Quasi-common-path laser feedback interferometry based on frequency shifting and multiplexing

We present a quasi-common-path laser feedback interferometer based on frequency shifting and multiplexing. The interferometer uses two acousto-optic modulators to shift the frequency of the target-generated feedback light by 2 Omega. A properly aligned mirror is inserted into the feedback path to generate a feedback light frequency shifted by Omega. Phase variations of the two quasi-common-path feedback light beams are simultaneously measured through heterodyne demodulation with two different reference signals. Their subtraction accurately reflects the target displacement. Under typical room conditions, the system's short-period resolution is better than 2 nm, and its 3 min displacement accuracy is 8 nm.