Thermal Effect of Acousto-Optic Frequency Shifter and Its Impact on Optical Metrology

Acousto-optic frequency shifter (AOFS) is the key component in optical metrology, encompassing applications such as length, velocity, and frequency measurements. The acousto-optic crystal suffers from thermal accumulation during operation, leading to significant changes in its optical and physical properties. As a result, the diffracted beam of an AOFS undergoes phase and frequency drift, degrading the measurement precision of optical metrology systems. In this letter, we evaluate the path length and optical frequency fluctuation of the $1^{\text {st}}$ -order diffracted beam passing through an AOFS due to thermal effect. Our experiment reveals linear correlation between both path length drift rate and frequency drift versus temperature drift rate. The measured proportionality factor are $0.791~\mu $ m/K and 1.019 Hz/K/s, respectively. Under thermal equilibrium conditions, AOFS-contributed noise floor in distance, velocity and frequency measurement are $\sim ~100$ pm at 100 ms averaging time, $2.5~\mu $ m/s at 1 ms averaging time, and $1.56\times 10 ^{-20}$ at 10,000 s averaging time, respectively.