Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-04-29
2003-09-02
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S743000, C369S112120
Reexamination Certificate
active
06614600
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an objective lens used for an optical pickup device and to the optical pickup device, and in particular, to an objective lens for an optical pickup device and to the optical pickup device.
There has been developed an optical pickup device which can record and reproduce information for optical information recording media of different types such as, for example, DVD and CD, and is used for various uses. The optical pickup device of this kind is required to have various kinds of demands including compactness in structure and low cost.
As a light source for an optical pickup device, a semiconductor laser is generally used, and two light sources including a light source for DVD and that for CD have so far been used, because an optimum wavelength for recording and/or reproduction of information for DVD is 635 nm or about 650 nm and an optimum wavelength for recording and/or reproduction of information for CD is about 780 nm.
However, an increase in cost of an optical pickup device is caused by providing two expensive semiconductor lasers. Therefore, it is requested to throw two semiconductor lasers into one. To satisfy this request, it is conceivable that a light source for DVD is used to reproduce information of CD. In a write-once compact disk (CD-R) developed recently, however, there is a problem that recording or reproduction of information cannot be performed because reflectance of light for DVD whose wavelength is about 635 nm is extremely low and reflected light with sufficient amount of light cannot be obtained accordingly.
With the background mentioned above, there are disclosed in TOKKAIHEI NO. 9-306014 by the applicant of this invention an optical pickup device and an objective lens to be used for the optical pickup device which can conduct recording or reproduction of information for DVD and CD-R by using a wavelength which lies between a wavelength of light for DVD and a wavelength of light for CD.
However, when light with the intermediate wavelength disclosed in TOKKAIHEI NO. 9-306014 is used, there still is a problem that it is difficult to conduct recording or reproduction of information for the compact disk (CD-RW) which is more strict than CD-R from a viewpoint of reflectance and can erase information. In addition, there is a fear that a unit cost of a semiconductor laser with the intermediate wavelength may turn out to be high because of less demand for that semiconductor laser.
In TOKKAIHEI NO. 11-339306, on the other hand, there is disclosed an optical pickup device which can conduct reproduction for DVD by using light with wavelength of 780 nm. However, the conventional technology of this kind is one to eliminate an influence of side lobe that is caused when conducting reproduction of DVD using light with wavelength of 780 nm, and it requires an additional circuit for that purpose, resulting in an increase in cost of the optical pickup device.
For these problems, there is an attempt to create an objective lens that shows excellent aberration even when a light flux with high NA and longer wavelength and conducts information recording and reproduction for DVD and CD, by forming plural optical functional regions on the optical surface of the objective lens in the direction perpendicular to the optical axis, and by using an inner optical functional region for both DVD and CD while using an outer optical functional region for DVD only.
For the attempt stated above, it is preferable that the objective lens is made of plastic materials, for carrying out mass production at lower cost. However, since a change in refractive index caused by ambient temperature is great in the case of plastic materials, it is difficult, even when plural optical functional regions are provided, to correct aberration caused by changes in refractive index under the environmental condition of intense changes in temperatures.
In particular, when NA grows to be higher, the tendency for spherical aberration changes caused by temperature changes to increase becomes more remarkable. In this case, amount of change &dgr;SA
3
of third-order component of spherical aberration caused by temperature change is expressed by the following expression when NA represents an image side numerical aperture of the objective lens, f represents a focal length, m represents an image forming magnification, and &lgr; represents a wavelength of the laser light source.
(&dgr;
SA
3
/&dgr;T
)∝
f·
(1−
m
)
4
·NA
4
/&lgr; (1)
It is apparent from the expression (1) that aberration correction is more difficult when numerical aperture NA of an objective lens grows greater, because the spherical aberration shows a tendency to deteriorate in proportion to the fourth power of the numerical aperture NA.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the problems in the conventional technology, and its object is to provide an optical pickup device and an objective lens to be used in the optical pickup device which can conduct recording and/or reproduction of information for optical information recording media each being different from others by using a single light source with a relatively long wavelength, without using an additional electric circuit, and can control a decline of efficiency caused by ambient changes.
The objective lens to be used in an optical pickup device described in Structure
1
is represented by an objective lens to be used in an optical pickup device having therein a light source and a light converging optical system including an objective lens to conduct recording and/or reproduction of information by making a light flux emitted from the light source to form an image on an image recording surface through transparent base boards of the first and second optical information recording media, wherein when &lgr; represents a wavelength of the light source, NA
1
represents a numerical aperture on the information recording surface side of the objective lens in the case of recording or reproduction of information for the first optical information recording medium, t
1
represents a thickness of the transparent base board of the first optical information recording medium and t
2
represents a thickness of the transparent base board of the second optical information recording medium,
&lgr;>700 nm (2)
NA
1
>0.65 (3)
t
1
<t
2
(4)
and a plurality of optical functional regions are formed on an optical surface of the objective lens so that a distance of each optical functional region from the optical axis in the direction perpendicular to the optical axis may be different, and an inner optical functional region closer to the optical axis among the aforesaid optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first and second optical information recording media, and an outer optical functional region which is farther than the inner optical functional region from the optical axis among the aforesaid optical functional regions is arranged to allow a light flux emitted from the light source to pass through for conducting recording or reproduction of information for the first optical information recording medium, and a diffractive structure is formed on the outer optical functional region.
The invention makes it possible to conduct recording or reproduction of information effectively even when CD-R or CD-RW is used for the second optical information recording medium, for example, because wavelength &lgr; of the light source exceeds 700 nm. On the other hand, when DVD is used as the first optical information recording medium, for example, there is a fear that an amount of light necessary for recording or reproduction of information may be insufficient for light whose wavelength &lgr; exceeds 700 nm. In the invention, therefore, the numerical aperture NA
1
is made to be greater than 0.65 so that sufficient amount of light may be obtained. Further, for deterioration of
Konica Corporation
Squire Sanders & Dempsey L.L.P.
Sugarman Scott J.
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