Reflective-type optical encoder based on fractional Talbot self-image effect using phase grating

This study presents a reflective-type optical encoder based on fractional Talbot self-image effect using a phase grating. An amplitude grating image is produced by the phase grating at a quarter or three quarters of Talbot distance. The encoder has the advantages of a large gap and more light efficiency. This work use a 850nm Light-Emitting Diode (LED) as light source, the light source will be collimated by a collimator, and incident on a phase grating by 45 degrees angle. The fractional Talbot self-image of phase grating is produced and projected on an amplitude index grating. The Moire interference is occurred when the both gratings displaced from each other. The encoder includes a reflective-type phase-grating scale with period 20 mu m and an index-amplitude grating. The gap between the phase index grating and mail scale was increased to three quarters of Talbot distance. The tolerance of gap between the index and main grating is 0.1 mm. The tolerance of yaw motion is +/- 0.25 degrees.