Objective lens for correcting chromatic aberration for use...

Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium

Reexamination Certificate

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C369S112080, C369S094000

Reexamination Certificate

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06370103

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a recording/reproducing method of optical information, wherein a light flux emitted from a light source is converged on an information recording plane and optical information to be reproduced is recorded on an optical information recording medium, or information on the information recording plane is reproduced, to an optical pickup apparatus, to a converging optical system, an objective lens to be used therefor, and to a recording/reproducing apparatus for optical information recording medium.
In recent years, with the practical use of a red semiconductor laser of a short wavelength, development of a DVD (digital video disk or called also a digital versatile disk), which is a high-density optical information recording medium having a larger capacity but a size of the same degree as a CD (compact disk) which is a conventional optical information recording medium (also called an optical disk), has been promoted. In this DVD, the numerical aperture NA of the objective lens at the side of the optical disk is made to be 0.6 in the case where a short wavelength semiconductor laser of 635 nm is used. Further, a DVD has a track pitch of 0.74 &mgr;m and a minimum pit length of 0.4 &mgr;m, and is made to have a high density more than twice in comparison with a CD, which has a track pitch of 1.6 &mgr;m and a minimum pit length of 0.83 &mgr;m. Moreover, in addition to the above-mentioned CD and DVD, optical disks having various standards, for instance, such as a CD-R (a direct read after write, writing once compact disk), a CD-RW, an LD, an MD (mini-disk), a DVD-RAM and an MO (magneto-optic disk) are merchandised and have come into general use. In Table 1, the thickness of the transparent substrate and the required numerical aperture of various optical disks are shown.
TABLE 1
Required
numerical
aperture NA
Thickness of
(wavelength of
transparent
light
Optical disk
substrate (mm)
source_nm)
CD, CD-R (reproducing only)
1.20
0.45 (1 = 780)
CD-R
1.20
0.50 (1 = 780)
(recording, reproducing)
LD
1.20
0.50 (1 = 780)
MD
1.20
0.45 (1 = 780)
MO (ISO 3.5 inch 230 MB)
1.20
0.55 (1 = 780)
MO (ISO 3.5 inch 640 MB)
1.20
0.55 (1 = 680)
DVD
0.60
0.60 (1 = 635)
Besides, with regard to the CD-R, it is necessary for the light source to have a wavelength &lgr;=780 (nm), but for the other optical disks, a light source having a wavelength other than those noted in Table 1 can be used; in this case, the required numerical aperture NA should be varied in accordance with the wavelength &lgr; of the light source used. For example, in the case of a CD, the required numerical aperture is approximated by NA=&mgr; (&mgr;m)/1.73, and in the case of a DVD, it is approximated by NA=&lgr; (&mgr;m)/1.06.
Further, the numerical aperture as referred to in this specification (for example, referred to as NA
1
, NA
2
, NAL, NAH, NA
3
, NA
4
, etc. hereinafter) means the numerical aperture of the converging optical system as seen from the transparent substrate side. NA
1
is a numeral aperture necessary for reproducing information from or recording information in the first optical information recording medium and NA
2
is a numeral aperture necessary for reproducing information from or recording information in the second optical information recording medium.
As described in the above, it is now an age when various kinds of optical disks, which are different in the size, the thickness of the transparent substrate, the recording density, the wavelength used, and so forth, are available on the market, and optical pickup apparatus capable of being used for various kinds of optical disks are proposed.
As one of them, it has been proposed an optical pickup apparatus which is equipped with converging optical systems corresponding to the different optical disks respectively, and the converging optical systems are switched over in accordance with the optical disk to be reproduced. However, in this optical pickup apparatus, a plurality of converging optical systems is required and it brings not only a high cost but also the necessity of a driving mechanism for switching over the converging optical systems; accordingly, the device is not desirable because of the complexity and the requirement of the precision in switching over.
Therefore, various kinds of optical pickup apparatus which can reproduce a plurality of optical disks by using a single converging optical system.
Incidentally, in this specification, the phrase ‘to reproduce an optical disk’, ‘to record an optical disk’ or the like is used in the sense ‘to reproduce information in an optical disk, ‘to record information in an optical disk’, or the like for simplicity's sake.
As one of them, in Japanese laid open patent H7-302437, an optical pickup apparatus, wherein the refracting surface of the objective lens is divided into a plurality of ring-shaped domain, and each of the divisional surface domains makes the beam converge on one of the optical disks having different thickness to reproduce information, is described.
Besides, in Japanese laid open patent H7-57271, an optical pickup apparatus, wherein, in the case of the first optical disk with the thickness of the transparent substrate t
1
, an objective lens designed to make the wave front aberration included in the converged beam not larger than 0.07&lgr; is employed, and in the case of the second optical disk having the thickness of the transparent substrate t
2
, the converged beam spot is formed in a little defocused state, is described.
However, in the optical pickup apparatus described in Japanese laid open patent H7-302437, it is necessary to make the laser output large because the incident light quantity is simultaneously divided into two focal points by a single objective lens, which brings about high cost. Further, in the optical pickup apparatus described in Japanese laid open patent H7-57271, increasing of jitter owing to side lobe occurs when the reproduction of the second disk is carried out. In particular, because the second disk is subjected to reproduction forcibly by the objective lens which makes the wave front aberration not larger than 0.07&lgr; for the first optical disk, there is a limit in the numerical aperture capable of reproducing the second optical disk.
With the background stated above, there are given methods proposed to solve the above-mentioned problems, which are disclosed in Japanese TOKKAIHEI Nos. 8-55363, 9-17023, 9-194975, 10-69675 and 11-96585.
However, when recording information on an optical information recording medium, another problem is further caused. In general, greater laser power is required for recording on an optical information recording medium than for reproducing, and this difference of power changes a wavelength of a light source (which is also called wavelength shift or wavelength drift), thus, chromatic aberration based on the wavelength change is caused. When chromatic aberration of an optical system is not corrected, a light converging position is changed by the wavelength shift, and a focal point is shifted, resulting in possibility of occurrence of an error in recording and/or reproducing of information.
For solving problems caused by wavelength changes, there have been proposed various optical systems each having a function of correcting chromatic aberration. However, none of them is one wherein reproducing/recording of plural optical information recording media can be conducted by one converging optical system.
In other words, in the prior art, it has been impossible to obtain an objective lens, an optical pickup apparatus and an optical information recording/reproducing apparatus wherein one converging optical system (with one or plural light sources) can cope with recording/reproducing of various optical disks each having different size, board thickness, recording density and wavelength to be used, a problem of chromatic aberration caused by wavelength shift derived from change of power in recording is solved, and thereby, recording/reprod

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