1. Field of the Invention
The present invention relates to an optical system of an optical pickup in an optical information recording and reproducing apparatus which records and reproduces information for optical discs having different corresponding wavelengths. More particularly, the present invention relates to an optical information recording and reproducing apparatus which allows compatibility for a plurality of optical recording mediums using laser light sources of different wavelengths, to an optical pickup, to an objective lens module, and to a diffractive optical element.
2. Description of Related Art
As an optical information recording and reproducing apparatus, an optical disc apparatus is known in which recorded information can be read from an optical recording medium, that is, an optical disc, such as digital versatile disc (hereinafter, referred to as DVD), compact disc (hereinafter, referred to as CD), or the like.
A compatible optical disc apparatus is known in which recorded information can be read from DVD and CD. As for DVD, the substrate thickness is 0.6 mm, the corresponding wavelength is in a range of 635 nm to 655 nm, and the numerical aperture (NA) of an objective lens is about 0.6. As for CD, the substrate thickness is 1.2 mm, the corresponding wavelength is in a range of 760 to 800 nm, and the numerical aperture of an objective lens is about 0.45. In the compatible optical disc apparatus, there is a case in which a laser light source having a wavelength λDVD in the vicinity of the wavelength 660 nm for DVD and a laser light source having a wavelength λCD in the vicinity of the wavelength 780 nm for CD are mounted.
For example, a technology is suggested in which an optical pickup device for allowing information to be recorded and reproduced for information recording mediums having different substrate thicknesses for DVD/CD, and an objective lens and an optical element used for the optical pickup device are provided (JP-A-2001-235676). The optical pickup device is suggested in which the objective lens having diffractive orbicular zones is used for the optical pickup device, such that, with an outside light flux of a predetermined numerical aperture in a use state of a small numerical aperture as a flare, recording and reproducing of information are performed for various information recording mediums having different thicknesses. The objective lens having such diffractive orbicular zones includes a diffraction surface having the diffractive orbicular zones. Here, when a function of an optical path difference of the diffraction surface is Φ(h) (where h is a distance from an optical axis), dΦ(h)/dh is a discontinuous or substantially discontinuous function at a place of a predetermined distance h.
On the other hand, as for blue-ray disc (hereinafter, referred to as BD), the thickness of a transmissive protection layer (which corresponds to the thickness of a transparent substrate of DVD or the like) is 0.1 mm, the corresponding wavelength is 408 nm, and the numerical aperture of an objective lens is about 0.85. Accordingly, in a BD/DVD/CD compatible optical disc apparatus, a laser light source which emits laser light of λBD in the vicinity of the wavelength 408 nm, that is, an optical system, needs to be mounted, in addition to the configuration of the above-described compatible optical disc apparatus. Further, since the optical discs of BD, DVD, and CD have different thicknesses, a unit for correcting three kinds of different spherical aberrations needs to be provided. In addition, since all of them have different numerical apertures, a corresponding unit also needs to be provided. However, in JP-A-2001-235676 described above, the specified descriptions of these units are not given. That is, it is difficult to realize compatibility of three or more kinds of recording mediums having different light source wavelengths, numerical apertures (effective diameters), optical disc thicknesses (the thickness of a transmissive protection layer), such as BD, DVD, CD, and the like by use of a single objective lens according to the related art.
In order to realize an optical pickup for a compatible apparatus, a method is suggested in which an objective lens exclusively used for BD and a DVD/CD compatible objective lens are used, and are switched according to wavelengths. In this case, however, since two objective lenses are used, a complex lens switching mechanism needs to be provided, which causes a problem in that manufacturing costs are increased. In addition, since an actuator is made large, it is disadvantageous to reduce the size of the apparatus. Further, a method may be considered in which an objective lens and a collimator lens are incorporated, but, since the collimator is fixed with respect to the objective lens, it may be difficult to maintain performance at the time of movement of the objective lens.
In any cases, if a plurality of light sources are used, and an optical system of exclusive prism, lens, and the like is configured in order to ensure compatibility of BD, DVD, and CD, an optical pickup or an overall optical head is complicated, and tends to have the large size.