| 摘要 |
PURPOSE:To detect the relative displacement of a diffraction grating in the lateral direction highly accurately, by changing the length of a light path in a gap between the diffraction gratings, and using the change in signal, whose period is 1/2 the pitch of the diffraction grating, which appeares in the average value of a diffraction Moire signal. CONSTITUTION:Laser light LB is projected on a diffraction grating 1. A transparent plate 3 having staircase shaped steps is attached on a diffraction grating 2. The staircase is symmetrically attached from the center of a high refractive index material of the plate 3 so that the range of a gap G is provided from Go to Go.2P<2>/lambda. The range of the distance 2P<2>/lambda is divided into five parts. The grating 2 is divided into equal regions A and B. The phase of the grating in the region B is shifted by P/4 from the phase of the grating in the region A. The light, which is converged by a lens group 4, is detected by a photodiode group 5. Signals V1 and V2, which are obtained by additions in adders 7, are outputted. The difference PD between the signals V1 and V2 is obtained by a subtractor 8. Based on the difference PD, the lateral displacement of the diffraction gratings can be detected highly accurately. |
