Optical encoder based on the fractional Talbot effect

This paper presents a reflective optical encoder based on the Talbot self-image of binary phase grating. The proposed encoder has several advantages in comparison to conventional Moiré effect based encoders, including larger gap between gratings, higher gap tolerance and greater light passage efficiency. This encoder with gap 220 μm can accurately measure the position of linear stages which move at high-speed. The encoder includes a grating scale with period 20 μm and provides a signal period of 20 μm with a frequency response of approximately 200 kHz. This system and the associated electronics provide an interpolation factor of 200 that gives a resolution of 0.1 μm. The repeatability is 0.2 μm for a traveling length of 600 mm.