The Design Of Fast THz-TDS Optical Delay Line And The Research On Digital Locked-in Amplifier Technology

In recent years,doing researches on THz has gradually become a hot trend among the institutes and universities both in China and abroad.In the THz-TDS technique,the detection speed and the signal-to-noise ratio of THz pulse signals are two main factors which influence the THz imaging and THz detection performance,so working to improve these two has significant application value.There are two parts in this article: the design of fast optical delay line device and the design of phase shifting compensation adaptive digital phase-locked amplifier algorithm.In order to improve the detection speed of the THz pulse signal,a fast optical delay line device is designed on the basis of the principle of involutes.The delay distance of this device is 40.036 mm while the delay time is 133.45 ps.The errors of linearity and stability are 0.419% and 0.806%.The scanning frequency reaches 20 Hz.The orthogonal vector type phase shifting compensation adaptive digital phase-locked amplifier algorithm is proposed to improve the performance of phase-locked amplifier.The signal-to-noise ratio of the THz signals collected reaches over 68 dB.In this paper,the main work and innovation points are as follows:1?According to the structure of photoconductive THz-TDS system,a fast optical delay line device on the basis of the principle of involutes is innovatively proposed after the analysis on the principle of equivalent time sampling.A mathematical model is built upon the analyzed basic principle of the delay line.The distance formula of the optical delay line device is deduced.The structural design of the delay device,the finite element model of rotating shaft one and the eccentricity errors between the rotating shaft and the involutes base circle are respectively analyzed.The light spot distortion resulted from the incident light going through the delay line device is theoretically and experimentally analyzed.2?Experiments are conducted on the built Michelson interference system to test the specifications of the device,including delay linearity,delay stability,delay distance and scanning frequency.Compared with common optical delay line,the device proposed in this paper provides with longer delay distance and higher scanning frequency.At the same time,it also has advantages in scanning linearity and stability.3?According to the principle of the PSD phase-locked amplifier,phase shifting compensation adaptive digital phase-locked amplifier algorithm is innovatively proposed to compensate the phase-shift error of the orthogonal vector type phase-locked amplifier.The oversampling method is applied to improve the ADC resolution and signal-to-noise ratio of the recovery signal.4?The phase shifting compensation adaptive digital phase-locked amplifier algorithm is used to recover the signal and verify the theory.The algorithm is verified by virtue of signal generator which is adopted to simulate the weak sinusoidal signal and the 8-f confocal THz-TDS system in terahertz research center of Tianjin University.Experimental results show that the signal-to-noise ratio of the THz recovery signal is obviously better than that of the signal collected from the analog phase-locked amplifier system.