| 摘要 |
A position detecting system is disclosed wherein lights being mutually coherent and having first and second frequencies, respectively, different from each other are produced, and the light of the first frequency is divided into a first light and a second light while the light of the second frequency is divided into a third light and a fourth light. The first, second, third and fourth lights are projected upon a diffraction grating on an object at different specific angles of incidence, respectively. A first beat signal is produced through interference between diffraction light resulting from diffraction of the first light by the diffraction grating and diffraction light resulting from diffraction of the fourth light by the diffraction grating, while a second beat signal is produced through interference between diffraction light resulting from diffraction of the second light by the diffraction grating and diffraction light resulting from diffraction of the third light by the diffraction grating. A phase difference between the first and second beat signals is detected to determine the position of the object, wherein the phases of the first and second beat signals advance oppositely with a displacement of the object.
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