| 摘要 |
PURPOSE:To obtain an electric signal having high S/N through a photodetector by forming an opening part to a diffraction grating holder to make only the light inside the luminous flux corresponding to the numerical aperture of a collimator lens incident on this lens and to cut off other light through a light shielding part. CONSTITUTION:The light delivered from a semiconductor laser light source 2 is made incident on a diffraction grating 4 and divided into a main beam and two secondary beams. While the light passed through the grating 4 are turned into parallel beams through a collimator lens 6 and transmitted through a beam splitter 8. These beams are focused on an optical disk 12 through an objective lens 10. The modulated light which is reflected by the disk 12 travels adversely the optical path and is separated by a splitter 8. These separated beams are led to a photodetector 14 and turned into the electric signals. here an opening part 21 is formed to a diffraction grating holder 20 so that only the light inside the luminous flux corresponding to the numerical aperture of the lens is made incident on the lens 6. While other light is cut off by a light shielding part 23. |
