| 摘要 |
PURPOSE:To obtain an accurate focal point error signal, by making the reflected light of the beams condensed on a subject to be irradiated incident to an optical member having a reflective surface which is set almost at the critical angle to a beam in the reflected light flux and then detecting the change of light volume of the reflected light or the transmitted light. CONSTITUTION:The P polarized beams emitted from a laser beam source 21 are condensed on a disk 26 via a collimator lens 22, a polarizing prism 23, a 1/4 wavelength plate 24 and an objective lens 25 respectively. The reflected light goes backward and is reflected by the prism 23 to be made incident to a detecting prism 27. A part of the incident beam is reflected on a reflective surface 27a which is set almost at the critical angle to the incident beam to be made incident to a photodetector 28. At the same time, the residual beams transmit through the prism 27 and are made incident to a photodetector 30. The output of the detector 30 applies an information signal via an amplifier 31; while the output of the detector 28 is compared with the set value through a focal point error detecting circuit 31 to deliver an accurate focal point error signal. |
