Improving SNR of Doppler signal and expanding dynamic measurement range of single-mode VCSEL self-mixing laser Doppler velocimetry

Abstract

The experiments show that the single-mode vertical-cavity surface-emitting laser (VCSEL) Laser Doppler Velocimetry (LDV) is better than the multilongitudinal-mode laser diode LDV in many characteristics, such as the accuracy of velocity measurement and the temperature range of the laser. Because the output power of the single-mode VCSEL is very low and only 0.7mW, the backscattered light received by the photodiode in the laser house is so weak that the signal-to-noise ratio (SNR) of Doppler signal is low. And the Doppler signal spectrum width spread and amplitude modulation envelope badly influence the velocity measurement accuracy. Analog phase-locked loop (PLL) has many characteristics, such as narrow band tracking filter, locking the signal with peak voltage and high rejection ratio of amplitude modulation. Using the analog PLL, the SNR of Doppler signal and the velocity measurement accuracy can be improved obviously. But because the locking range of the common analog PLL is narrow, we use difference frequency analog PLL to expand the locking range, and the dynamic range of velocity measurement can be greatly expanded. As a result, the velocity measurement accuracy of the single-mode VCSEL self-mixing LDV is better than 1% when the velocity range is from 30mm/s to 480mm/s.