A micro-displacement encoder using a thin-film resonant photodetector with directional sensitivity

A micro-displacement encoder based on compact Moire configuration is developed using a resonant photodetector with directional sensitivity for oblique incidence. The photodetector consists of a thin-film silicon photodiode and a resonant grating (index grating). The fabricated photodetector is directly located in a Fresnel image zone of a scale grating. Therefore, the optical arrangement of the proposed encoder is simple. Using a guided-mode resonance, the photodetector detects selectively the interference of the ± 1st-order beams diffracted from the scale grating, while suppressing the sensitivity for the 0th-order beam. The 0th-order beam causes undesired signal modulation for the air gap fluctuation of the encoder. In the experiment, sub-micron resolution and robustness to the air gap variation have been confirmed. The directional sensitivity and the encoder performances have also been analyzed theoretically.