Optical: systems and elements – Diffraction
Reexamination Certificate
1999-09-28
2002-11-26
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
C359S556000, C359S569000, C359S575000, C359S015000, C369S044140
Reexamination Certificate
active
06487015
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to diffraction gratings for generating a plurality of beams and multi-beam pickups for recording/reproducing simultaneously to/from a plurality of tracks on an optical recording medium. The invention relates particularly to a diffraction grating having a plurality of gratings with different cycles for generating a plurality of beams and an optical pickup using such a diffraction grating.
2. Description of the Background Art
Various optics-related techniques have been researched, developed and reduced to practice in a variety of fields including communication, measurement and processing, because they permit operations at high a frequencies (high speeds), spatial information processing, phase processing and the like.
Among such techniques, the use of diffraction gratings and holograms as elements for selecting wavelengths, deflecting light, and generating a plurality of beams is known. For example, a diffraction grating in a spectroscope is used for separating a light component with a different wavelength in an incident light beam, and has a constant grating cycle.
Japanese Patent Laying-Open No. 5-264923 proposes a hologram scanner using a hologram for deflecting and collecting light as an application to a laser printer. According to this document, a light beam emitted from a semiconductor laser as an optical source is allowed to come into a plurality of scan holograms formed on a hologram disc. A light beam diffracted and deflected by the scan holograms is used to form an image on a scanning surface (photoreceptor drum), and the deflection angle of the diffracted light beam is changed by the rotation of the hologram disc for scanning. In order to correct characteristics on the scanning surface such as astigmatism and curvature of field, a grating pitch for the scan hologram is two-dimensionally modulated.
There is known such an information recording/reproducing apparatus in which an optical information recording medium (optical disc) is used, a plurality of beams are generated in an optical pickup for reading information, and the plurality of beams are directed at the optical disc at a time to obtain a signal. In the optical pickup, a diffraction grating is used to generate a plurality of beams (see Japanese Patent Laying-Open No. 1-269239 for example). Such a diffraction grating used for an optical pickup will be now described in detail.
First Conventional Example
Japanese Patent Laying-Open No. 1-269239 discloses an optical pickup including a diffraction grating for dividing a beam, a hologram element for dividing light for RES (Radial Error Signal) and FES (Focus Error Signal), and a laser source as they are integrally formed.
FIG. 1
is a view of the 3-beam optical pickup described above. Divergent light emitted from a semiconductor laser
112
as an optical source is divided into a plurality of beams by a diffraction grating
113
, each transmitted through hologram element
114
as the zeroth-order diffracted light and comes into a collimator lens
115
. The beams formed into parallel rays by collimator lens
115
are collected into a small enough spot on a disc
117
by an objective lens
116
, and reflected as an optical beam reflecting information on disc
117
. The reflected light is passed through objective lens
116
and collimator lens
115
and divided into the zeroth-order diffracted light beam and the first-order diffracted light beam by hologram element
114
. The first-order diffracted light beam is allowed to come into an internal light receiving portion
118
including an RES light receiving portion, an FES light receiving portion and an RF (Radio Frequency) signal light receiving portion.
Herein, semiconductor laser
112
, diffraction grating
113
, hologram element
114
and internal light receiving portion
118
are built integrally as a hologram laser unit
119
. Diffraction grating
113
divides a light beam into three beams, the zeroth-order diffracted light beam and the ±first-order diffracted light beams, and the zeroth-order light beam is used as an RF signal and an FES, while the ±first-order diffracted light beams are used as an RES. Therefore, the tracking employs the 3-beam method.
Second Conventional Example
A multi-beam optical pickup is proposed by which an optical recording medium is irradiated with a plurality of beams, which are used to record/reproduce information simultaneously to/from a plurality of tracks on the optical recording medium (Japanese Patent Laying-Open No. 1-248329).
The multi-beam optical pickup will be now described in conjunction with related figures.
FIG. 2
is a view of an optical system. A light beam emitted from a semiconductor laser
102
as an optical source is formed into parallel light with a collimator lens
104
, then let into a diffraction grating
103
and divided into a plurality of beams. Then, each of the beams is passed through a beam splitter
105
, collected into an optical spot small enough on a disc
107
by an objective lens
106
and reflected and passed through objective lens
106
as a light beam reflecting information on disc
107
. The light beam is reflected by beam splitter
105
, and let into a light receiving portion
110
through a light collecting lens
108
and a cylindrical lens
109
.
When a light beam is divided into for example three beams with a diffraction grating, the zeroth-order diffracted light beam (referred to as main beam) of diffraction grating
103
in the center on disc
107
, and the ±first-order diffracted light beams (referred to as sub beams) on opposite sides thereof are arranged in order to simultaneously perform reading with the three beams in total.
In the diffraction gratings used in the conventional pickups according to the first and second conventional examples, as shown in the plan view in FIG.
3
and the cross sectional view in
FIG. 4
, a grating recess
121
and a grating ridge
122
in diffraction grating substrate
130
are formed along straight lines and over an aperture surface in a constant grating cycle pn.
In the hologram scanner used for a laser printer disclosed by Japanese Patent Laying-Open No. 5-264923, the diffraction grating formed in the hologram is used for deflecting a single incident beam and does not divide this single beam into a plurality of beams. This is because this application requires a great beam intensity after diffraction, so if the beam is divided into a plurality of beams, the intensity of each diffracted light beam could be lowered, and a necessary beam intensity could not be obtained on the scanning surface. Therefore, only a diffracted light beam of the lowest order is typically used.
As a result, there is not disclosed in the document any such concept of correcting aberration in the diffracted light beam of the highest order when a plurality of high order diffracted light beams are used. The grating is formed to have curves in order to converge a beam spot in the X-direction (main scanning) and the Y-direction (sub scanning) on the scanning surface.
As described above, the diffraction grating of the optical pickup according to the first and second examples typically includes a straight line grating of an equal pitch as shown in
FIGS. 3 and 4
. In such a case, particularly in a multi-beam optical pickup which records/reproduces information to/from a plurality of tracks using a plurality of optical beams, the following problem is encountered.
In a multi-beam optical pickup, if the intervals Pd of beams on the disk is equal, the relation between Pd and the intervals Ph of the beams at the internal light receiving portion is given as follows:
Pd=Ph·fOL/fCL
wherein the focal distance of the objective lens is fOL and the focal distance of the collimator lens is fCL.
In order to use a number of beams for recording/reproducing at a time, Pd should be small. To reduce Pd, Ph must be reduced or fCL must be increased. However, Ph is under restrictions as to the photoelectric conversion sensitivity and processi
Kitamura Kazuya
Kurata Yukio
Ueyama Tetsuo
Yamaguchi Takeshi
Assaf Fayez
Birch & Stewart Kolasch & Birch, LLP
Sharp Kabushiki Kaisha
Spyrou Cassandra
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