to-noise ratio.The noise-hidden signals are difficult to be effectively distinguished by the traditional filtering methods,because of the high intensity and relation of noise to signal.While in some nonlinear optical systems,the output signal-to-noise ratio has a peak value with the increase of the noise intensity,which dynamics is described as “stochastic resonance(SR)”.The weak signal can be effectively recovered from an initial noise-hidden situation via SR.While as a novel detection method,the fundamental mechanism and practical realization of stochastic resonance are still has obvious gap,and further research works are essential.In this dissertation,the main work is the research on effective reconstruction of weak signal in various noisy environments with a high quality via the optical stochastic resonance based on nonlinear optical effects.The detail thesis is structured as follows:In chapter one,we analyze the meaning of research on detecting weak optical signals from noisy environments,and conclude the urgent problem of detection methods.Based on the previous discussion,we introduce the optical stochastic resonance,including the three key elements of statistical noise,regular signal and nonlinear system,and the evaluation index of capability of stochastic resonance.At last,we generally introduce the application of stochastic resonance in weak optical signal processing.In chapter two,we firstly study the bistability stochastic resonance based on typical two-level medium ring cavity,and demonstrate that the main reason of the slow response time is induced the temporal delay induced by “critical slowing down” effect.Then we pertinently propose the optical bistability stochastic resonance based on surface plasmon.With establishing the optical bistability equation,designing the optical system including a Kretschmann structure,optimizing the thickness of metal layer and incident angle can mostly trigger the surface plasmon to change the refraction and phase delay,and contribute to the extraction of initial nanosecond pulse signal from high noisy environment.Meanwhile,the approach overcomes the time delay of trailing edge induced by “critical slowing down” effect and has higher cross-correlation gain about 8 times.In chapter three,we propose an approach of obtaing stochastic resonance based on modulation instability in optical spatiotempral chaos.The instability growth rate of partially coherent wave is derived from complex Landau-Ginzburg equation.By adjusting the nonlinear coefficient,diffraction coefficient and correlation length,the initial signal are hidden and regenerated from increasing noise intensity.Meanwhile,it is found that the signal-to-noise ratio grows with noise intensity with a highest value of 5 dB and temporal window about 100 ms.In chapter four,we investigate the reason of spatial coherency decreasing of optical image propagating in atmosphere,and then research the dynamics of extracting weak pulse signal images via stochastic resonance.Based on spatial modulation instability,photorefractive band transport theory and radiation model Wigner-Moyal transform approach,we obtain the instability growth rate of noisy signal,and analyze the influence of the system paramaters including the applied electric field across SBN crystal and correlation length.For the input signal-to-noise intensity ratio 1:25,the reconstruction of nanosecond pulsed images is obtained by parameter tuning stochastic resonance with a cross-correlation gain about 10 times.In chapter five,considering the vital problem of detecting multiplicative noisy image via traditional detection method,and serious distortion of image high-frequency information via SR based on single beam self modulation instability,we propose the mechanism of reconstruction based on the spatial cross phase modulation instability.A theory of instability growth is derived for coupling between partially spatial coherent beams.It is found that by coupling an additional noise beam which is highly-incoherent than the multiplicative noisy signal beam,the energy can directionally transform form the former to the latter.With designing the coupling optical paths,tuning the correlation length of noise and signal beams,the reconstruction of multiplicative noisy image can be obtained with a high cross-correlation gain 3.2.Additionally,it is demonstrated that the approach can simultaneously preserve and reconstruct the low-frequency image shape and high-frequency image detail.