Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2002-04-22
2004-04-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S719000, C359S634000, C369S112030
Reexamination Certificate
active
06728042
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates, to an optical pick-up device and an objective lens used therein, for example, to an optical pick-up device which can perform information recording and/or information reproduction for two optical information recording media each having a different thickness of a transparent base board and an objective lens used therein.
With the practical use of a short wave red semiconductor laser in recent years, there has been developed a high density optical disk DVD (digital video disk) that is the same in size as a conventional optical disk CD (compact disk) representing an information recording medium, and has a larger capacity. In order to attain high density of a recording signal, it is requested to make a spot formed on a recording medium by an objective lens to be smaller in an optical system of an optical information recording/reproducing device employing such an optical disk. Therefore, there are the facts that a wavelength of a laser representing a light source is made to be shorter and a numerical aperture of an objective lens is made to be higher.
For example, in an optical pick-up device that performs information recording and/or reproduction for DVD, when a short wave semiconductor laser ray having a wavelength of about 650 nm is used as a light source, numerical aperture NA closer to the optical disk is approximately 0.6 for the objective lens that converges the laser ray. Incidentally, in CD and DVD, there are optical disks under various kinds of standards, including, for example, CD-R (write-once read-many compact disk), and in addition to CD and DVD, there is MD (mini disk) that has been commercialized and has become popular.
On the other hand, in the optical pick-up device that performs information recording and/or reproduction for CD-R, it is necessary to use the light source having a wavelength of about 780 nm. For the other optical disks, it is possible to use the light source having a wavelength other than the specific wavelength. In this case, a necessary numerical aperture NA can be changed in accordance with the wavelength &lgr; of the light source used therein. For example, the necessary numerical aperture is NA=&lgr; (&mgr;m)/1.73 approximately for CD, while the necessary numerical aperture is NA=&lgr; (&mgr;m)/1.06 approximately for DVD. The numerical aperture in the present specification means a numerical aperture of the converging optical system viewed from the optical disk side. The necessary numerical aperture means a numerical aperture that is calculated by both spot size d required on the recording surface of the optical disk and wavelength &lgr; in use, and that is commonly shown by the formula NA=0.83×&lgr;/d.
As mentioned above, in the market, there exist various kinds of the optical disks having different transparent base board thicknesses, different recording densities and different wavelengths in use. However it is too heavy a burden for the user to purchase the exclusive information recording and reproduction device which can perform information recording and/or reproduction for each optical disk. Therefore, there is proposed the information recording/reproduction device provided with the interchangeable optical pick-up device that can cope with the various optical disks.
In such an optical disk, when light fluxes each having a different wavelength are made to enter the base boards each having a different thickness, it is necessary to correct spherical aberration to be the prescribed value or less, and to control a spot diameter of each light flux to be within the prescribed range, in order to perform information writing and reading properly.
As a solution for the foregoing, there is proposed the optical pick-up device wherein converging optical systems are provided for different optical disks respectively, and optical converging systems are switched in accordance with the optical disks to be reproduced. The optical pick-up device mentioned above makes it possible to correct the spherical aberration to be under the prescribed value, and to make the spot diameter of each light flux to be within the prescribed range, when the light fluxes having different wavelengths are incident on the base boards having different thicknesses. However, this optical pick-up device requires a plurality of light converging optical systems, to cause the complicated structure and higher cost, which is not preferable.
For this problem, there is developed an objective lens having a diffractive structure that can converge the light flux passing within the prescribed numerical aperture to spot light and that can make the light flux passing beyond the prescribed numerical aperture to be flared light. This objective lens can obtain spot light having a necessary diameter for the information recording or reproduction for the various optical information recording media such as, for example, CD and DVD.
Here, in the objective lens having the diffractive structure stated above, wave front aberration caused by the difference of the wavelength of the light source is generated originally, because the light flux having the numerical aperture smaller than the prescribed numerical aperture is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly. Accordingly, it is necessary to make the characteristic of the wave front aberration to be better by some means. Further, there is request to increase the spot light amount for the optical information recording media having thick base board such as CD.
SUMMARY OF THE INVENTION
The object of the present invention is to provide the optical pick-up device and the objective lens which can control a phase shift of a wave front, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
The object of the present invention is to provide the optical pick-up device and the objective lens which have the better characteristic of the wave front aberration, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
Further, the object of the present invention is to provide the optical pick-up device and the objective lens which can increase the spot light amount, for the optical information recording medium such as CD having the thick base board, in the objective lens which is used for the information recording or reproduction for the various optical information recording media such as CD and DVD commonly.
An optical pick-up device has a first light source having wavelength &lgr;
1
, a second light source having wavelength &lgr;
2
(&lgr;
1
<&lgr;
2
) and a converging optical system including an objective lens which converges the light fluxes emitted from the first light source and the second light source onto an information recording surface of the optical information recording medium, and being capable of performing the information recording and/or reproduction for the first optical information recording medium whose transparent base board thickness is t
1
, by using the first light source and the converging optical system, and being capable of performing the information recording and/or reproduction for tile second optical information recording medium whose transparent base board thickness is t
2
(t
1
<t
2
), by using the second light source and the converging optical system, wherein when NA
1
is a first necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the first optical information recording medium, using the first light source and tile converging optical system, and when NA
2
(NA
1
<NA
2
) is a second necessary numerical aperture of the objective lens which performs the information recording and/or reproduction for the second optical information recording medium, using the second light source and at the convergin
Arai Norikazu
Ota Kohei
Collins Darryl J.
Epps Georgia
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
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