An optical disk device has an aperture of an objective lens in an incoming path
of a beam from a semiconductor laser to an optical disk formed larger than an aperture
in a return path from the optical disk or an aperture is varied in recording and
in reproduction. This configuration improves recording/reproducing ability since
light is focused on an optical disk with high numerical aperture. In addition,
since reflected light from the optical disk is detected with low numerical aperture,
margins for tilt and defocus are not reduced. Furthermore, since unnecessary signal
components contained in the reflected light can be eliminated, a S/N (signal-to-noise
ratio) of an information signal also increases. Thus, a high-performance optical
disk device can be obtained. Alternatively, by varying the aperture of an objective
lens in recording and in reproduction, an optical disk device in which recording
density and recording quality are increased without deteriorating reproduction
quality can be obtained.
