Study Of Joint Modulation Scheme Of Acousto-optic And Magneto-optic Modulator In Continuous Variable Light Communication

Single spatial mode continuous variable quantum key distribution(CV-QKD)can be realized with modern coherent light communication technologies which does not need single-photon source and detectors.This is one of the advantages that discrete variable quantum key distribution(DV-QKD)protocol cannot be matched.So QKD protocol of this type is possible to be applied in practical communication in the near future.The amplitude and phase as well as the polarization components of light can be used as the pair of canonical variable of CV-QKD encoding scheme.In the encoding mode of light amplitude and phase,two laser beams called local oscillator(LO)light and weak signal light should propagate in the atmosphere separately.Shortcoming of such scheme is that it's hard to maintain the coherence and the synchronization between local oscillator and signal.This drawback affects the stability of communication using such protocol.By modulating polarization components,single spatial mode CV-QKD are capable of containing the LO and signal in the same spatial mode,which overcomes the problems exist in protocol using light amplitude and phase.Whatever the physical variables of the communication system are,encoding procedure is always done by light modulators.Usually,the modulation system of CV-QKD consists of an electro-optic modulator(EOM)or a magneto-optic modulator(MOM)or a combination of them.Based on the drawback of EOM and MOM,the thesis proposes a new modulation scheme using acousto-optic modulator(AOM)and MOM.Main obstacle of such scheme is how to control polarization via AOM.According to the idea before,the thesis is arranged as follow:Firstly,the function of several light modulators frequently used nowadays is introduced.Principle of how EOM,MOM and photo-elastic modulator(PEM)modulates the polarization state is shown.Key job of the thesis includes analyzing the photo-elastic effect,the method of solving acousto-optic coupling wave equation and the principle of how AOM modulates polarization state.These principles serve as the basic design reference for an acousto optic polarization modulator.Secondly,several frequently used method of solving the intermediate diffraction differential equation is analyzed using the principles and equations in part I.N Order Approximation(NOA)method is selected for higher accuracy.This method is programmed to simulate the polarization distribution of normal and oblique incidence condition of Raman-Nath diffraction and intermediate diffraction regime.Influence of incident angle on Bragg diffraction is also considered.Parameters of acousto-optic polarization modulator is determined through these simulation results.It's found that polarization effect of light diffracted by ultrasound is related to the photo-elastic parameters,the dimension of interaction medium,the frequency of ultrasound as well as the angle of incident light.An AOM is designed according to the simulation results.Experimental results demonstrate the tendency of such acousto-optic modulator fits the simulation results.At last,Jones matrix of AOM and MOM modulation system as well as the Stokes parameter is derived.Four states protocol can be achieved with such joint modulation system.Joint modulation system is established to verify the modulation scheme.By analyzing the experimental results,one can draw the conclusion that this system achieve the aims of the thesis.