| 摘要 |
PURPOSE:To obtain a low-frequency beat signal with a simple constitution under a condition where such a feature of an optical system that it can be adjusted easily and it can use a plurality of light rays of different wavelengths is maintained. CONSTITUTION:A luminous flux from a white light source 10 is split into two parts through a diffraction grating 14 and the two luminous fluxes are made incident to an acoustooptical modulation element (AOM) 17 at prescribed crossing angles through relay optical systems 15a and 15b. Since driving signals SF1 and SF2 having frequencies which are slightly different from each other are supplied to the AOM 17 from the opposite direction, diffracted light rays L1(1) and L2(-1) having a small frequency difference between them are emitted from the AOM 17. The positions of diffraction grating marks RM and WM are detected by the heterodyne interference method by irradiating the marks RM and WM with the diffracted light rays L1(1) and L1(-1) at a prescribed crossing angle through an objective lens 38, etc. |
