1. Field of the Invention
The present invention relates to an actuator used with an optical pickup, and more particularly, to an actuator used with an optical pickup having a plurality of objective lenses so that a plurality of optical discs having different recording densities can be recorded and/or reproduced.
2. Description of the Related Art
It is well-known that digital versatile discs (DVDs) are recorded and/or reproduced using light having a wavelength of 650 nm (or 635 nm) and using an objective lens having a numeral aperture of 0.6 (0.65 in the case of a recordable disc). In the case of DVDs having a diameter of 120 mm and a track pitch of 0.74 μm, DVDs have a capacity of 4.7 or more gigabytes for a single surface. Accordingly, DVDs are not suitable for a recording medium on which high definition (HD) moving picture information is to be recorded. This is the reason that a recording capacity of 23 or more gigabytes for a single surface is required so that 135-minute HD moving picture information can be recorded on the recording medium.
To meet the need of a high density recording capacity, high density optical discs using light having a wavelength shorter than that of a red color, i.e., blue light, and an objective lens having a numerical aperture greater than 0.6 and having a more narrow track have been developed and standardized.
In order to obtain an adequate tolerance with respect to tilt of optical discs, as the numerical aperture of an objective lens increases for high density, the thickness of optical discs should be reduced. Considering allowable tolerance with respect to the tilt of optical discs, the 1.2 mm thickness of compact discs (CDs) is reduced to 0.6 mm for DVDs, and there are further possibilities that the thickness of blu-ray discs (BDs) is reduced to 0.1 mm. Regarding the numerical aperture of an objective lens, the 0.45 numerical aperture of CDs is increased to 0.6 for DVDs, and there are further possibilities that the numerical aperture for BDs is 0.85. Also, considering a recording capacity, there are good possibilities of adopting a light source such as a blue light source for high density discs, i.e., BD and HD DVD. Here, high density discs include a BD and a high definition DVD (HD DVD). The thickness of HD DVDs is 0.6 mm. The numerical aperture for HD DVDs is 0.65. In the development of optical discs with new specifications, compatibility with existing optical discs is important.
However, a special technique is required to design and manufacture an objective lens having a high numerical aperture of about 0.85 as a single lens. Besides, it is difficult to make the working distance of an objective lens having a high numerical aperture be as long as that of an objective lens for DVDs.
Accordingly, to solve the working distance problem, in a compatible optical pickup in which optical discs having high densities can be recorded and/or reproduced, at least one objective lens used to record and/or reproduce CDs and/or DVDs, and an objective lens for high density recording having a higher numerical aperture than the previous objective lens should be separately provided.
For driving in both directions, i.e., a focusing direction and a tracking direction, an actuator for an optical pickup has a magnetic circuit. The actuator maintains an interval between an optical disc and an objective lens in the focusing direction and moves the objective lens in a desired track position (center of a track). As described above, however, the above-mentioned optical pickup adopting a plurality of optical discs having different recording densities requires objective lenses corresponding to the plurality of optical discs having different recording densities, and thus an actuator used with an optical pickup having a plurality of objective lenses loads a plurality of objective lenses on a moving portion so as to move in the focusing and tracking directions.
Referring to FIG. 1, an actuator disclosed in Japanese Patent Publication No. Hei 10-106001 includes a lens holder (bobbin: 7) at which two objective lenses 2 and 3 are installed, a rotation axis 1 which rotatively guides the lens holder 7, and a magnetic circuit which drives the objective lenses 2 and 3 in focusing and tracking directions.
The two objective lenses 2 and 3 are arranged at a predetermined angle θ with respect to the center of the rotation axis 1 and rotate simultaneously at the predetermined angle θ with respect to the rotation axis 1.
The magnetic circuit includes focusing and tracking coils 8 and 9 installed at the lens holder 7, yokes 5a and 5b, and a plurality of magnets 6. The plurality of magnets 6 are arranged at an angle greater than the predetermined angle of the objective lenses 2 and 3 with respect to the rotation axis 1. The tracking coil 9 is provided to correspond to the magnets 6.
In a conventional actuator having the above structure as illustrated in FIG. 1, the two objective lenses 2 and 3, arranged at the predetermined angle θ with respect to the rotation axis 1 within one lens holder 7, rotate simultaneously by the predetermined angle θ with respect to the rotation axis 1 such that a plurality of optical discs having different recording densities and thicknesses can be recorded and/or reproduced. Detailed descriptions of the conventional actuator illustrated in FIG. 1 and disclosed in Japanese Patent Publication No. Hei 10-106001 will be omitted.
Since the above conventional actuator requires a complicated magnetic circuit to control the positions of the objective lenses 2 and 3 before and after the objective lenses 2 and 3 rotate simultaneously by the predetermined angle θ with respect to the rotation axis 1, and an additional structure in which the objective lenses 2 and 3 rotate simultaneously by the predetermined angle θ with respect to the rotation axis 1, the entire structure of the conventional actuator is complicated.
Since the above conventional actuator presents an axis perturbation type design, sensitivity is relatively low and non-linear properties are relatively large, the above conventional actuator is less suitable to record and/or reproduce high density optical discs than DVDs.
Moreover, in the above conventional actuator, compared to the structure of an existing actuator in which one objective lens is installed in a bobbin, the moving portion of the above conventional actuator is large, and heavier, and thus the actuator sensitivity is further degraded.