| 摘要 |
PURPOSE:To realize a small-sized structure capable of detecting an in-focus state accurately all the time by causing only the circumferential luminous flux of luminous flux reflected by an irradiated body. CONSTITUTION:Through a collimator lens 22, a P-polarized beam radiated from a laser beam source 21 is condensed into parallel luminous flux, with which a video disk 26 is irradiated as a spot through a polarizing prism 23, a quarter-wavelength plate 24 and an objective lens 26. The light reflected by the disk 26 is condensed by the objective lens 25, changed into S-polarized light through the quarter-wavelength plate 24, and reflected by the polarizing prism 23 to strike a detection prism 27. The reflecting surface 27a of the detection prism 27 is set nearly at a critical angle to the parallel luminous flux in an in-focus state. The circumferential luminous flux of the luminous flux reflected by the reflecting surface 27a is photodetected by a photodetector 28 which has photodetection areas 28A and 28B separated mutually. An output difference between the photodetection areas 28A and 28B is found by a differential amplifier 29 to obtain a focus error signal. |
