A resolution improvement of measuring the pitch deviation of a diffraction scale grating based on the change in angles of diffraction of diffracted beams

Abstract

An investigation is carried out through experiments and numerical calculations for the improvement of the resolution of measuring the pitch deviation of a diffraction scale grating with the diffracted beams. Since the changes in angles of diffraction of the diffracted beams detected by an optical head are utilized to evaluate the pitch deviation in the method, the sensitivity of the optical head directly affects the resolution for measurement of the pitch deviation. Meanwhile, in the previous studies, the resolution of a laser-autocollimation unit in the optical head for the detection of the change in the angle of diffraction of a diffracted beam has been limited to approximately 0.1 arc-second; this has been a bottleneck in increasing the resolution for measurement of the pitch deviation. In this paper, an investigation using a prototype optical head is first conducted in preliminary experiments to clarify the cause of the degradation of the resolution for measurement of the pitch deviation. Numerical calculations are also carried out to confirm the influence of the multiple longitudinal modes (MLM) of a laser beam, which is suggested as the main cause of the degradation of resolution in the results of preliminary experiments. Based on the results of experiments and numerical calculations, an attempt is made to employ a laser beam having a single longitudinal mode (SLM) in the optical head. The pitch deviation of a scale grating with a nominal pitch of 4 μm is measured by a prototype optical head with an SLM laser to verify the improvement of the resolution for measurement of the pitch deviation.

Keywords

• diffraction scale grating
• pitch deviation
• angle of diffraction
• resolution
• longitudinal mode