Methods of using optical frequency comb for angle measurement are discussed. Autocollimation is the most commonly used method for angular displacement measurement. Here, a diffraction grating is attached to the measurement target instead of a mirror, which is used in the conventional autocollimation method. Each mode of the optical frequency comb with a wide wavelength bandwidth is angularly dispersed by the grating. The angular displacement of the target is determined from the most strongly detected optical frequency modes.
An optical setup for the nonorthogonal two-axis Lloyd’s mirror interferometer, which is composed of a nonorthogonal type of mirror-substrate assembly and a newly designed beam expansion assembly, is proposed to fabricate highly precise two-dimensional (2-D) diffraction scale gratings having uniform pattern structures over a wide area. The beam expansion assembly is designed in such a way that a thin collimated laser beam is at first generated by a small collimating lens to carry out a precise polarization modulation control of subdivided wavefronts in the beam, followed by the generation of a collimated laser beam with the expansion of the thin collimated laser beam by using a beam expander. Since a pair of high-precision half-wavelength plates made of crystalline quartz can be employed in the newly designed optical setup, a much better polarization modulation control can be expected, compared to the conventional optical setup in which low-precision large-size half-wavelength plates made of polycarbonate should have been employed. To verify the effectiveness of the proposed optical setup, theoretical calculations and some basic experiments are carried out by using a prototype optical setup to demonstrate the feasibility of the proposed optical setup for fabrication of highly precise 2-D scale gratings.
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