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Laser interferometry is one of the most useful measurement techniques due to its advantages of long measurement range, high measurement resolution, and flexible arrangement. Various kinds of laser interferometers are designed according to their application fields or corresponding objectives such as for measuring displacement, velocity, refractive index, wafer warpage, or geometry errors. According to the measurement principles, interferometers can be classified into two measurement types, wavelength-based and grating-based. The problem of poor measurement stability caused by the unstable wavelength of the light source is difficult to avoid. This problem might affect the accuracy of measurement results obtained from interferometer if compensation technique of unstable wavelength light source is not utilized. Grating-based laser interferometer, also known as laser encoder, is proposed to overcome the problem caused by the unstable wavelength due to the phase variation of grating-based interferometer is independent from the wavelength of the light source. Many studies based on the measurement principle of grating-based interferometer have been carried out to measure displacement precisely. Each method comes with its own merits and limitations.
In this study, a grating-based interferometer based on recurring-diffraction technique is proposed for displacement measurement. This interferometer has the merits of grating interferometry, heterodyne interferometry, and recurring-diffraction optical configuration. It has the capability of measuring in-plane displacement with high resolution and stability. The proposed system takes advantage of a “recurring-diffraction” optical configuration, which directs diffracted light to pass through two gratings triple times without additional optical components, thereby enhancing the phase change induced by grating displacement, effectively improving the resolution of the grating-based interferometer. Both the feasibility and performance of the proposed grating-based interferometer have been addressed and demonstrated in several experiments. The experimental results show that our proposed interferometer is capable of measuring in-plane displacement with the resolution of 2 nm, and the repeatability better than 1 nm. The proposed grating-based interferometer has excellent measurement properties, and is well-suited for applications within precision manufacturing, tool machine industry, automatic optical measurement, nanotechnology, semi-conductor technology and other related fields. Compared with other measurement instructions, this interferometer has the advantages of high resolution, high stability, relatively straightforward operation, and high flexibility.
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