A laser incremental encoder with autofocus operation is proposed. In the encoder, an optical read head focuses three laser beams upon a disk with a scaling grating. The scaling grating is a reflection grating for which the height difference between adjacent pixels is one quarter of the wavelength of the laser. Each of the two laser beams is focused onto one of the two sections of the reflective grating, whereby the pitch of the grating can be detected by interferometric reflection. The third beam acts to provide autofocus and is reflected from a smooth part between the two sections of the reflective grating. Therefore the encoder is less sensitive to the gap between the optical read head and the scaling grating, and no precision adjustment is required. Also, the reading data are free from the vibration and waviness of the grating surface. In the experiments an encoder disk was fabricated from a wafer by lithographic techniques. The encoder with an 18-mm radius of the disk can reach a resolution of 216 = 65,536 pulses per revolution without electronic interpolation.
© 1998 Optical Society of America
Ji-hua Zhang and Lilong Cai, "Autofocus Laser Rotary Encoder," Appl. Opt. 37, 2691-2695 (1998)