Optical information processor and optical element

Details Optical information processor and optical element Optical information processor and optical element

1999-11-09

2003-09-09

Hindi, Nabil (Department: 2655)

Dynamic information storage or retrieval

Specific detail of information handling portion of system

Radiation beam modification of or by storage medium

C369S112080, C369S112230, C369S112020

Reexamination Certificate

active

06618343

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical information processor in which information is optically recorded on or reproduced from an optical disk, and to an optical element used in an optical pick-up.
BACKGROUND OF THE INVENTION
The operation of an optical head that is one of conventional optical information processors is described with reference to FIGS.
18
(
a
) and
18
(
b
). Light emitted from a semiconductor laser
18
-
1
, an exemplary light source, passes through a hologram
18
-
5
as a separation element and then is focused on an optical disk
18
-
2
, as an exemplary information recording media, by an objective lens
18
-
3
. After passing through the objective lens, light reflected from the optical disk is diffracted by the hologram and is incident onto first photodetectors
18
-
4
-
1
and
18
-
4
-
2
. An aperture in an optical path from the light source to the optical disk (hereinafter referred to simply as an “incoming path”) through which light passes is determined by an objective lens holder
18
-
6
. A circular aperture is used in many cases. An aperture NA corresponds to a diameter of light being incident onto the objective lens. The diameter D satisfies the relationship of
D
=2
×f×NA,
wherein f represents a focal length of the objective lens. Since the focal length f is constant, the size of the NA corresponds to the size of the diameter D. The aperture in an optical path from the optical disk to the photodetectors (hereinafter referred to simply as a “return path”) through which light reflected from the optical disk passes also is determined by the objective lens holder
18
-
6
. Therefore, the apertures in the incoming and return paths are equal.
A detection case of various signals is described. When the hologram is formed of a part of a Fresnel lens, it can be formed so that diffracted light in one side is focused before reaching the photodetector
18
-
4
-
1
and diffracted light in the other side is focused at a position behind the photodetector
18
-
4
-
2
as shown in FIG.
18
. As shown in a view seen from the A direction in
FIG. 18
, when the respective photodetectors
18
-
4
-
1
and
18
-
4
-
2
are formed while being divided into three parts, a focus error signal FE in a SSD (spot size detection) system can be detected from the calculation result of outputs from the respective photodetectors. The FE can be obtained from either:
FE
=(
18
-
4
-
1
b
)−(
18
-
4
-
2
b
)  (1) or
FE
=((
18
-
4
-
1
a
)+(
18
-
4
-
1
c
)+(
18
-
4
-
2
b
))−((
18
-
4
-
1
b
)+(
18
-
4
-
2
a
)+(
18
-
4
-
2
c
))  (2).
When a track direction on an optical disk coincides with the information track direction shown in FIGS.
18
(
a
) and
18
(
b
), a far field pattern as a diffraction pattern produced by a track is formed at spots on the photodetectors as shown in the view seen from the A direction. Therefore, the tracking error signal TE can be obtained from any one of:
TE
=(
18
-
4
-
1
a
)−(
18
-
4
-
1
c
)  (3),
TE
=(
18
-
4
-
2
a
)−(
18
-
4
-
2
c
)  (4), and
TE
=((
18
-
4
-
1
a
)+(
18
-
4
-
2
c
))−((
18
-
4
-
1
c
)+(
18
-
4
-
2
a
))  (5).
A data information signal RF of an optical disk can be obtained from all the outputs of the photodetector
18
-
4
-
1
or
18
-
4
-
2
, or the total outputs of the photodetectors
18
-
4
-
1
and
18
-
4
-
2
.
FIG. 19
shows a configuration of an optical disk device in another conventional example using two laser beam sources that emit beams with different wavelengths from each other. This optical disk device has two laser beam sources
19
-
1
(emitting a beam with a wavelength &lgr;
1
) and
19
-
2
(emitting a beam with a wavelength &lgr;
2
) that emit beams with different wavelengths from each other. The laser beam
19
-
21
with a wavelength &lgr;
1
(in the case of DVD or the like, &lgr;
1
=660 nm) emitted from the laser beam source
19
-
1
passes through a polarization hologram element
19
-
3
.
This polarization hologram element is formed by forming a grating with a depth of din a substrate made of an anisotropic material such as lithium niobate and filling groove parts of the grating with an isotropic material (with a refractive index of n
1
). Generally, given the phase difference &phgr; between a beam passing through a groove portion and a beam passing between the groove portions, transmittance is represented by cos
2
(&phgr;/2). When the substrate has refractive indexes of n
1
and n
2
with respect to polarized lights parallel and perpendicular to the grating grooves respectively, &phgr;=0 holds with respect to the polarized light parallel to the grating grooves and therefore the transmittance is 1. On the other hand, with respect to the polarized light perpendicular to the grating grooves, &phgr;=2&pgr;(n
1
'n
2
) d/&lgr;. Therefore, when the depth d is set to obtain &phgr;=&pgr;, the transmittance is 0, i.e. the polarized light is totally diffracted.
Consequently, when considering the polarization direction of the beam
19
-
21
emitted from the laser beam source
19
-
1
and groove orientation of the polarization hologram element
19
-
3
, the laser beam
19
-
21
is allowed to pass through the element
19
-
3
without being diffracted. The transmitted light
19
-
22
is converted from linearly polarized light (S-wave) into circularly polarized light
19
-
23
by a 1/4 wave plate
19
-
4
, is reflected by a surface of a prism
19
-
5
, and then is collimated into parallel light
19
-
24
by a collimator lens
19
-
6
. The parallel light
19
-
24
enters an objective lens
19
-
8
mounted on a moving element
19
-
14
of an actuator via a mirror
19
-
7
for bending an optical path and is incident onto a signal surface
19
-
9
of the optical disk.
In the case of recording on the signal surface, by increasing the power for emitting beams of the laser beam source
19
-
1
and modulating light corresponding to a recording signal, a required signal is recorded on the signal surface
19
-
9
.
The light
19
-
25
reflected from the signal surface
19
-
9
travels in the opposite direction to the incoming path. The light
19
-
25
is converted to linearly polarized (P-wave) light
19
-
26
by the ¼ wave plate
19
-
4
and passes through the polarization hologram element
19
-
3
. In this case, due to polarization dependability of the element
19
-
3
the light is branched into a positive first-order diffracted light
19
-
27
and a negative first-order diffracted light
19
-
28
whose symmetry axis is the incident-light axis. The lights
19
-
27
and
19
-
28
are incident onto detection surfaces on photodetectors
19
-
105
provided adjacently to the laser beam source
19
-
1
. Thus, a control signal and a reproduction signal are obtained to reproduce information.
On the other hand, a laser beam
19
-
29
emitted from the semiconductor laser beam source
19
-
2
emitting a beam with the other wavelength &lgr;
2
(in the case of CD or the like, 790 nm) passes through a hologram element
19
-
11
to be diffracted and branched into three beams (a positive first-order diffracted light, a negative first-order diffracted light, a zeroth-order light). The three beams pass through the prism
19
-
5
while being limited by an aperture element
19
-
12
provided on a light-incident surface of the prism
19
-
5
and are collimated by the collimator lens
19
-
6
into convergent light
19
-
30
. Then, the convergent light passes through the objective lens
8
via a mirror
19
-
7
for bending an optical path, thus being incident onto a signal surface
19
-
15
of an optical disk whose substrate has a different thickness from that when using the laser beam source
19
-
1
. In this case, the diffracted light caused by the hologram element
19
-
11
is allocated to three spots on the signal surface and is used for the detection of a tracking control signal and a reproduction signal by a so-called three-beam tracking meth

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