The present invention relates to an optical system used to record data to and/or reproduce data from one or more types of optical discs having different thicknesses of cover layers and/or different data recording densities.
There are various types of optical discs such as a CD (compact disc) and a DVD (digital versatile disc). The DVD has the cover layer thinner than that of the CD, and has a data recording density higher than that of the CD. For supporting both of the CD and DVD, the optical system for optical discs (i.e. an optical pick-up) is required to correct a spherical aberration which changes depending on the thickness of the cover layer of an optical disc being used and to change NA (numerical aperture) of a light beam so as to attain an effective beam diameter suitable for recording/reproducing operation of the optical disc being used.
In general, the optical system for optical discs has a light source for emitting the light beam, a coupling lens, and an objective lens. The coupling lens has the function of collimating the light beam emitted by the light source, or changing the degree of divergence of the light beam emitted by the light source. That is, the coupling lens is employed in the optical system to suppress aberrations or to enhance the efficiency of use of light.
There is a demand for decreasing the number of optical components in the optical system to decrease the cost of an optical pick-up and to further downsize the optical pick-up. Each of Japanese Patent Provisional Publications No. HEI 8-62496, HEI 8-334686, SHO 64-25113 and HEI 2-223906 discloses an optical pick-up which does not require the coupling lens. The optical system disclosed in each of the publications is configured to form a relatively large beam spot suitable for an optical disc having a relatively low recoding density and having a relatively thick cover layer (e.g. CD or CD-R). Since the optical system has a relatively small NA, the relatively large beam spot is attained.
It is noted that aberrations are caused in the optical system disclosed in each of the publications if a position of the objective lens is changed with respect to a position of the light source. More specifically, a spherical aberration is caused when the objective lens is moved in a direction of its central axis (optical axis) for a focusing operation. A coma and astigmatism are caused when the objective lens is moved in a direction perpendicular to the optical axis from a reference axis for a tracking operation because in this case off-axis light is incident on the objective lens.
As used herein, the term reference axis represents an axis including the optical axis of the objective lens and passing through the light source in a condition in which the objective lens is not moved by the tracking operation in the direction perpendicular to the optical axis. Also, the term “reference position” of the objective lens is defined as an initial position of the tracking operation.
It is required that aberrations are sufficiently suppressed for performing the recording/reproducing operation with high accuracy. In particular, aberrations having an asymmetric property such as a coma and astigmatism are main factors that deteriorate quality of a recording (optical) signal and a reproducing (optical) signal. Therefore, it is particularly required that the optical system is configured to sufficiently suppress the aberrations having an asymmetric property.
Hereafter, the term “tracking shift movement” represents movement of the objective lens in the direction (i.e. a radial direction of an optical disc) perpendicular to the optical axis due to the tracking operation, and a term “focusing shift movements” represents movement of the objective lens in the direction of the optical axis due to the focusing operation.
The coma caused in a cover layer of the optical disc when the optical disc tilts relative to the optical axis of the objective lens has such a property that it changes depending on the thickness of the cover layer. For this reason, both surfaces of the objective lens provided in the optical system disclosed in each of the publications are configured to be aspherical surfaces which cancel the coma caused in the cover layer of the optical disc. That is, the objective lens is configured as an aplanatic lens.
By contrast, sufficient correction of another aberration having an asymmetric property, i.e. astigmatism, is very difficult even if the objective lens of which both surfaces are asymmetrical surfaces is employed in the optical system. Therefore, the optical system disclosed in each of the publications can be used only for an optical disc, such as a CD, having relatively large tolerance to aberrations. That is, the optical system is specialized for an optical disc drive used for the optical disc having a relatively large tolerance to aberrations.
In other words, the optical system can not be used for an optical disc (such as a DVD) which has a relatively large recording density and requires a relatively small beam spot diameter.
Recently, optical systems which support both of the CD and DVD (hereafter, referred to as a CD/DVD-compatible optical system) have been provided. However, such a conventional CD/DVD-compatible optical system can not correct a coma which changes depending on the thickness of the cover layer of the optical disc. Further, astigmatism is not corrected in the CD/DVD compatible optical system as in the case of the optical system of each of the publications.
It is understood that the optical system disclosed in each of the publications can not be used as the CD/DVD-compatible optical system because the amount of a coma changes depending on the thickness of the cover layer of the optical disc.