| 摘要 |
PURPOSE:To obtain a stable displacement signal by arranging a parallel plate element and an incident face of a protection layer of a photodetector while being tilted with respect to an optical axis of a reflected luminous flux in opposite direction so as to prevent the interference of the luminous flux in the photodetector and correct coma aberration of the reflected luminous flux. CONSTITUTION:The incident face 7a-a of the protection layer 7a of the photodetector 7 is tilted to the optical axis of the reflected luminous flux 6 in the opposite direction to the parallel plate element 3. After the reflected luminous flux 6 is made incident in the protection layer 7a of the photodetector 7, the result reaches the photodetecting face 7b-b of the photodetection layer 7b, a part is absorbed by the photodetection layer 7b and converted into an electric signal. The remaining part is reflected in the photodetection face 7b-b, returned to the incident face 7a-a of the protection layer 7a, reflected again and comes to the photodetection face 7b-b, but a spot is formed to a position different from the spot formed at first. Thus, the interference of the luminous flux in the photodetector is prevented and the excellent spot without coma aberration is formed on the photodetection face and a stable displacement signal is obtained. |
