| 摘要 |
PURPOSE:To prevent the error of an out-of-focus detecting signal from being generated by comparing the levels of reflected light signals from a recessed part and a projecting part provided on an information recording plane and with diameters larger than that of a spot and with stages larger than the focusing depth of the spot. CONSTITUTION:The recessed part 15 or the projecting part 16 with the diameter larger than that of the spot and with the stage larger than focusing depth of the spot is provided on the information recording plane. And reflected light from the recessed part 15 or the projecting part 16 is received by the one end 6a of an optical fiber 6, and it is radiated to an optical detector 10 provided in a fixed optical system, and the signal level of the optical detector 10 at a time when the spot exists in the recessed part 15 is compared with that of the optical detector 10 at the time when the spot exists in the projecting part 16. In such a way, it is possible to make optical adjustment for focusing unnecessary, and to prevent the error of the detecting signal from being generated even when aberration in an optical component due to the change of a temperature or vibration is generated. |
