| 摘要 |
PURPOSE:To stably obtain an autofocus error signal with a simple constitution suitable for mass production by setting specific conditional expressions to an angle made by the optical axis of a laser beam making incident on a diffraction grating and that of a photodetector which receives reflected light from a recording medium. CONSTITUTION:Light from a semiconductor laser 1 is reflected by a reflection type diffraction grating lens 16, and condensed on a disk 10 through an objective lens 18. The optical path of the reflected light is bent by a grating, and received by a four-division photodetector 14. Inequalities I and II are satisfied where the magnification of the lens 16 is denoted as (m), a laser oscillation wavelength as lambda, a lens opening part as NAO, a disk moving amount as DELTAZd, and the angle which the optical axis of the light source for the grating lens 16 and the optical axis incident on the photodetector make is denoted as theta. Consequently, the autofocus error signal is stabilized even if the oscillation wavelength changes. What this device has to do is merely to drive the lens 18, and its constitution is simple. The grating lens 16 can be mass-produced at high accuracy by projecting formation and the evaporation of a metallic film, thereby cutting down the cost. |
