Study Of Dual Channel Digital Lock-in Amplifier Based On Spot Position Detection System

The atmospheric laser communication technology which takes laser as the information carrier has become a new trend of the development of communication technology due to its high communication speed,large communication capacity,strong anti-interference ability and good secrecy.In the laser communication system,the performance of the precision tracking system directly determines the real-time and stability of the laser link.The four quadrant detector(QD),a position detector with high sensitivity and high resolution,is used as precision tracking detector to extract the centroid position of light spot in this topic.Affected by the background light noise,the weak beacon optical signals received by QD are usually submerged in a large amount of noises and are hard to detect.To improve the detection accuracy of QD,we need to extract weak light signals from strong background noise.According to the characteristics that the useful signal and noise signal are both in the low frequency section,a dual channel digital lock-in amplifier based on FPGA is applied to spot detection system in this paper.The basic principle of four quadrant detector and lock-in amplifier are studied at first.Then we analyzed the feasibility of applying dual channel digital lock-in amplifier to the spot detection system and its suppression ability on different kinds of noise.According to the basic structure of the dual channel digital lock-in amplifier,the hardware circuit system and the FPGA data processing module for dual channel digital lock-in amplifier are designed.In order to verify the dual channel digital lock-in amplifier can effectively enhance the spot detection system's ability to suppress low-frequency noise,the experimental platform for spot detection system is built.By comparing with traditional method of direct amplification,it is proved that whether the carrier signal of dual channel digital lock-in amplifier is sinusoidal or square wave,it can both effectively suppress low frequency background noise.Experimental results show that the precision of the spot position detection system using dual channel digital lock-in amplifier can reach 0.2?m which satisfies the system's requirement for spot detection accuracy.