Research On High Sensitivity Intradyne Laser Coherent Detection Technology

Intradyne detection not only has the advantages of low bandwidth like homodyne detection and heterodyne detection without phase-locked loop,but also can demodulate all the information of the intermediate frequency signal after the interference of the local oscillator light and the signal light.Theoretically,the limit sensitivity of intradyne detection can reach the level of single photon,so intradyne detection is suitable for weak light detection fields such as coherent optical communication and radar detection.In this thesis,a pulse intradyne coherent detection method based on quadrature phase detection is proposed.The frequency of the intermediate frequency signal is determined by the frequency drift of the laser.The structure is simple and high sensitivity detection can be realized.The focus of this thesis is to develop high sensitivity pulse intradyne coherent detection methods.By analyzing the factors affecting the intradyne coherent detection system,and optimizing the structure to improve the sensitivity of the system,and the intradyne coherent detection system is applied to the vibration monitoring based on coherent optical time domain reflectometer.The main research contents of this thesis are:1.The thesis establishes a theoretical analysis model of intradyne coherent detection.Firstly,the noise in the coherent detection system and the demodulation method of the intradyne signal are theoretically deduced,and the detection sensitivity of the intradyne detection and the heterodyne detection is analyzed.Theoretically,the limit sensitivity of intradyne detection can reach-95 d Bm(single photon level),which is higher than that of heterodyne detection.Secondly,the influence of different factors on the signal-to-noise ratio of the intradyne coherent detection system is tested on the optical simulation software.Finally,a simulation system for intradyne detection based on polarization diversity reception is also built.This receiving method can effectively reduce the loss of signal light and improve the detection signal-to-noise ratio of the system.2.The thesis completes the experimental construction of the pulse intradyne coherent detection system and the pulse heterodyne coherent detection system.The intradyne detection system adopts pulse signal light source and quadrature phase detection method,which can significantly improve the receiving signal-to-noise ratio and sensitivity of the system.The design and testing of the optical circuit and circuit of the system are completed,and the high sensitivity of pulse intradyne detection is verified.The sensitivity can reach-88.14 d Bm(5 photons),which is 8.46 d B higher than pulse heterodyne detection.The experiments also verified the effects of laser linewidth and local oscillator optical power on the sensitivity of the intradyne detection.The narrower the line width,the higher the system sensitivity.Moreover,the optimal local oscillator optical power values for both intradyne coherent detection and heterodyne coherent detection are also obtained experimentally.3.In this thesis,based on the method of pulse intradyne detection,a vibration monitoring and positioning system based on coherent optical time domain reflectometry is built,and studies the propagation mechanism of the scattered echo signal and the vibration monitoring principle.Compared with the traditional heterodyne detection method,the thermal noise is small and the detection sensitivity is high.The intensity and position information of the vibration signal are obtained by squaring and demodulating the IQ two-way quadrature signals.The weak echo signal is processed by the digital averaging method,which can effectively locate the vibration at 3 km,12.5 km and 15.5km.The longer the positioning distance,the worse the positioning accuracy.When the signal pulse width is 200 ns and the vibration distance is 12.5 km,the spatial resolution is 18.5 m.when the signal-to-noise ratio is reduced to 13.5 d B,the detection sensitivity is-84.32 d Bm,and the detection distance can reach 18 km.Finally,the experiment also proves that the positioning error of different types of vibration signals is the same.