Dynamic information storage or retrieval – Specific detail of information handling portion of system – Radiation beam modification of or by storage medium
Reexamination Certificate
2002-04-08
2004-10-19
Tran, Thang V. (Department: 2653)
Dynamic information storage or retrieval
Specific detail of information handling portion of system
Radiation beam modification of or by storage medium
Reexamination Certificate
active
06807139
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens used for an optical pickup apparatus, an optical pickup apparatus (device) and information recording and reproducing method, and in particular, to an objective lens wherein a single objective lens performs recording/reproduction for optical information recording media having various recording density, an optical pickup device and an information recording and reproducing method, and further relates to an objective lens, an optical pickup device, and an information recording and reproducing method, the objective lens is provided with a diffractive structure and is used within a finite magnification (a magnification for a finite light source), the optical information recording medium having a high recording density satisfies the excellent temperature characteristic, and it is possible to reduce the noise signal generated when the focusing movement is performed for an optical information recording medium having low recording density.
At the present time, there are many kinds of the optical information recording media, whose standards are determined as shown in Table 1. Incidentally, hereinafter (including lens data of tables), E (for example, 2.5×E-3) shows powers of 10 (for example, 2.5×10
−3
).
TABLE 1
Thickness
Necessary numerical
(mm) of
aperture NA (wave
transparent
length of light
Optical disk
base board
source &lgr; nm)
CD, CD-R
1.20
0.45 (&lgr; = 780 nm)
(reproduction)
CD-R (recording,
1.20
0.50 (&lgr; = 780 nm)
reproduction)
DVD
0.60
0.60 (&lgr; = 635 nm)
As an example wherein there is required an interchangeability among optical information recording media having various recording densities, there are DVD and CD. As shown in Table 1, optical information recording media are different each other in terms of thickness of transparent base board. In order to obtain the interchangeability, it is necessary to correct spherical aberration which is caused by the difference of the transparent base board thickness, by using some means. Further, since there is a difference between the required numerical apertures for DVD and CD, it is necessary to take some measures to meet the situation.
As an example of the objective lens which obtains the interchangeability between DVD and CD, there is developed the objective lens having a diffractive structure. As such an objective lens, for example, there is an objective lens wherein, there are provided different diffractive structures for inner and outer areas on either one of the lens surfaces which are separated at specific distance h from an optical axis, and the inner area corrects the spherical aberration for each of the thickness of the transparent base boards, while the outer area corrects the spherical aberration for DVD only and makes flare without making correction of the spherical aberration for CD. By the above-mentioned construction of the objective lens, it is possible to form a light converged spot required for recording or reproduction of information on each of the optical information recording media.
By the way, for the flare formation for CD on the outer area, it is conceivable to form an over flare and an under flare. When the under flare is taken, the design turns out to be of a strong effect of diffraction, and a diffraction pitch has a tendency to be narrow. Accordingly, it is preferable for the design to take the over flare, from the view point of diffraction efficiency.
On the other hand, for the objective lens of the recent optical pickup device which performs information recording or reproduction for DVD, infinite magnifications are employed widely, and thereby, a collimator lens is usually necessary for radiating a parallel light flux to the objective lens. However, from the viewpoint of the structure, a finite magnification structure without the collimator lens has the advantage of designing an optical system of the optical pickup device to be simple. Incidentally, from the viewpoint of the optics, due to the shift from the infinite magnification to the finite magnification, there are problems that the shift to a tracking direction of the objective lens generates coma, and that the deterioration becomes greater in the spherical aberration generated by the change of temperature.
Using an image side numerical aperture NA of the objective lens, focal length f, image forming magnification m and wavelength &lgr; of laser light source, the three order changed amount of the aberration caused by temperature change is shown by the following formula,
&dgr;
SA
3
/&dgr;T∝f
·(1
−m
)
4
·NA
4
/&lgr; (1)
and it can be understood that the change of the temperature characteristic becomes excessive with more finite magnification, shorter wavelength and higher numerical aperture in the optical pickup device.
Especially when the objective lens is structured by plastic, it is regarded as questionable that the spherical aberration deteriorates due to change of refractive index caused by the change of temperature. In
FIG. 16
, when temperature changes from 25° C. to 55° C., there is shown the changed amount of spherical aberration of the plastic objective lens which is corrected for aberration in m=−1/7.2, NA=0.60, and &lgr;=650 nm under the standard temperature 25° C. (the change of refractive index to the change of material temperature dn/dT=−0.00012 (/° C.)).
The formation of diffractive structure on the objective lens can reduce spherical aberration caused by the change of temperature, though the objective lens is made of plastic. The reason for this is as follows. That is, power of the diffractive structure has character to be proportional to the wavelength, for example, when temperature rises, refractive index of plastic drops to generate the over spherical aberration, while on the diffractive section, a laser oscillated wavelength is shifted to the longer wavelength side to generate the under spherical aberration, which means that both of them have the effect to cancel spherical aberration each other.
However, the correction of the temperature characteristic by diffraction has following problems on the plastic objective lens having large percentage of the finite magnification.
As understood from
FIG. 15
, when an over spherical aberration is highly generated at a refractive section of the objective lens, it is impossible to cancel the spherical aberration by the objective lens only, unless an under spherical aberration is generated highly at the diffractive section. Since above-mentioned matter is the same meaning as the matter that the change of refractive index corresponding to the change of temperature (this is wavelength dependence) is made to be high at the diffractive section, the effect of diffraction becomes higher. Then, an over flare section for CD changes to the under direction at the outer area, and as a result, the flare section nears the optical axis.
It means on CD that a distance between a light converged spot formed by the light flux passing through the inner area and an inside diameter of flare formed by light flux passing through the outer area becomes shorter. The nearer the inside diameter approaches the converged spot, the more flare light which may be detected by an optical detector increases, which is not desirable, because of increment of noise signals in focusing. That is, if the effect of diffraction is made to be higher for the purpose of improving the temperature characteristic, the focusing noise is worsened, which is a problem.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the above-mentioned problems, and the object of the invention is to provide the objective lens for the optical pickup device which can perform the recording or reproduction of information properly though having simple construction, for the various types of optical information recording media, the optical pickup device and information recording and reproducing method.
The objective lens mentioned in (1) is one which is used for the o
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Tran Thang V.
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