Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2001-09-28
2003-10-21
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S565000, C359S721000
Reexamination Certificate
active
06636365
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens made of a plastic material that is used in a optical pickup apparatus, wherein, a light flux radiated from a laser light source is formed into a light spot by an objective lens on an information recording surface of an optical information recording medium through its transparent board, to record and/or reproduce information optically, and more particularly, to an objective lens wherein a first light source with a wavelength of &lgr;
1
is used for recording and/or reproducing a first optical information recording medium, and a second light source with a wavelength of &lgr;
2
is used for recording and/or reproducing a second optical information recording media, and corrects by a diffraction a spherical aberration that is produced by the difference between a thickness t
1
of the transparent base board of the first optical information recording medium and a thickness t
2
of the transparent base board of the second optical information recording medium.
With a short wavelength red semiconductor laser which has recently been put to practical use, there has been commercialized a high density DVD (digital versatile disc) that is the same in size as a conventional CD (compact disc) and has a larger capacity as an optical information recording medium. When information is recorded on this DVD or reproduced from this DVD, a semiconductor laser with a short wavelength of 635 nm or 650 nm and an objective lens with a numerical aperture (NA) of an approximate value of 0.6 are necessary.
Further, with a spread of a readable optical information recording medium CD-R (write-once read-many compact disc), an optical pickup apparatus is required to have the so called interchangeability of information recording medium that makes it possible to record and/or reproduce information for both CD-R and DVD.
However, since the necessary signals (reproducing signal, focus error signal and tracking error signal) cannot be obtained by a drop in the reflectance of CD-R in the short wavelength band, the semiconductor laser of wavelength 780 nm is prepared for CD-R, separately from the short wavelength semiconductor laser for DVD,.
Incidentally, for recording and/or reproducing information for CD-R (hereinafter referred to also as CD), the numerical aperture of the objective lens is required to have an approximate value of 0.45.
Incidentally, the required numerical aperture mentioned in the present specification is one that is calculated by a spot size d required on the optical information recording medium and wavelength &lgr; to be used and can be generally expressed by a formula NA=0.83&lgr;/d.
Information recording surfaces of these optical information recording media are protected by the transparent base boards, and the thicknesses of the transparent base boards are determined by the specifications of each optical information recording medium, to be 0.6 mm for DVD and 1.2 mm for CD system respectively.
If there is no need to consider the interchangeability, the spherical aberration of the pick-up optical system may be designed to be optimum to the thickness of each transparent base board.
However, in order to realize the interchangeability between DVD and CD each having a different thickness of the transparent base board, some means are needed for the above-mentioned correction of the spherical aberration.
For example, in the official gazette of TOKUKAI No. 2000-81566, there is described the objective lens using the diffracted light in the optical pickup apparatus in which DVD and CD are different each other in terms of a wavelength to be used, as an example of means for correction of the spherical aberration.
A power of the diffractive lens can be divided into a power of a refractive section representing a basic aspheric surface and a power of a diffractive section. That is, a degree of freedom in designing can be increased even for a single lens, by introducing the diffractive action. Further, in the diffractive section, there is an effect that the power is proportional to the wavelength which is different from that in the refractive optical system.
By properly allocating the power of the diffractive section and the refractive section, the spherical aberration correction can be performed.
However, in case of recording and/or reproducing information for CD, if the spherical aberration correction is conducted on the outer side of the area covered by the necessary numerical number NA
2
=0.45, a spot light is stopped down to be too small. Accordingly, in order to use a diaphragm having the common aperture size for DVD and CD, the spherical aberration must be designed so as to let the area outside NA
2
=0.45 be a flare, when CD is used. Thus, in the so-called diffraction interchangeable lens that can converge the light flux properly for both of DVD and CD, the optical pickup apparatus can be constructed with a single objective lens and a single simple aperture diaphragm, by jointing the diffractive surfaces each being designed optically based on a different concept.
As mentioned above, the diffractive lens power is generally expressed by the sum of the diffractive section power and the refractive section power of the diffractive base surface. However, when a diffractive lens is made of a plastic material, there is observed a distinctive mark that the temperature characteristic and the wavelength characteristic are contrary each other. The reason will be explained as follows.
Let it be considered that the temperature of the diffractive lens has risen. By the increment of temperature, the laser wavelength becomes longer and the over spherical aberration is generated on the diffractive section, while the under spherical aberration is generated on the base refractive section due to the reduction of the refractive index caused by the temperature rise, therefore, the spherical aberrations are produced on the refractive section and the diffractive section wherein both of the spherical aberrations are canceling each other.
Accordingly, to perform the temperature compensation, the absolute values of the spherical aberrations generated on the refractive section and the diffractive section should be close to each other.
On the other hand, when the wavelength is shifted toward the longer wavelength side simply by an unevenness of the laser oscillating wavelength, the over spherical aberration is generated in the diffractive section, but a chromatic aberration in the diffractive section is smaller than the over spherical aberration, resulting in the over spherical aberration in the diffractive section which remains without being cancelled.
Consequently, there is a tendency that, when improving the wavelength characteristics to exceed the certain level, the temperature characteristic begins to go down, while, when improving the temperature characteristics to exceed the certain level, the wavelength characteristic begins to go down, which requires that both of them are balanced.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the above-mentioned problems, and an object of the invention is, in the interchangeable objective lens which is using diffraction, to provide with an objective lens, wherein, in addition to the interchangeability between DVD and CD, temperature characteristics is considered, and an objective lens wherein wavelength characteristic is considered and an objective lens wherein these characteristics are well-balanced, and further, to provide with an optical pickup apparatus using these objective lenses.
(1) The optical pickup apparatus described in (1) is represented by an optical pickup apparatus that is provided with a first light source with wavelength &lgr;
1
that records and/or reproduces information by radiating the light flux on the first optical information recording medium with a transparent base board having thickness t
1,
a second light source with wavelength &lgr;
2
(&lgr;
1
<&lgr;
2
) that records and/or reproduces information by radiating the light flux on the second
Saito Shin'ichiro
Sakamoto Katsuya
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Schwartz Jordan M.
Stultz Jessica
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