Research On The Heterodyne Detection System Based On Balanced Detector

Optical heterodyne detection technology has been widely used in weak signal detection,which is characterized by strong detection ability,high conversion gain,good filtering performance and high stability in the detection of amplitude,intensity,phase etc.Due to the drawbacks as low utilization ratio and large noise of reference light,the common heterodyne detection can not meet the requirements of the detection accuracy and sensitivity,which leads to the emergence of balanced detection.The Doppler speed measurement system equipped with balanced detection technology of high precision and high sensitivity could further improve the performance of the speed measurement system.Firstly,the principle of direct detection,optical heterodyne detection,and balanced detection are introduced;the expression of the photocurrent is derived and the advantages of different detection methods are pointed out.After the summary of balanced detection methods,their advantages and disadvantages are analyzed respectively.The concrete implementation methods of optical path and circuit in the balanced detection are elaborated and their advantages are given.Secondly,a mathematical model of balanced detection is established.The SNR expression of balanced detection is deduced in detail.The quantum noise caused by the optical signal in detection system and its influence on the signal-to-noise are analyzed theoretically.The analyses indicate that the noise of excess intensity can be eliminated and the remaining noise can be suppressed by same-phase signal differential subtraction.Based on the expression of SNR,the relationship between influence factors and the SNR is presented in the form of curves via Matlab simulation.The simulation shows that the SNR of balanced detection is higher than that of the heterodyne detection when multiple relationship of conversion coefficient of two detectors ranges from 0.467 to 1.Finally,the balanced detection circuit is designed by Altium Designer Winter,and latterly applied in laser Doppler velocimetry system for relevant validation experiments.Experimental results show that when the two same detection systems share a set of signal amplification circuit,the noise suppression performances are optimized.