Optical pickup optical system, optical pickup apparatus,...

Details Optical pickup optical system, optical pickup apparatus,... Optical pickup optical system, optical pickup apparatus,...

2000-10-05

2002-06-25

Edun, Muhammad (Department: 2653)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S044230, C369S044120, C369S112200

Reexamination Certificate

active

06411587

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an optical system for optical pickup, an optical pickup apparatus, a coupling optical system, a coupling optical system lens and a recording/reproduction apparatus, and in particular, to an optical system for optical pickup, an optical pickup apparatus, a coupling optical system, a coupling optical system lens and a recording/reproduction apparatus which are used suitably for a high density optical disk apparatus employing a light source wherein monochromaticity is poor, or a wavelength fluctuates quickly.
In recent years, there has been developed, as an optical information recording medium, a DVD (storage capacity: 4.7 GB) which is similar in terms of size to a CD (storage capacity: 640 MB) and is enhanced in terms of recording density, and it has spread rapidly. For reproduction of the DVD, there is used a laser beam emitted from a light source wherein a wavelength is 635 nm-660 nm. A divergent light flux emitted from the laser light source is collimated to a collimated light flux by a collimator lens, and then, enters an objective lens whose numerical aperture (NA) on the part of DVD is 0.6 to be converged on an image recording surface through a transparent base board of DVD.
Recently, optical information recording media similar to CD and DVD having storage capacity of 10-30 GB are developed actively, by the use of an objective lens with further higher NA and a light source with further shorter wavelength. As a short wavelength light source which is considered to be hopeful, there is a GaN blue semiconductor laser having an oscillation wavelength of about 400 nm.
However, the oscillation wavelength of the GaN blue semiconductor laser is temperature-dependent in the same way as in other semiconductor lasers, and in addition, fluctuation of the wavelength is caused by the mode hop or by laser output, and high frequency superimposing is necessary for reducing laser noise, resulting in poor monochromaticity of the oscillation wavelength, which are specific characters. It is therefore predicted that correction of longitudinal chromatic aberration (longitudinal spherical aberration) is necessary in the light-converging optical system of high density optical disk employing the GaN blue semiconductor laser.
In Session WD 26 of ISOM/ODS' 99 (Joint International Symposium on Optical Memory and Optical Data Storage 1999) Postdeadline Poster Papers, there was shown an example of experiments wherein, when a high density optical disk with storage capacity of 17 GB was reproduced by an ordinary objective lens whose numerical aperture on the part of a disk is 0.6, multi-mode oscillation is caused by high frequency superimposing to make the spectral band width to be 0.7 nm (FWHM), and jitters were worsened, compared with an occasion where no high frequency superimposing was carried out, and there was proposed an objective lens of a junction doublet type wherein longitudinal chromatic aberration is improved, and the surface on the part of a light source and the surface on the part of a disk are aspherical, and the junction surface is spherical.
As an objective lens of a junction doublet type for an optical disk wherein chromatic aberration is corrected, those described in Japanese TOKKAISHO Nos. 61-3110 and 62-286009 are known. Further, Japanese TOKKAIHEI No. 9-311271 discloses an objective lens wherein both surfaces are aspherical and represent a phase type diffraction element, and a numerical aperture on the part of an optical information recording medium is 0.7 or more.
There are further proposed an optical disk device employing an objective lens of a two-element structure whose numerical aperture on the part of an optical information recording medium is 0.85, and a high density disk. For example, in Session WC 1 (on page 50 of the preliminary report) of ISOM/ODS' 99, there was announced an optical disk device having storage capacity of 9.2 GB wherein a red semiconductor laser with wavelength of 65.0 nm was used. Further, in Session WD 1 (on page 28 of the preliminary report 2) of ISOM/ODS' 99, it was announced that the storage capacity of 20 GB was attained by using a GaN semiconductor laser with wavelength of 400 nm. With regard to these semiconductor lasers, there was no announcement about an influence of wavelength fluctuation.
In Japanese TOKKAIHEI No. 11-174318, there is described an objective lens of a two-element structure wherein the numerical aperture on the part of an optical information recording medium is 0.85 and a hologram is provided on the optical surface for correcting chromatic aberration. As stated above, it is possible to correct chromatic aberration by introducing a diffraction element in the objective lens optical system, but, manufacturing cost of a diffraction lens made of glass is very high in general.
On the other hand, as an objective lens made of plastic wherein chromatic aberration is corrected by a diffraction element, those wherein the numerical aperture on the part of an optical information recording medium is about 0.55 are put to practical use. However, with regard to an objective lens made of plastic, its spherical aberration fluctuates when a refractive index of the objective lens is varied by temperature change. Therefore, the objective lens made of plastic is not suited for a converging optical system of a high density optical disk wherein a short wavelength is required for a light source and a high NA is required for the objective lens. Though spherical aberration fluctuation caused by temperature change can be lightened by the two-element structure even in the case of an objective lens made of plastic, change in an outside diameter caused by temperature change is great because the coefficient of expansion of plastic material is greater than that of glass, and thereby, the lens tends to be distorted by temperature change to low temperature or to high temperature after being incorporated in the lens frame, which is a problem. Therefore, when the spherical aberration fluctuation caused by temperature change also is considered, it is preferable that at least one of two lenses is made to be a glass lens.
Being affected by heat generation from an actuator for driving an objective lens, temperature distribution is caused on the objective lens, and this causes distribution of refractive index when the objective lens is a plastic lens, thus, wavefront aberration of a light flux that has passed through the objective lens is worsened to cause problems easily. Accordingly, even in the case of an objective lens of a two-element structure, it is preferable that both of the two elements are represented by a glass lens when temperature characteristics are taken in consideration. However, the glass lens has a problem of manufacturing cost as stated above, and it is actually difficult to collect chromatic aberration by only an objective lens for high density disk.
As represented by an objective lens of a junction doublet type for optical disk, a lens wherein a lens made of low dispersion material having positive refracting power and a lens wherein a lens made of high dispersion material having negative refracting power are combined without using a diffraction element, is not suited for an objective lens for optical disk which is definitely required to be light in weight. The reason for the foregoing is that the refracting power of each lens is required to be great to attain high NA of the lens because dispersion of a material is limited, thereby, weight of the lens itself tends to be increased.
Japanese TOKKAISHO No. 62-269922 discloses a method to correct longitudinal spherical aberration of the total light-converging optical system for recording/reproducing an optical information recording medium, by correcting chromatic aberration of an objective lens insufficiently and by correcting chromatic aberration of a collimator lens excessively.
In view of the circumstances mentioned above, an object of the invention is to provide an optical pickup optical system which can be manufactured at low

Affiliated with

Also associated with

Rating

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

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2917937