Optical pickup apparatus and objective lens

Details Optical pickup apparatus and objective lens Optical pickup apparatus and objective lens

2000-11-15

2002-04-02

Edun, Muhammad (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S112010, C369S112230

Reexamination Certificate

active

06366542

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical pickup apparatus and a resin-made objective lens, and in particular, to an objective lens improved in terms of temperature characteristics and aberrations and to an optical pickup apparatus which employs the objective lens mentioned above and is used favorably for recording and/or reproducing an optical information recording medium.
As an optical system for recording and reproducing an optical information recording medium having accuracy required for a conventional CD reproducing apparatus (incidentally, the optical system for recording and reproducing mentioned in the present specification includes an optical system for recording, an optical system for reproducing, an optical system for recording and reproducing, or an apparatus employing them), an optical system of an infinite conjugation type is disclosed in TOKKAISHO No. 57-76512, and an optical system of a finite conjugation type is disclosed in TOKKAISHO No. 61-56314. Further, TOKKAIHEI No. 6-258573 discloses one employing a coupling lens for reducing occurrence of aberrations which are caused by temperature changes when a resin-made objective lens is used. In recent years, however, a lens made of resin (plastic) material is commonly used for the requirement of low cost with regard to an optical system for recording and reproducing, especially to an objective lens for the optical system for recording and reproducing.
However, an objective lens made of resin materials has a problem that an aberration generated by refractive index changes caused by temperature changes is greater than that for a lens made of glass material. In general, a refractive index change for resin materials is ten times or more that for glass materials. When &Dgr;T represents a difference of temperature between standard design temperature and actual ambient temperature used, an aberration is changed by this temperature difference &Dgr;T is mainly third order spherical aberration. When SA represents third order spherical aberration components of wavefront aberration expressed in an rms value, a sign is defined so that SA is greater than zero when spherical aberration is positive (over) and SA is smaller than zero when spherical aberration is negative (under). It is possible to express third order spherical aberration &Dgr;SA rms that changes depending on temperature change &Dgr;T with the following expression, by using numerical aperture NA of an objective lens on the optical information recording medium side (image side), focal length f, image forming magnification m, factor of proportionality k and wavelength of light &lgr;.
&Dgr;
SA/&Dgr;T=k·f
(1
−m
)4(
NA
)4/&lgr;  (1)
Incidentally, when a lens made of resin materials has positive refracting power, the third order spherical aberration turns out to be over when a temperature rises. Namely, in the above expression (1), factor of proportionality k takes a positive value. When a single lens made of resin materials is made to be an objective lens, the factor of proportionality k takes a large positive value.
In the objective lens for a compact disk that is presently used commonly, aberrations generated by changes in ambient temperatures used do not go up to the problematic level, because NA is about 0.45. However, at the present time wherein optical information recording media are in the trend toward high density, an objective lens constituting an optical system of an apparatus for recording and reproducing is also requested to comply with the trend of high density.
To be concrete, there has been developed, as an optical information recording medium, a DVD (storage capacity: 640 MB) which is similar to CD (storage capacity: 4.7 GB) in terms of size and has been enhanced in terms of recording density, and the DVD is spreading rapidly. For reproducing the DVD, it is general to use a laser beam with prescribed wavelength emitted from a light source whose wavelength is within a range from 635 nm to 660 nm. Further, it is general that a divergent light flux emitted from a laser light source is collimated by a collimator lens, then, is made to enter an objective lens whose NA on the DVD side is 0.6 or more, and is converged on an information recording surface through a transparent base board of DVD.
In recent years, in particular, an optical information recording medium which is the same as CD and DVD having a storage capacity of 10-30 GB is being developed actively by using an objective lens having higher NA and a light source having a shorter wavelength. As a light source having a short wavelength which is considered to be promising, there are available GaN blue semiconductor laser-with oscillation wavelength of about 400 nm and SHG blue laser. Namely, an optical system in an apparatus for recording and reproducing is requested to have high NA and requested to comply with a laser beam whose wavelength is further short.
In consideration of the foregoing from the viewpoint of wavefront aberration, when NA is increased from 0.45 to 0.6 and wavelength &lgr; of the laser beam is shortened from 660 nm to 400 nm, for example, wavefront aberration Wrms is increased by (0.6/0.45)4÷400/660=5.17 times.
Though it is considered that focal length f is made small for controlling wavefront aberration to be small based on expression (1), it is actually difficult to make f to be smaller than the current size, because of necessity to secure a focusing operation distance. Further, in the optical system of a finite conjugation type of m<0 and in the optical system of a infinite conjugation type of m=0, aberrations caused by temperature changes are serious problems in the case of high NA. Though it is considered that temperature characteristics are improved by realizing 0<m<1 in the optical system employing a coupling lens, there is a problem, in this case, that the optical system, and further, the apparatus need to be large in size, because a distance between a subject and an image of an optical system needs to be increased in length, or a coupling lens with high NA is needed, for securing an operation distance necessary for focusing.
As stated above, it has been difficult to realize an optical system having high NA in the lens system wherein a conventional resin-made objective lens is used, because of occurrence of an aberration that is in proportion to 4
th
power of numerical aperture NA of an objective lens on the image side caused by refractive index change &Dgr;n of resin material that is further caused by temperature change.
In the optical system of an optical information recording and reproducing apparatus that is requested to attain high density information recording by the short wavelength of a laser light source and by high NA, it is unavoidable to give up the use of resin-made objective lens and to use a glass-mold lens or a glass combined lens wherein a refractive index change for temperature change is small but the cost is high.
For the problems mentioned above, TOKKAIHEI No. 11-337818, for example, discloses a technology for correcting aberrations for temperature change by providing a diffraction lens structure having a prescribed spherical aberration characteristics on an objective lens for an optical head.
However, there also is a demand to reduce the number of lenses in a light-converging optical system for the purpose of achieving compactness and low cost of a recording and reproducing apparatus for an optical information recording medium. To satisfy this demand, it is necessary to constitute an optical system of the so-called finite conjugation type by the use of a single objective lens. However, the objective lens disclosed in TOKKAIHEI No. 11-337818 does not comply with the optical system of a finite conjugation type.
SUMMARY OF THE INVENTION
In an objective lens for a recording and reproducing apparatus (optical pickup apparatus) for an optical information recording medium, an object of the invention is to provide a resin-made objective lens that can secure sufficient capacity for

Affiliated with

Also associated with

Rating

Optical pickup apparatus and objective lens does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Optical pickup apparatus and objective lens, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical pickup apparatus and objective lens will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2898234