Optimized all-fiber laser Doppler velocimeter with large depth of field

Abstract

• The structure of the existing laser Doppler velocimeter has been improved to measure a greater depth of field: 0–25 m. • For the first time, the changes of the echo signal strength of the laser Doppler velocimeter at different distances were tested and given graphically, and the signal-to-noise ratio is higher than 15 dB within the whole detection range. • An apparatus for calibrating the speed accuracy of all fiber laser Doppler velocimeter was designed and made. The experimental results showed that the speed accuracy was better than ±0.1 m/s at different distances. Due to the low spatial coupling efficiency of optical fiber and large energy loss, the depth of field in the all-fiber laser Doppler velocimeter is short, and the focus needs to be adjusted before measurement, so it is impossible to continuously measure the velocity of object moving along the optical axis. To solve this problem, an all-fiber Doppler velocimeter based on the Mach-Zehnder interferometer structure is designed, also, in the optical antenna section, C-lens is used to change the divergence angle of the light coming out of the optical fiber. The optimized design achieves the detection with large depth of field. The experiment is designed to verify the intensity of the echo signal, signal-to-noise ratio (SNR) and the speed accuracy of the velocimeter taken in different depths of field. The test results show that the depth of field in this system is superior to 25 m. The SNR is stable around 16 dB with small fluctuation in the range of depth of field, and the speed accuracy is better than ±0.1 m/s.