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Laser feedback interferometry based on phase difference of orthogonally polarized lights in external birefringence cavity.

A simple and effective displacement sensor based on external birefringent feedback in Nd:YAG lasers is demonstrated. The measurement is based on the principle that, when linearly polarized light passes through the birefringent external cavity and then is fed back into laser resonator by external object, a phase difference is generated between laser sinusoidal-modulated intensities in the two orthogonal directions. These two sinusoidal intensities with lambda/2 period can be subdivided to lambda/8 after 4-fold evaluation. Moreover, the directional discrimination can be easily obtained according to the phase relationship between them. The chief advantages of the sensor are that it is compact, small size, flexible, low cost, and robust. Experimental results have shown that the standard deviation of displacement measurement is 0.093 microm in a 7 mm range and 0.34 microm in a 20mm range.