Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-11-09
2004-04-27
Edun, Muhammad (Department: 2655)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S112260, C369S112230
Reexamination Certificate
active
06728172
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens for an optical pickup device and an optical pickup device, and for example, to an objective lens for an optical pickup device and an optical pickup device which can record on and/or reproduce from optical information recording media each having different density for information recording.
With a recent practical use of a short wavelength red semiconductor laser, there has been developed a high density optical disc DVD (digital versatile disc) that is the same in size as a CD (compact disc) which is a conventional optical disc representing an optical information recording medium, and is designed to have larger capacity. In order to attain high density of a recording signal, in an optical system of an optical information recording/reproducing apparatus employing such an optical disc, a spot formed on a recording medium by an objective lens through light convergence is requested to smaller.
Since there are available various optical discs each having different recording density on the market, it is a heavy burden for a user to purchase an exclusive information recording/reproducing apparatus which can record and/or reproduce information for each optical disc. Due to this, there has been developed an information recording/reproducing apparatus having an optical pickup device which can record or reproduce information for CD with the use of the optical system for recording or reproducing for DVD, for example.
In the optical pickup device stated above, a parallel light enters the objective lens to converge light and form a spot on a recording surface of DVD, while a divergent light enters to converge light and form an appropriate spot on a recording surface of CD, because a parallel light generates an aberration due to the difference of the thickness of a transparent base board for CD, and that for DVD.
Incidentally, since necessary numeral apertures are different each other for DVD and CD, a dichroic filter is used to adjust the required numeral apertures. The dichroic filter has a function that makes a light flux having a wavelength for recording or reproducing of information for DVD to pass through without intercepting the light flux, while makes a light flux having a wavelength for recording or reproducing of information for CD to be shaded so as to have a required numeral aperture for CD. From the view point of cost, it is preferable that information is recorded on or reproduced from optical information recording media each having different density for information recording, without providing the dichroic filter.
SUMMARY OF THE INVENTION
An object of the invention is to provide an objective lens for an optical pickup device and an optical pickup device which can record and/or reproduce (hereinafter referred to also as recording and reproducing) information for optical information recording media each having different density for information recording.
The objective lens for the optical pickup device stated in (1) is the one having therein a first light source which emits a light flux having wavelength &lgr;
1
, a second light source which emits a light flux having wavelength &lgr;
2
(&lgr;
1
is not equal to &lgr;
2
), a light-converging optical system including at least the objective lens which converges the light flux emitted from the first light source on the information recording surface via the transparent base board of the first optical information recording medium, in case of recording or reproducing information for the first optical information recording medium, and converges the light flux emitted from the second light source on the information recording surface via the transparent base board of the second optical information recording medium, in case of recording or reproducing information for the second optical information recording medium having density for information recording different from that of the first optical information recording medium, and a photo detector which receives a reflected light or a transmitted light from the first and second optical information recording media, wherein at least one of optical surfaces of the objective lens is provided with a central region without a diffractive structure and a peripheral region with diffractive structure adjacent to the central region, and when NA
1
represents a prescribed numerical aperture on the image side of the objective lens which is necessary for recording or reproducing information for the first optical information recording medium using the first light source, a first spot represents a spot formed by the light flux having passed through the central region, and an m
th
order (m represents nonzero integers) represents a diffracted ray having the maximum diffracted ray amount among diffracted rays (0
th
order diffracted ray is included, if any) generated by the diffractive structure from the light flux having passed through the peripheral region, and when NA
2
(NA
2
<NA
1
) represents a prescribed numerical aperture on the image side of the objective lens which is necessary for recording or reproducing information for the second optical information recording medium using the second light source, a second spot represents a spot formed by the light flux having passed through the central region, and an n
th
diffracted ray (n is nonzero integers) represents a diffracted ray having the maximum diffracted ray amount among diffracted rays (0
th
order diffracted ray is included, if any) generated by the diffractive structure from the light flux having passed through the spherical area, the above-mentioned central region nearly corresponds to a region through which the light flux in the numerical aperture NA
2
passes, an amount of the n
th
order diffracted ray which reaches the inside of the second spot is less than that of the m
th
order diffracted ray which reaches the inside of the first spot, and the m
th
order diffracted ray and the n
th
order diffracted ray satisfy the relationship of m=n.
That is, when the light flux having passed through the peripheral region having the diffractive structure reaches the inside of the spot formed by the light flux having passed through the central region, the spot diameter of the light flux passed through the spherical area becomes smaller, because its numeral aperture NA is larger. In the objective lens stated in (1), the amount of the light flux of the n
th
order diffracted ray reaching the inside of the second spot is smaller than that of the light flux of the m
th
order diffracted ray reaching the inside of the first spot. Due to this, when the light flux having wavelength &lgr;
1
enters the objective lens, the spot diameter becomes so small that it is possible to record or reproduce for the optical information recording medium having high recording density, and on the other hand, when the light flux having wavelength &lgr;
2
(&lgr;
2
>&lgr;
1
) enters the objective lens, the spot diameter becomes so large that it is possible to record or reproduce for the optical information recording medium having low recording density. In particular, it is preferable that the light amount of the light flux of the nth order diffracted ray reaching the inside of the second spot is small to the extent which does not affect substantially (to the level that the spot diameter does not become too small) the spot diameter of the second spot which is necessary for recording or reproducing for the second optical information recording medium. And further, it is preferable that the amount of the light reaching the inside of the second spot is small to the extent which does not change the spot diameter substantially for all light fluxes outside the numerical aperture NA
1
, including the light flux of the n
th
order diffracted ray.
In the diffractive structure satisfying the relationship of order m=n, the sectional form including an optical axis is nearly like a saw-tooth, and since it is possible to make the step amount in the direction of the optical axis to be relatively small,
Honda Koji
Ikenaka Kiyono
Edun Muhammad
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
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