Double-diffraction planar encoder by conjugate optics

A planar encoder using conjugate optics is proposed for sens- ing the 2-D displacement of a 2-D grating. A Doppler frequency shift of diffracted light is generated when the grating moves. The optical conju- gate path can compensate for the error arising from the relative tilt be- tween the optical head and the scale (head-to-scale tilt). Additionally, the optical head is easily integrated, having high tolerance in component-to- component placement. The 2-D displacement system with the 2-D grat- ing, which has period of 1.6 mm in both the X and Y directions, provides a signal period of 0.4 mm by using a double-diffraction configuration. This system and associated electronics provide interpolation with a factor of 400, corresponding to a measurement resolution of 1 nm. © 2005 Society