Micro-encoder based on higher-order diffracted light interference

A micro-encoder based on higher-order diffracted light interference has been developed for use in high-precision positioning. It uses ±3-order diffracted beams to obtain interference fringes for displacement detection and provides six signal periods for moving the scale grating by one grating period (3.2 µm) without an interpolation electric circuit. This micro-encoder head includes a distributed feedback laser diode, edge-illuminated refracting-facet photodiodes, a pair of polyimide waveguides, each with a total internal reflection mirror, and a micro-grating that causes ±3-order beams, diffracted by the scale grating, to interfere coaxially. All of these components are on a micromachined Si optical bench (3 × 2 mm2).