| 摘要 |
PURPOSE:To attain to enhance resolving power enhancing an S/N ratio, by providing such a focus servo mechanism that the diameter of a beam spot formed on a slit as an image becomes always constant. CONSTITUTION:Light beam is allowed to irradiate a rotary disc 23 through an optical system and an object lens 28 to which a drive mechanism capable of regulating the focus thereof is attached and the reflected beam is received by a plurality of divided beam receiving elements 32 while the outputs of the divided light receiving elements 32 are synthesized to form a control signal. Next, the drive mechanism of the object lens 28 is driven by using said control signal to obtain a beam spot with a definite diameter on the rotary disc 23 while the control signal is subjected to wave form shaping to take out an output pulse. Because the distance between the lens 28 and the disc 23 is kept constant, the diameter of the beam spot does not change even if the slit of the disc 23 is displaced by an X-amount. Hereupon, if the spot diameter is made min., max. resolving power is obtained. Therefore, S/N and resolving power can be enhanced. |
