We present a laser interferometer for displacement measurement requiring high precision. Consisting of a small number of optical parts, the instrument is simple, compact, and not affected by environmental conditions. Further, with a birefringent double-focus lens, the two interfering beams share a common path and the interference signal is immune to fluctuations of the optical path length. Peak-to-peak noise in a dc to 1 kHz bandwidth is less than 0.02 Å. We applied this interferometer to scanning force microscopy and tested the performance.
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