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 . A properly aligned mirror is inserted into the feedback path to generate a feedback light frequency shifted by Ω. 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 , and its displacement accuracy is .
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