Laser Doppler velocimeter with variable optical frequency shift

An opto-electronic system that allows a stable accurate frequency shift, variable from 10 kHz to 50 MHz, to be introduced in a laser Doppler velocimeter (LDV) has been developed. The system combines a fixed optical-frequency shift obtained using a Bragg cell with an electronic shift which results from mixing the detector signal with a variable local oscillator frequency phase locked to the Bragg cell driver. This paper discusses the advantages of the system in both a tracker and a counter based LDV.