| Optical heterodyne detection technology is widely used in areas such as laser radar,laser communication,atmospheric monitoring,and target detection,with many advantages such as sensitivity higher than that of direct detection methods by several orders of magnitude.It was proposed to greatly increase the range of system ranging of high-sensitivity,long-distance,high-precision laser-amplitude-modulation heterodyne detection based on heterodyne detection and high-speed distance measurement.However,in the long-range detection under the atmospheric environment,the random fluctuation of the refractive index caused by atmospheric turbulence will cause the laser beam's arrival angle fluctuations,the coherence degradation,the light intensity flicker,etc.These seriously affect the detection performance of the heterodyne detection system of laser chirp amplitude modulation.The theoretical research of the effect of atmospheric turbulence on the heterodyne detection is relatively mature,and the relevant experimental research is relatively lagging.However,experiments conducted on turbulence in the actual atmosphere are not only time consuming and costly,but the randomness of actual atmospheric turbulence makes the controllability of turbulence intensity poor.Therefore,This thesis simulates atmospheric turbulence based on the indoor hot air turbulence simulator,and carries out an experimental study on the influence of turbulence on the heterodyne detection system of laser chirp amplitude modulation.This thesis theoretically has deduced the formula for the influence of the arrival angle fluctuations and the atmospheric dry length on the coupling efficiency of the all-fiber heterodyne detection system,and carried out the corresponding simulation analysis.Based on the angle-of-arrival probability density distribution function of the plane wave under turbulent flow,the formula for the relationship between the angle of arrival fluctuation and the average coupling efficiency of the system is deduced.The simulation results show that when the angle of arrival fluctuation is equal to the square of the ratio of the fiber mode field radius to the coupling lens focal length,the average coupling efficiency has droped to about10%.At the same time,based on the complex coherence function of plane waves under turbulent flow,the formulae for the relationship between atmospheric dry length and system coupling efficiency is deduced.The simulation results show that when the ratio of system receiving aperture to atmospheric dry length is equal to 2,the system coupling efficiency is reduced to about 10%.The drop in coupling efficiency will directly lead to a decrease in the output signal-to-noise ratio of the system.This thesis has designed and built a 1.55?m band heterodyne detection system of laser chirp amplitude modulation,and used a balanced detection method to reduce shot noise caused by local oscillation.The system can achieve a distance resolution of 0.3m,and the minimum detectable optical power of the system can reach1.47?10-1 3W.It has been set up of a turbulent laser heterodyne detection experiment system which uses the indoor hot air turbulence simulator atmospheric turbulence.The experimental results show that when the atmospheric dry length of the turbulent flow in the simulator is reduced from 0.217m to0.014m,the turbulence intensity is enhanced and optical power coupled into single-mode fiber is decreased.And the signal-to-noise ratio of the Doppler signal is reduced by about 14dB.At the same time,a heterodyne detection system of laser chirp amplitude modulation under turbulent flow has been built.Based on the above-mentioned atmospheric turbulence,the signal-to-noise ratio of the Doppler frequency shift signal is decreased by 13dB,and the signal-to-noise ratio of the frequency shift signal caused by the target distance is decreased by 11dB.This thesis has not only test the influence of different atmospheric turbulence intensity in the simulator on the output signal-to-noise ratio of the heterodyne detection system of laser chirp amplitude modulation,and provided the experimental basis for subsequent theoretical verification.Besides but also,it provides a reference for the experimental study and system performance test under turbulence of the application system of the heterodyne detection system of laser chirp amplitude modulation. |
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