| 摘要 |
PURPOSE:To improve the reliability by providing a parallel plate type dual grating in which a spherical lens is clipped between the 1st and 2nd diffraction gratings and providing a photodetector to a focus of the transmitted light and the diffracted light therethrough respectively. CONSTITUTION:The light made incident in the 1st diffraction grating 17 of the parallel plate type dual grating 16 is separated into two lights, transmitted light TP and a diffracted light KP, they are made incident in a spherical lens 19 and made incident in the 2nd diffraction grating 18 after through it. The grating 18 acts like a cylindrical lens and then most of the light TP even through the grating 18 is the transmitted light TP, collected according to the light collecting capability of the lens 19, the light KP is collected unidirectionally to cause astigmatism. Thus, when the 1st photodetector 20 having a 2-split light receiving face is arranged to a light collection point P on the optical path of the light TP transmitted through the grating 18, the difference from the light received quantity is obtained to detect a track error signal. Moreover, the 2nd photodetector 21 is arranged, the difference of the received lights is obtained to detect a focus error. |
