A Digital Phase-Lock Method for AOD Scanning Measurement System

An acoustic-optic deflector (AOD) is used as a beam scanning actuator in a heterodyne interferometer for nano-scale surface measurement. In order to obtain the phase difference between the reference signal and measurement signals, a digital phase-locking method is proposed using high speed A/D sampling and digital signal processing. Considering the measured signal frequency incident upon the photo-detector is double that of the reference signal, the latter is first squared in order to match the measured signal frequency. The phase difference can then be obtained by calculating the correlation coefficient between the two signals. Experimental results show this method is flexible and can be used in real time. The measurement results are not affected by the variation of the DC offset and amplitude of the measured signals. Error analysis also demonstrates the proposed digital phase-lock method, which is accurate with resolution of 0.25nm. It can be used to replace the lock-in amplifier which is traditionally used in this kind of application and can therefore greatly reduce the cost of the measurement system.