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A simple and effective displacement sensor based on external anisotropic feedback in Nd:YAG lasers has been presented
and demonstrated. When the system operates in anisotropic feedback induced by placing a birefringence element with
phase difference about 45 degree(such as a wave plate) in the external cavity, both the laser intensities in two orthogonal
directions are sinusoidal-modulated by the external reflector with a period of half wavelength displacement, but with a
phase difference about 90 degree between them. When threshold intensity is introduced, a period of intensity fringe can
be divided into four equal zones. Each zone corresponds to &lgr;/8 displacement of the external feedback reflector.
According to the appearing sequence of the four states, the movement direction of external reflector can be discriminated.
Thus, a novel displacement sensor with a resolution of up to 133 nm, as well as a function of direction discrimination, is
believed to be achieved. The chief advantages of this sensor are that it is compact, small size, flexible, low cost, and
robust. Most importantly, this sensor has a great potential to be improved in resolution by electric subdivision methods
applied in the grating encoders. Experimental results have shown that the uncertainty (3&sgr;) of displacement measurement
is 0.2&mgr;m in a 7mm range, and the linearity is better than 2.5710-5.
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