OPTICAL DISK APPARATUS WITH DIFFERENT TRANSMISSION FACTORS FOR POLARIZED LIGHT
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention
[0002] The present invention relates to an optical disk apparatus, and in particular to apparatus provided with an objective lens system of a high numerical aperture (NA) and with a light source of short wavelength for an optical pickup.
[0003] 2. Description of the Related Art
[0004] As known in the art, one way for achieving highdensity data recording or high-density data reading with respect to an optical disk, is to reduce the diameter of the light spot formed on the recording layer of the optical disk. There are two options for effecting the diameter reduction. The first option is to decrease the wavelength of the irradiating light (by using a blue laser diode, for example) The second option is to increase the NA of the objective lens system.
[0005] Regarding the first option, a blue laser diode or LD (wavelength: 400-405 nm) used for the light source is unpreferable to a red LD (wavelength: 660-685 nm) since the sensitivity of a detector may deteriorate by about 40%. Disadvantageously, the reduced sensitivity entails the decline of S/N ratio for reproduction signals. Besides, light of a shorter wavelength is more likely to be absorbed by transparent components of the disk apparatus, such as lenses and light splitting prisms. This may also result in the decline of S/N ratio for reproduction signals.
[0006] Regarding the second option, an objective lens system having a great NA causes light to converge sharply, as shown in FIG. 1, in comparison with a small NA system. The sharp conversion of light, however, can make it difficult to keep the information about the polarization of light passing through transparent components (such as an objective lens or a coil carriage facing the storage disk) or being reflected on an optical disk. Such insufficient information about polarization of light will lead to the deterioration of the S/N ratio for reproduction signals.
SUMMARY OF THE INVENTION
[0007] The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide an optical disk apparatus that makes use of an objective lens of a great NAbut still can enjoy a good S/N ratio for reproduction signals.
[0008] According to a first aspect of the present invention, there is provided an optical disk apparatus including: a light source for emitting light in a prescribed wavelength range; an objective lens system for causing the light emitted from the light source to be focused onto a storage disk; and a reproduction signal system for causing return light from the storage disk to be conduced to a detector. The light emitted from the light source has a direction of polarization parallel to a recoding track of the storage disk. The objective lens system has transmission factors for s-polarized light and p-polarized light that are defined relative to the objective lens system. The transmission factor for s-polarized light is greater than the transmission factor for p-polarized light.
[0009] Preferably, the objective lens system may include an objective lens and a transparent member located between the objective lens and the storage disk. The objective lens and/or the transparent member is formed with a transmission control film.
[0010] According to a second aspect of the present invention, there is provided an optical disk apparatus including: a light source for emitting light in a prescribed wavelength range; an objective lens system for causing the light emitted from the light source to be focused onto a storage disk; and a reproduction signal system for causing return light from the storage disk to be conducted to a detector. The light emitted from the light source has a direction of polarization perpendicular to a recoding track of the storage disk. The objective lens system has transmission factors for s-polarized light and p-polarized light that are defined relative to the objective lens system. The transmission factor for p-polarized light is greater than the transmission factor for s-polarized light.
[0011] According to a third aspect of the present invention, there is provided an optical disk apparatus including: a light source; an objective lens system for causing light emitted from the light source to be focused onto a storage disk; and a reproduction signal system for causing return light from the storage disk to be conducted to a detector, the reproduction signal system being provided with a transparent member having an incident surface and an exit surface. The incident surface and/or the exit surface is an inclined surface that is slant relative to the return light and covered by a dielectric layer.
[0012] Preferably, the return light may include a p-polarized component and an s-polarized component that are defined relative to the inclined surface. The transparent member may be provided with a plurality of return light irradiating regions having different transmission factors resulting from the dielectric layer.
[0013] Preferably, the light emitted from the light source may have a direction of polarization parallel to a recording track of the storage disk. The return light irradiating regions may include first marginal regions spaced from each other in a direction corresponding to a track-crossing direction and a second marginal region different from the first marginal regions. The first marginal regions are smaller in transmission factor for polarized light than the second marginal region.
[0014] Preferably, the light emitted from the light source may have a direction of polarization perpendicular to a recording track of the storage disk. The return light irradiating regions may include first marginal regions spaced from each other in a direction corresponding to a track-crossing direction and a second marginal region different from the first marginal regions. The second marginal region may be smaller in transmission factor for polarized light than the first marginal regions.
[0015] Preferably, the transparent member may be provided with a plurality of regions having different P-S phase differences with respect to p-polarized and s-polarized components defined relative to the inclined surface.
[0016] Preferably, the plurality of regions may include a first region corresponding to a center of the return light, second regions spaced in a direction corresponding to a
