Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2001-08-10
2002-12-24
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S661000, C369S112010
Reexamination Certificate
active
06498689
ABSTRACT:
RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 2000-245194 filed on Aug. 11, 2000, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens and optical pickup apparatus for recording and reproducing signals, which are commonly usable for two or more kinds of optical recording media having thicknesses different from each other. In particular, the present invention relates to an objective lens which, when light beams for irradiating the optical recording media have wavelengths different from each other according to the kinds of optical recording media, can efficiently converge the respective light beams onto their corresponding optical recording media; and an optical pickup apparatus using the same.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other.
In the two optical recording media exemplified above, however, the disc thickness of the CD-R is fixed to 1.2 mm whereas the DVD in use mostly has a thickness of 0.6 mm. Therefore, the individual wavelengths of light for reproducing or recording are needed to be securely converged at their predetermined positions different from each other.
Under such circumstances, a system in which two objective lenses having converging actions different from each other are made switchable depending on the optical recording media subjected to reproducing or recording has been known. However, it complicates the structure of optical pickup apparatus and opposes the demands for compactness and lower cost.
On the other hand, it has recently been known to use a multifocal lens composed of a diffractive optical element or a plurality of circular strip divisions for an objective lens part of an optical pickup apparatus (Japanese Unexamined Patent Publication No. 8-62493, Japanese Unexamined Patent Publication No. 9-145994, and the like).
In the apparatus using the above-mentioned diffractive optical element, however, a noise light component of an order different from that contributing to the recording and reproducing is emitted from the diffractive optical element. In the apparatus using the above-mentioned multifocal lens, the light transmitted through a circular strip division other than that contributing to the recording and reproducing at that time is noise light. Due to these noise light components, the S/N ratio at the recording and reproducing deteriorates.
SUMMARY OF THE INVENTION
In view of the circumstances mentioned above, it is an object of the present invention to provide a compact, inexpensive objective lens with a favorable S/N ratio for optical recording media, which can favorably converge two wavelengths of light onto their corresponding recording surfaces of optical recording media having respective thicknesses without complicating the structure of optical pickup apparatus; and an optical pickup apparatus using the same.
The present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
the objective lens satisfying the following conditional expression (1) with respect to a light beam having a wavelength &lgr;
1
used for recording or reproducing one of the two optical recording media, and
the following conditional expression (2) with respect to a light beam having a wavelength &lgr;
2
used for recording or reproducing the other of the two optical recording media:
N
L&lgr;1
>N
D&lgr;1
(1)
N
L&lgr;2
<N
D&lgr;2
(2)
where
N
L&lgr;1
is the refractive index of the lens on the light source side with respect to the light beam having the wavelength &lgr;
1
;
N
D&lgr;1
is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength &lgr;
1
;
N
L&lgr;2
is the refractive index of the lens on the light source side with respect to the light beam having the wavelength &lgr;
2
; and
N
D&lgr;2
is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength &lgr;
2
.
Also, the present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
the objective lens satisfying the following conditional expression (3) with respect to a light beam having a wavelength &lgr;
1
used for recording or reproducing one of the two optical recording media, and
the following conditional expression (4) with respect to a light beam having a wavelength &lgr;
2
used for recording or reproducing the other of the two optical recording media:
N
L&lgr;1
<N
D&lgr;1
(3)
N
L&lgr;2
>N
D&lgr;2
(4)
where
N
L
&lgr;
1
is the refractive index of the lens on the light source side with respect to the light beam having the wavelength &lgr;
1
;
N
D
&lgr;
1
is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength &lgr;
1
;
N
L
&lgr;
2
is the refractive index of the lens on the light source side with respect to the light beam having the wavelength &lgr;
2
; and
N
D
&lgr;
2
is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength &lgr;
2
.
Further, the present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
assuming that the optical recording medium recorded or reproduced with a light beam having a wavelength &lgr;
1
and the optical recording medium recorded or reproduced with a light beam having a wavelength &lgr;
2
have respective thicknesses t
1
and t
2
satisfying the condition of t
1
<t
2
,
the objective lens satisfying the following conditional expression (5) when the cemented face of the cemented lenses is convex toward the light source, and
the following conditional expression (6) when the cemented face of the cemented lenses is convex toward the optical recording media:
N
L&lgr;1
>N
D&lgr;1
and N
L&lgr;2
<N
D&lgr;
2 (5)
N
L&lgr;1
<N
D&lgr;1
and N
L&lgr;2
>N
D&lgr;2
(6)
where
N
L
&lgr;
1
is the refractive index of the lens on the light source side with respect to the light beam having the wavelength &lgr;
1
;
N
D
&lgr;
2
is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength &lgr;
1
;
N
L
&lgr;
2
is the refractive index of the lens on the light source side with respect to the light beam
Fuji Photo Optical Co., Ltd.
Snider Ronald R.
Snider & Associates
Sugarman Scott J.
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