Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1998-10-29
2001-01-09
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S201200, C369S044140
Reexamination Certificate
active
06172350
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical disk device, and particularly to an optical head of the optical disk device making use of holograms and an optical information reading method applied in the optical head.
Along with progress of high-density and large-capacity technology of memory devices, narrower tracks and finer pits are pursued in optical disks. For enabling the finer pit size, technical improvement of the optical head performance such as converging performance for irradiating a micro-spot, fine focusing control, accurate tracking servo or high sensitivity to the pit signal is indispensable. For realizing the high performance and the miniaturization as well of the optical head, application of the hologram to the optical head has been earnestly studied.
FIG. 6
is a perspective view schematically illustrating an optical head making use of a hologram which is described in “Optical Disk System for Multimedia”, by Imanaka et al., National Technical Report, Vol. 40, No. 6, pp. 771-778, December 1994, (hereinafter called the first prior art). In the first prior art of
FIG. 6
, a laser light which is reflected by a recording medium
602
and made parallel through an object lens
605
, is divided into two beams by a two-portion grid-type hologram
606
and converged by a collimate lens
604
. The converged two beams of +1
st
order waves are detected by a pair of photo-detectors
607
to be processed by a signal processor. This is a typical example of a CFT (Correct Far-field Tracking) hologram head.
FIG. 7
is a side view illustrating another prior art example of the hologram head, which is disclosed in a Japanese patent application laid open as a Provisional Publication No. 248144/'87 (hereinafter called the second prior art). For miniaturizing and lightening optical pickup-mechanism, the collimate lens and the object lens are replaced with phase-type holograms, whereby an incident laser light is diffracted and converged on a recording medium, in the second prior art.
Referring to
FIG. 7
, a laser diode
703
generates a laser light S-polarized having an oscillation plane perpendicular to the drawing sheet, which penetrates a first phase-type hologram
706
b
being incident with a non-Bragg angle, and diffracted by a second phase-type hologram
706
a
having interference fringes perpendicular to the sheet to be converged on an optical disk
702
. Between the second phase type hologram
706
a
and the optical disk
702
, there is provided a quarter-wavelength plate
705
, which makes circularly-polarized the laser light to be converged on the optical disk
702
, and makes P-polarized the laser light reflected by the optical disk
702
to have an oscillation plane parallel to the sheet, so that the reflected laser light penetrates the second phase-type hologram
706
a
and is diffracted by the first phase-type hologram
706
b
being incident with a Bragg angle to be converged on a photo-detector
707
.
In order to miniaturizing the optical head, the size of its optical system, that is, the light-path length between the light source or the photo-detector and the recording medium should be reduced. However, the reduction of the light-path length is limited in the first prior art of
FIG. 6
, because the collimate lens and the object lens inserting the hologram between them should be arranged at a certain distance to the light source and the photo-detector adjoining with each other.
For miniaturizing and thinning the optical system, the collimate lens and the object lens are replaced with holograms in the second prior art of FIG.
7
. However, the second prior art requires high precision phase-type holograms minutely arranged, considering complex polarization components of the holograms whereon the laser light is incident with the Bragg angle. Further, the complexity of the diffraction efficiency may cause output instability of the optical head.
Still further, the servomechanism of the second prior art depends on photo-detection of a single reflection beam. Therefore, the focusing and tracking accuracy should be degraded compared to ordinary servomechanism wherein the reflection beam is split into two beams for improving detection accuracy.
SUMMARY OF THE INVENTION
Therefore, a primary object of the present invention is to provide an optical head and an optical information reading method wherein no collimate lens nor object lens is used and photo-detection can be made with more than one reflection beams, for enabling further miniaturization of the optical head without degrading focusing and tracking precision or output stability.
In order to achieve the object, an optical head of the invention has an optical system comprising:
a light source for generating a laser light;
a collimate hologram working as a collimate lens for converting the laser light into first plane-wave light;
an object hologram working as an object lens for converting the first plane-wave light into first spherical-wave light converging on a recording medium and converting the first spherical-wave light reflected from the recording medium into second plane-wave light; and
a splitting hologram for converting the second plane-wave light into second spherical-wave light converging on a photo-detecting means.
The splitting hologram has preferably two symmetrical portions each having a carrier component symmetrical to each other and each diffracting the second plane-wave light into a part of the second spherical-wave light converging on a position which is shifted from a center line of respective one of the two symmetrical portions by a carrier angle of the respective one.
Therefore, the photo-detecting means can be arranged apart from two optical centers of the splitting hologram by the carrier angles, without needing collimate lenses for the photo-detection, which gives larger freedom in the arrangement of the optical system compared to the prior art of
FIG. 6
, enabling to shorten the light-path length from the light source and the collimate hologram to the splitting hologram. Hence, further down-sizing of the optical head can be realized.
The photo-detecting means preferably comprises a first and a second photo-detector each having two detection regions, for enabling the focusing of the optical system to be controlled by the servo controller according to a double-knife-edge method so that a sum of a difference of light intensity between the two detection regions of the first photo-detector and that of the second photo-detector becomes zero, and enabling the tracking of the optical system to be controlled by the servo controller according to a push-pull method so that a difference between a sum of light intensity of the two detection regions of the first photo-detector and that of the second photo-detector becomes zero.
Therefore, more accurate and robust focusing control is enabled against shift of the photo-detectors which may be caused along with long time usage.
As to each of the two symmetrical portions of the splitting hologram, a Leith-type ring-band-plate hologram, which can be fabricated by giving a carrier component to a Gabor-type ring-band-plate hologram, can be employed.
The collimate hologram, the object hologram or the splitting hologram may be an amplitude hologram, a phase-type hologram or a phase-type volume hologram.
Furthermore, the splitting hologram may have four symmetrical portions each having a carrier component symmetrical to each other and each diffracting the second plane-wave light into a part of the second spherical-wave light converging on a position which is shifted from a center line of respective one of the four symmetrical portions by a carrier angle of the respective one.
REFERENCES:
patent: 4929823 (1990-05-01), Kato et al.
patent: 59-160166 (1984-09-01), None
patent: 62-248144 (1987-10-01), None
patent: 8-77578 (1996-03-01), None
Imanaka et al., “Optical Disk System for Multimedia”, National Technical Report, vol. 40, No. 6, pp. 771-778, Dec. 1994.
Le Que T.
McGinn & Gibb PLLC
NEC Corporation
LandOfFree
Optical head and an optical information reading method... 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 head and an optical information reading method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical head and an optical information reading method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2447428