Photon-counting laser interferometer for absolute distance measurement on rough surface

We designed a dual-wavelength photon-counting laser interferometer for absolute distance measurement of noncooperative targets. The weak optical interference on the rough surface was measured by a single-photon detector. The range of nonambiguity of the dual-wavelength interferometer was less than 1.2 μm, as the maximum errors of L_g and L_r were 7.8 nm and 9.1 nm caused by the photon-counting measurement and the frequency shift of the two unlocked lasers. We integrated laser triangulation into the interferometer as a coarse measurement, thus increasing the range of nonambiguity to 6.5 mm. As a result, a measurement standard deviation of ∼18 nm was achieved within a range of 1.1 mm in the experiment.