Validation of pitch deviation measurement of a diffraction scale grating using robust and highly-sensitive optical angle sensors

A method for evaluating the pitch deviation of a diffraction scale grating based on the changes in the angles of diffraction of diffracted beams is a promising one for measurement of the pitch deviation over the entire length of the scale. In this method, differential operations with the readouts from two laser autocollimation (LA) units are performed so that the influences of the angular error motion of the scale scanning and the scale local slope can be reduced. Meanwhile, the sensitivity of each LA unit should be accurately calibrated, since the influence of the sensitivity fluctuation will become apparent, especially under the condition where the nominal grating pitch is much larger with respect to the light wavelength of the laser beam for measurement. Also, the verification of the pitch deviation obtained by the proposed method remains an issue to be addressed. In this paper, at first, the allowable sensitivity fluctuation of the LA unit is estimated based on numerical calculations. After that, by using the prototype optical system, the sensitivity fluctuations of LA units are evaluated in experiments. Furthermore, the pitch deviation of a diffraction scale grating with a nominal pitch of 4 μm is evaluated by the proposed method, and the accumulated pitch deviation is compared with the calibration curve provided by a scale manufacturer, as well as the calibration result obtained by a home-made calibration system with a laser interferometer.