A compact optical encoder with micromachined photodetector

A compact optical encoder has been fabricated using a micromachining technique for the measurement of linear displacement. The index grating for detecting the Moire signal from the superimposed gratings consists of transmission type silicon grids, which are etched through by the reactive ion plasma. An array of line photodetectors is installed on the silicon grids by ion implantation. A scale grating is illuminated by the light passing through the slits of the transmission index grating, and thus the light source can be placed just behind the index grating. Therefore the structure of the proposed optical encoder is compact. In the experiment, the second order grating imaging phenomenon under incoherent illumination has been applied to the displacement sensing. The encoder signal with a high contrast is obtained at a large air gap between the two gratings.