| Summary: | 碩士 === 國立臺灣科技大學 === 機械工程系 === 107 === Laser interferometers have come to be broadly utilized in the fields of semiconductor industry, precision machinery industry, microscopic techniques, optical alignment systems, and medical research. According to the signal detection scheme, interferometers can be categorized into the types of homodyne and heterodyne. In comparison to homodyne detection interferometer, heterodyne detection interferometer proves a superior anti-noise ability. If high frequency noise signal is considered, nanometer resolution is easily being achieved by using heterodyne detection interferometer. Several heterodyne interferometers have been developed to provide exact displacement information with high resolution. However, the problem of poor measurement stability caused by the unstable wavelength of the light source is still difficult to avoid. Grating interferometry, one of the well-known interferometries, is proposed to overcome the problems resulting from the unstable wavelength of the light source. Many methods based on the measurement principle of grating interferometry have been carried out to measure displacement with high system stability.
In this study, a laser interferometer based on the techniques of heterodyne sinusoidal modulation and double-diffraction is proposed for displacement measurement. This interferometer has the advantages of grating interferometry, heterodyne interferometry, and double-diffraction technique. According to the optical configuration of our proposed interferometer, a novel heterodyne light source is obtained by using a sinusoidal modulation method to modulate a Wollaston prism. The heterodyne light source is then passed through a beam splitter, and the resulting transmission beam moves to the diffraction grating, and is diffracted. The diffracted beams are reflected back into the diffraction grating along the original optical path so that the grating phase is introduced into the diffracted beams two times, forming a double-diffraction optical configuration. System sensitivity and resolution can be effectively enhanced by using this method. The second diffraction also causes the two beams to overlap to form interference, and after passing through an analyzer, the interference signal can be received by a photodetector. The in-plane displacement of the grating can be obtained by calculating the phase variation of the interference signal.
In order to verify the measurement performance of the proposed measurement technique, several experiments are conducted and compared the results obtained from our double-diffraction interferometer with a commercial measuring instrument. The experiment results show that this double-diffraction interferometer is able to measure displacement with the resolution of 10 nm. This double-diffraction interferometer can be widely used in the fields of precision positioning systems, laser interference lithography, near-field optical microscopy, and automated optical detection.
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