| In recent years, the laser communication has taken widespread attention because of its advantages, such as large communication capacity, strong anti-interference ability, light structure, and economic equipment, etc., and has become a research hot spot for many scholars. Spot position detection system is an important part of laser communication, and its detection accuracy will directly affect the quality of communication. Four quadrant detector(QD) as one of the commonly used feedback components of the spot position detection system, the weak conversion photocurrent is amplified to calculate the position of facula, and then feedback to the servo system to complete the precision tracking. Due to the influence of background light and circuit noise, the weak current of the photoelectric conversion mainly contains low frequency noise, which will greatly reduce the detection accuracy of QD. In addition, the working frequency of QD is also in the low frequency band, it is difficult to effectively suppress the noise by the traditional amplification technology. The lock-in amplifier moves the frequency of useful signal from low to high by frequency spectrum shift, and it has great advantages in inhibition of low-frequency noise. It can measure weak sinusoidal signal with SNR as low as 10-5 and has very high accuracy. With the development of digital technology, the lock-in amplifier is developed from the analog to the digital type. The digital lock-in amplifier has overcome the temperature drift and other non-ideal low frequency characteristics, which greatly improve the performance of the lock-in amplifier, so that research development and application of lock-in amplifier has been greatly improved.In this paper, the digital lock-in amplifier is applied to the spot position detection system. Firstly, the theory of spot position detection and lock-in amplifier are studied; Secondly, the digital lock-in amplifier system is designed and implemented, which is composed of two parts: the low noise, high gain preamplifier; and high precision data acquiring, processing system; Finally, the system is tested. The experimental results show that the digital lock-in amplifier can effectively suppress the background light noise, improve the SNR of system and meet engineering requirements. The main research contents include:1. The basic principle of position detection of Gauss spot model is studied, and the mathematical model of the position detection accuracy is derived. The research results show that the main factors affecting the accuracy of QD position detection are spot radius, centroid position and SNR of system. The most important factor is the SNR of system. Therefore, digital lock-in amplifier technology applied to light spot position detection system is proposed.2. The detection principle of the lock-in amplifier is studied, the correlation detection technique and correlation demodulation algorithm are analyzed, the modeling and simulation of the single channel and dual channel lock-in amplifiers are carried out with Matlab/simulink. It provides a theoretical basis for the design of digital lock-in amplifier system.3. A low noise, high gain preamplifier is designed, which converts the weak light current into voltage, for facilitating the acquisition of AD. The circuit is composed of three parts which are cross resistance amplifying circuit, zero- voltage adjustment circuit and active band pass filter. The open circuit noise is about 35 uV when the amplification factor is 2.5×107V/A, and is consistent with the theoretical calculation value which is 30 uV.4. High precision data acquiring and processing system is designed. The output noise is 10~20uV when the input is short, and the effective number of AD can be up to 18~19 bits. In addition, the AD controller, digital correlation demodulation, reference signal generator and digital low pass filter are designed in FPGA. And signal demodulation function is realized.5. The digital lock-in amplifier is tested by the spot position detection platform,and then compared with the traditional test method. The comparison results show that the digital lock-in amplifier can effectively suppress the background light noise, improve the SNR of system, and then improve accuracy of the spot detection. And the accuracy of the spot detection is higher than 1μm. |
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