| 摘要 |
PURPOSE:To obtain an inexpensive focal point detector having high sensitivity and a facilitated production, by forming both a beam splitter that leads the reflected light of the beams focused on a subject to be irradiated to a photodetector and a detecting prism into a body. CONSTITUTION:The beams (linearly polarized within the paper surface) emitted from a laser beam source 1 are focused on a disk 6 including an information track via a collimator lens 2, a block 14, a beam splitting surface 3a, a block 13, a 1/4 wavelength plate 4 and an objective lens 5 respectively. The reflected light is made incident to the block 13 via the plate 4 and reflected on the surface 3a and a reflecting surface 15 to be received at a photodetector 12. The surface 15 is set equal to or little smaller than the critical angle to the incident light in a focusing state. Thus the volume of incident light varies to light receiving regions 12A and 12B if the disk 6 has a displacement toward the direction (a) or (b) and becomes out of focus. In such way, an inexpensive focus detector can be obtained with high sensitivity and a facilitated production. |
