Optical EngineeringTwo-grating interferometer with sinusoidal phase modulation for surface profile measurement
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A two-grating interferometer for measurement of profiles of polished surfaces is proposed. The ±first-order beams diffracted by the first grating produce parallel fringes of about 50-µm spacing on an object surface. By using the second grating, we eliminate the parallel fringes and extract a phase distribution produced by the surface profile of the object. Moreover, the vibration of the second grating of a small size enables us to use the sinusoidal phase-modulating interferometry. Measurement resolution is higher than 0.3 µm at the equivalent wavelength of 35 µm.