Study On The Mechanism Of Reconstruction Of Weak Optical Signal Via Stochastic Resonance In Nematic Liquid Crystals

Weak light signal detection has a very broad application prospect.The research on the extraction of weak light signal under the background of noise is a key component of all-optical communication technology.Traditionally,background noise is harmful to signal detection.The stronger the noise,the more difficult it is to detect the signal.The traditional weak light signal detection technology mostly adopts simple filtering method,but the detection effect is poor when the signal is seriously disturbed or even submerged by noise.In some nonlinear systems,if a certain type of noise is added,the effect of weak signal extraction will be better with the increase of noise intensity,which is called stochastic resonance.In this thesis,the nonlinear effect in nematic liquid crystals is used to realize stochastic resonance,and then the effective extraction of weak signals in noisy environment is realized.The details are as follows:In chapter one,several weak light detection techniques,including mean value estimation,cross correlation detection,narrowband filtering,chaos theory and stochastic resonance detection,are summarized.The three elements of stochastic resonance,including modulated signal,nonlinear system and noise,are described.Three methods of system evaluation are introduced: signal to noise ratio,correlation and peak signal statistics.The important progress in the development of stochastic resonance is reviewed.It can be seen that stochastic resonance phenomenon exists widely in all kinds of systems and plays a unique role in the field of weak signal detection.It has a good development prospect in the future.In chapter two,the work of stochastic resonance reconstruction of noisy image based on nematic liquid crystal is discussed in detail.We employ nematic liquid crystals as the nonlinear medium to recover noisy images via stochastic resonance,in which nonlinear coupling allows signals to grow at the expense of noise.The process is theoretically analyzed and the cross-correlation is numerically calculated.It is found that the quality of output images is affected by the input noise intensity,the applied voltage and the correlation length of noise light.Noise-hidden images can be effectively recovered by optimizing these parameters.The results suggest that nematic liquid crystals can be used for reconstruction of noisy images via stochastic resonance based on modulation instability with molecule reorientation nonlinearity.In chapter three,we theoretically propose a method to restore weak pulse signals submerged in noise via stochastic resonance,which is based on the optical bistability induced by the molecule reorientation in a Fabry-Perot cavity with an intracavity nematic liquid-crystal film.The bistable properties of this cavity are analyzed with different reflectance of the mirrors,initial phase shift and initial angle between the phase propagation vector and the director.The cross-correlation coefficient between pure input pulses and output is calculated to quantitatively evaluate the influence of noise intensity on output.The simulation results show a cross-correlation gain of 3.2 and that the buried signal can be recovered effectively by this device.It proves the potential of using this structure to recover noise-hidden pulse signals in an all-optical system.