Recently, as well known, various types of optical recording media for recording and/or reproducing information and optical head devices capable of recording and/or reproducing an information to the optical recording media have been developed and used.
Among these, an optical recording medium (hereinafter referred to as “optical disk”) for CD is an optical disk having a cover thickness of 1.2 mm for protecting the information recording plane, and a semiconductor laser of a 780 nm wavelength band as a light source and an objective lens having an NA (numerical aperture) of from 0.44 to 0.51 are employed for recording and/or reproducing an information.
On the other hand, an optical disk for DVD is an optical disk having a cover thickness of 0.6 mm, and a semiconductor laser of 650 nm wavelength band as a light source and an objective lens having a NA of from 0.60 to 0.65 are employed for recording and/or reproducing an information.
Further, in order to increase recordable information volume, an optical disk having a cover thickness of 0.1 mm for which a semiconductor laser of 410 nm wavelength band as a light source and an objective lens having a NA of 0.85 are employed, is proposed. Hereinafter, an optical disk for which a semiconductor laser of 410 nm wavelength band is employed is specifically referred to as an optical disk for HD.
Here, light in a λ1=410 nm wavelength band means light having wavelength from about 390 nm to about 430 nm, light in a λ2=650 nm wavelength band means light having wavelength from about 630 nm to about 680 nm, and light in a λ3=780 nm wavelength band means light having wavelength from about 760 nm to about 820 nm.
Further, numerical apertures NA of objective lenses to be employed for HD, DVD and CD are designated as NA1, NA2 and NA3 respectively. NA1 is about 0.85, NA2 is about from 0.60 to 0.65, and NA3 is about from 0.44 to 0.51.
Further, a phase difference caused by the difference between an ordinary refractive index and an extraordinary refractive index of a birefringent material for an ordinarily polarized light and an extraordinarily polarized light respectively, is referred to as “birefringent phase difference”, the terms being used to distinguish from a normal phase difference corresponding to an optical path difference not caused by the dependency of refractive index on polarization. Further, “phase difference” is shown by a unit of radian (rad), and it is referred to as “wavelength phase difference” when it is described by a wavelength unit.
By the way, three types of optical disks for CD, for DVD and for HD have different cover thicknesses and wavelengths to be used from one another. Accordingly, there has been a problem that in an optical head device for recording and/or reproducing an information, when an objective lens designed for any one type of optical disk is mounted for recording and/or reproducing an information from these optical disks compatibly, for example, when the optical head device is used for recording and/or reproducing an information from a different type of optical disk from the above type of optical disk, a large spherical aberration is generated and recording and/or reproducing of the information can not be performed.
To cope with this problem, in order to perform recording and/or reproducing an information from optical disks having different cover thicknesses by employing a single objective lens in the optical head device, various solutions for reducing the spherical aberration generated have been proposed. (For example, JP-B-2713257 and JP-B-2725653.)
As a conventional example, JP-B-2713257 proposes an aperture-limiting element comprising a substrate and a multi-layer film filter which is a lamination of transparent dielectric films having different refractive indexes, or a diffraction grating formed in the periphery of the substrate. The aperture-limiting element switches NA by transmitting light having one wavelength and reflecting or diffracting light having the other wavelength.
FIG. 20 shows an example of a cross-sectional view of a conventional aperture-limiting element 1000 which transmits light having a wavelength of λ2 for DVD and reflects light having a wavelength of λ3 for CD. A multi-layer film filter 1200 is formed in an annular region (middle region) obtained by subtracting a circular region of numerical aperture NA3 from a circular region of numerical aperture NA2 on the surface of a transparent substrate (glass substrate) 1100, which constitutes an aperture-limiting element transmitting incident light having a wavelength of λ2 and not transmitting incident light having a wavelength of λ3.
Here, a phase compensation film 1300 for phase adjustment is formed on a multi-layer film filter 1200 having an annular region so as to align the phases of transmitted light having a wavelength of λ2 between a circular region of numerical aperture NA3 and the annular region in which the multi-layer film filter 1200 is formed.
Optical head device reducing spherical aberration caused by the difference of cover thickness of optical disks, can be constructed by employing the above aperture-limiting element 1000 together with an objective lens and by switching NA of light beam to be converged on an information recording plane depending on the difference of wavelengths for DVD and CD. Here, residual spherical aberration is reduced by making incident light having a wavelength of λ3 into the objective lens, to be diverging light beam.
As a conventional example 2, JP-B-2725653 proposes a phase correction element comprising a hologram optical element with an aperture-limiting function having a concentric circular interference fringe pattern whose cross-sectional shape is a form of steps, in addition to the aperture-limiting element. The phase correction element transmits light having a first wavelength and diffracts light having a second wavelength different from the first wavelength to generate a spherical aberration canceling a spherical aberration of an objective lens.
Further, an optical head device for recording and/or reproducing an information in an optical recording medium of an optical disk such as a CD or a DVD, has a construction that light emitted from a semiconductor laser as a light source is converged on the optical recording medium by an objective lens, and returning light reflected by the optical recording medium is introduced into a photo-acceptance element as a photodetector by a beam splitter, and the information in the optical recording medium is transformed into an electrical signal.
Here, in order to effectively converge emitting light from the semiconductor laser on the optical recording medium of the optical disk and to effectively detect signal light from the optical recording medium by the photodetector, it is effective to employ a polarizing beam splitter. The polarizing beam splitter transmits in an incoming path linearly polarized light emitted from the light source and having a polarization plane in a predetermined direction, and reflects or diffracts in a returning path linearly polarized light reflected by the optical recording medium and thereby having a polarization plane perpendicular to that of the incoming path, whereby the polarizing beam splitter can switch the direction of light to the photodetector. Here, in order to change the polarization plane of the linearly polarized light in the returning path perpendicular to the polarization plane in the incoming path, a phase plate (¼ waveplate) generating a birefringent phase difference of π/2 for the wavelength of incident light, is employed, which is disposed in the optical path between the polarizing beam splitter and the optical recording medium.
However, as shown in the conventional examples 1 and 2, although there is an aperture-limiting element or a phase correction element applicable for recording and/or reproducing informations of two types of optical disks by employing a single objective lens, there is no phase correction element for three wavelengths applicable for recording and/or reproducing informations of three types of optical disks of HD, DVD and CD, it has been difficult to record and/or reproduce informations of these three types of optical disks by employing a single objective lens.
Further, when the aperture-limiting element of the above conventional example 1 is employed as a compatible element for three types of optical disks of HD, DVD and CD, it is necessary to add a function of wavelength selection filter which transmits incident light having a wavelength of λ1 and does not transmits incident light having wavelengths of λ2 and λ3, in a first annular region obtained by subtracting a circular region of numerical aperture NA2 for DVD from a circular region of numerical aperture NA1 for HD (here, NA1>NA2), of the aperture-limiting element 1000 shown in FIG. 20. Further, in the same manner, it is necessary to add a function of wavelength selection filter which transmits incident light having wavelengths of λ1 and λ2 and does not transmit incident light having a wavelength of λ3, in a second annular region obtained by subtracting a circular region of numerical aperture NA3 for CD from a circular region of numerical aperture NA2 for DVD (here, NA2>NA3). Further, it is necessary for the circular region of numerical aperture NA3 to have a function of transmitting all of incident light having wavelengths of λ1, λ2 and λ3.
Then, in a case of applying a conventional technique employing a multi-layer film filter for the wavelength selection filter, it is necessary to deposit multi-layer films having different spectral transmittances in the first annular region and the second annular region separately in the divided regions, which requires an extremely complicated process. Therefore, it has been difficult to stably produce an aperture-limiting element which does not deteriorate a transmitted wavefront aberration of incident light having a wavelength of λ1 in the entire region of aperture NA1. Here, a transmitted wavefront means a wavefront of light after the light is transmitted through an optical element such as a phase correction element. “A transmitted wavefront is changed” means that a wavefront of light is changed while the light is being transmitted through an optical element and the light is output with a changed wavefront.
Further, a phase correction element producing a birefringent phase difference of π/2 for a wavelength of λ1 and having non-deteriorated property of phase correction element for wavelengths of λ2 and λ3 and with which a phase plate is integrally formed, has been demanded.
The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a phase correction element and an optical head device applicable for recording and/or reproducing an information in three types of optical disks of HD, DVD and CD employing a single objective lens for HD.