In this paper, we report a novel quasi-common-path laser feedback interferometer (QLFI)
for highly stable, high-resolution and non-contact displacement measurement. QLFI measures the
displacement of the target by measuring the phase of feedback light. In addition to the
target-generated feedback light (frequency shifted by 2#), a reference mirror generates a reference
feedback light which is frequency shifted by #. The phase variations of both feedback lights are
measured by heterodyne detection simultaneously and their difference offers the phase variations
caused only by target displacement. When the optical path lengths of the reference and measuring
feedback light are nearly the same, the phase fluctuations caused by the environment and laser
instability are effectively removed. The heat-induced deformation of a He-Ne laser tube is
successfully in-line measured using QLFI.