U.S. patent application number 11/045343 was filed with the patent office on 2005-06-23 for optical system for detecting data signal and tracking error signal.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Katayama, Ryuichi.
Application Number | 20050135207 11/045343 |
Document ID | / |
Family ID | 18212705 |
Filed Date | 2005-06-23 |
United States Patent
Application |
20050135207 |
Kind Code |
A1 |
Katayama, Ryuichi |
June 23, 2005 |
Optical system for detecting data signal and tracking error
signal
Abstract
In an optical system, reflected light from the disk is
diffracted by a hologram optical element, and received by an
optical detector. A focusing error signal is detected from
-1st-order diffracted light of the hologram optical element. A
tracking error signal by the differential phase method, a tracking
error signal by a push-pull method and the data signal recorded on
the disk are detected from +1st-order diffracted light of the
hologram optical element. A diffraction efficiency of the
+1st-order diffracted light is higher than a diffraction efficiency
of the -1st-order diffracted light.
Inventors: |
Katayama, Ryuichi; (Tokyo,
JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NEC CORPORATION
|
Family ID: |
18212705 |
Appl. No.: |
11/045343 |
Filed: |
January 31, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11045343 |
Jan 31, 2005 |
|
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09442773 |
Nov 18, 1999 |
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Current U.S.
Class: |
369/44.23 ;
250/201.5; G9B/7.113; G9B/7.114; G9B/7.124; G9B/7.134 |
Current CPC
Class: |
G11B 7/0901 20130101;
G11B 7/131 20130101; G11B 7/0908 20130101; G11B 7/1353 20130101;
G11B 7/123 20130101; G11B 7/1356 20130101; G11B 7/0943 20130101;
G11B 7/1381 20130101 |
Class at
Publication: |
369/044.23 ;
250/201.5 |
International
Class: |
G11B 007/00; G02B
007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 1998 |
JP |
10-328656 |
Claims
What is claimed is:
1. An optical system comprising: a light source; an object lens for
focusing emitted light from the light source onto an optical
recording medium; first optical separating means which is provided
between the light source and the object lens and which separates an
optical path of reflected light from the optical recording medium,
from an optical path of the emitted light from the light source;
second optical separating means which separates the reflected light
from the optical recording medium via the first optical separating
means into a first group of light and a second group of light; and
an optical detector for receiving the first group of light and the
second group of light; wherein an optical signal strength of the
first group of light is larger than an optical signal strength of
the second group of light.
2. An optical system as claimed in claim 1, wherein: wherein the
optical system is constituted such that a tracking error signal by
a differential phase method, a tracking error signal by a push-pull
method and a data signal recorded on the optical recording medium
are detected from the first group of light while a focusing error
signal is detected from the second group of light.
3. An optical system as claimed in claim 1, wherein: the first
optical separating means and the second optical separating means
are an integrated polarizing hologram optical element, the
polarizing hologram optical element transmits the emitted light
from the light source and diffracts the reflected light from the
optical recording medium, and the first group of light is
+1st-order diffracted light of the polarizing hologram optical
element while the second group of light is -1st-order diffracted
light of the polarizing hologram optical element.
4. An optical system as claimed in claim 3, wherein: the polarizing
hologram optical element is divided into four regions by two
divided lines respectively in parallel with a radial direction and
a tangential direction of the optical recording medium, and
directions of lattices or pitches of the lattices of the four
regions are different from each other.
5. An optical system as claimed in claim 3, wherein: a phase
distribution of lattices in the polarizing hologram optical element
is formed in a step-like shape of four levels, when phase
differences of light transmitting through the two contiguous levels
for ordinary light and extraordinary light are designated
respectively by notation .phi.o and .phi.e and widths of the
lattices of a 1-st stage through a 4-th stage are respectively
designated by notations p/2-w, w, p/2-w and w, .phi.o is
substantially equal to 0, .phi.e is substantially equal to .pi./2
and w/p falls within the range of 0<w/p<0.25 or
0.25<w/p<0.5, and the emitted light from the light source is
incident on the polarizing hologram optical element as the ordinary
light while the reflected light from the optical recording medium
is incident on the polarizing hologram optical element as the
extraordinary light.
6. An optical system as claimed in claim 3, wherein: a phase
distribution of lattices in the polarizing hologram optical element
is formed in a step-like shape of four levels, when phase
differences of light transmitting through the two contiguous levels
for ordinary light and extraordinary light are designated
respectively by notations .phi.o and .phi.e and widths of the
lattices of a 1-st stage through a 4-th stage are respectively
designated by notations p/2-w, w, p/2-w and w, .phi.o is
substantially equal to .pi./2, .phi.e is substantially equal to 0
and w/p falls within the range of 0<w/p<0.25 or
0.25<w/p<0.5, and the emitted light from the light source is
incident on the polarizing hologram optical element as the
extraordinary light while the reflected light from the optical
recording medium is incident on the polarizing hologram optical
element as the ordinary light.
7. An optical system as claimed in claim 1, wherein: the second
optical separating means comprises a Wollaston prism, the first
group of light is one of two refracted lights of the Wollaston
prism, and the second group of light is the other of two refracted
lights of the Wollaston prism.
8. An optical system as claimed in claim 7, wherein: the Wollaston
prism includes a first prism disposed on an incident side of the
reflected light from the optical recording medium and a second
prism disposed on an emitting side of the reflected light from the
optical recording medium, an optical axis of the first prism is
inclined by an angle .theta. to a direction in parallel with a
polarizing direction of the reflected light from the optical
recording medium, an optical axis of the second prism is inclined
by the angle .theta. to a direction orthogonal to the polarizing
direction of the reflected light from the optical recording medium,
the first group of light is refracted light constituting
extraordinary light in the first prism and constituting ordinary
light in the second prism of the reflected lights from the optical
recording medium, the second group of light is refracted light
constituting the ordinary light in the first prism and constituting
the extraordinary light in the second prism in the reflected light
from the optical recording medium, and .theta. falls within the
range of -45.degree.<.theta.<0.degree. or
0.degree.<.theta.<.theta.<45.degree..
9. An optical system as claimed in claim 7, wherein: the Wollaston
prism includes a first prism disposed on an incident side of the
reflected light from the optical recording medium and a second
prism disposed on an emitting side of the reflected light from the
optical recording medium, an optical axis of the first prism is
inclined by an angle .theta. to a direction in parallel with a
polarizing direction of the reflected light from the optical
recording medium, an optical axis of the second prism is inclined
by the angle .theta. to a direction orthogonal to the polarizing
direction of the reflected light from the optical recording medium,
the first group of light is refracted light constituting ordinary
light in the first prism and constituting extraordinary light in
the second prism in the reflected light from the optical recording
medium, the second group of light is refracted light constituting
the extraordinary light in the first prism and constituting the
ordinary light in the second prism of the reflected lights from the
optical recording medium, and .theta. falls within the range of
-90.degree.<.theta.<-45.degree. or
45.degree.<.theta.<90.degree..
10. An optical system as claimed in claim 7, wherein: a four
division prism for refracting the reflected light from the optical
recording medium is provided between the Wollaston prism and the
optical detector or between the first optical separating means and
the Wollaston prism, the four division prism is divided into four
regions by two dividing lines respectively in parallel with a
radial direction and a tangential direction of the optical
recording medium, and directions of inclination of the emitting
faces in respect of the incident faces or angles made by the
emitting faces and the incident faces of the four regions are
different from each other.
11. An optical system as claimed in claim 7, wherein: a hologram
optical element for diffracting the reflected light from the
optical recording medium as +1st-order diffracted light is provided
between the Wollaston prism and the optical detector or between the
first optical separating means and the Wollaston prism, the
hologram optical element is divided into four regions by two
dividing lines respectively in parallel with a radial direction and
a tangential direction of the optical recording medium, and
directions of lattices, pitches of the lattices or phase
distributions of the lattices are different from each other.
12. An optical system as claimed in claim 11, wherein: the phase
distribution of the lattices in the hologram optical element is
formed in a step-like shape of N levels (N is an integer equal to
or larger than 3), and when a phase difference of light
transmitting through the two contiguous levels is designated by a
notation .phi. and all of widths of the lattices of a 1-st stage
through an N-th stage are designated by a notation p/N, .phi. is
substantially equal to 2.pi./N.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS
[0001] This application is a division of application Ser. No.
09/442,773, filed Nov. 18, 1999, now pending, and based on Japanese
Patent Application No. 10-328656, filed Nov. 18, 1998, by Ryuichi
KATAYAMA. This application claims only subject matter disclosed in
the parent application and therefore presents no new matter.
BACKGROUND OF THE INVENTION
[0002] This invention relates to an optical head apparatus for
carrying out recording and reproducing for an optical recording
medium, and in particular, to an optical head apparatus capable of
detecting both of a tracking error signal by a differential phase
method and a tracking error signal by a push-pull method.
[0003] For an optical recording medium of a read only type, such
as, DVD-ROM, a differential phase method is generally used as a
method of detecting a tracking error signal.
[0004] On the other hand, for an optical recording medium of a
rewritable type, such as, DVD-RAM, a push-pull method is generally
used as a method of detecting a tracking error signal.
[0005] Accordingly, in order to deal with both of an optical
recording medium of the read only type and an optical recording
medium of the rewritable type by a single optical head apparatus,
both of a tracking error signal by the differential phase method
and a tracking error signal by the push-pull method are required to
be detected.
[0006] Further, as a method of detecting a focusing error signal,
there are generally used a Foucault method (or double knife edge
method), an astigmatism method and a spot size method.
[0007] In this case, the Foucault method is featured in that noise
of the focusing error signal in traversing tracks is smaller than
those of the astigmatism method and the spot size method.
[0008] Japanese Unexamined Patent Publication (JP-A) No.
143878/1998 and Japanese Unexamined Patent Publication (JP-A) No.
143883/1998 disclose an optical head apparatus capable of detecting
both of a tracking error signal by the differential phase method
and a tracking error signal by the push-pull method, and also
capable of detecting a focusing error signal by the Foucault
method.
[0009] FIG. 1 shows a structure of a first conventional optical
head apparatus disclosed in Japanese Unexamined Patent Publication
No. 143878/1998.
[0010] Emitted light from a semiconductor laser 1 is formed into a
parallel ray by a collimator lens 2, is incident on a polarization
beam splitter 3 as p-polarized light, transmits therethrough
substantially by 100%, is converted from linearly polarized light
into circularly polarized light by a quarter-wave plate 4, and is
focused onto a disk 6 by an object lens 5.
[0011] Reflected light from the disk 6 transmits through the object
lens 5 in a reverse direction, is converted from circularly
polarized light into linearly polarized light by the quarter-wave
plate 4, is incident on the polarization beam splitter 3 as
s-polarized light, reflected thereby substantially by 100%, is
diffracted by a hologram optical element 158, transmits through a
lens 8 and is received by an optical detector 159.
[0012] FIG. 2 is a plane view of the hologram optical element 158.
The hologram optical element 158 is divided into four of a region
160 through a region 163 by two dividing lines respectively in
parallel with a radial direction and a tangential direction of the
disk 6.
[0013] FIG. 3 shows a pattern of the optical detector 159 and light
spots on the optical detector 159.
[0014] The light detector 159 has a light receiving portion 164
through a light receiving portion 171. With this structure,
+1st-order diffracted light from the region 160 of the hologram
optical element 158 forms a light spot 173 on a boundary line
between the light receiving portion 164 and the light receiving
portion 165. -1st-order diffracted light therefrom forms a light
spot 178 on the light receiving portion 170.
[0015] +1st-order diffracted light from the region 161 of the
hologram optical element 158 forms a light spot 172 outside of the
light receiving portions while -1st-order diffracted light
therefrom forms a light spot 179 on the light receiving portion
171.
[0016] +1st-order diffracted light from the region 162 of the
hologram optical element 158 forms a light spot 174 on a boundary
between the light receiving portion 166 and the light receiving
portion 167 while -1st-order diffracted light therefrom forms a
light spot 177 on the light receiving portion 169.
[0017] +1st-order diffracted light from the region 163 of the
hologram optical element 158 forms a light spot 175 outside of the
light receiving portions while -1st-order diffracted light
therefrom forms a light spot 176 on the light receiving portion
168.
[0018] When outputs from the light receiving portion 164 through
the light receiving portion 171 are respectively designated by
notations V164 through V171, the focusing error signal by the
Foucault method is obtained by calculation of
(V164+V167)-(V165+V166).
[0019] The tracking error signal by the differential phase method
is obtained from a phase difference between V168+V170 and
V169+V171.
[0020] Further, the tracking error signal by the push-pull method
is obtained from calculation of (V168+V171)-(V169+V170).
[0021] Moreover, a data signal recorded on the disk 6 is obtained
from calculation of V168+V169+V170+V171 or
V164+V165+V166+V167+V168+V169+V170+- V171.
[0022] FIG. 4 shows a structure of a module 180 which is a
principal portion of the second conventional optical head disclosed
in Japanese Unexamined Patent Publication No. 143883/1998.
[0023] A semiconductor laser 181 and an optical detector 182 are
installed inside the module 180, and a hologram optical element 183
is arranged at a window portion of the module 180.
[0024] Emitted light from the semiconductor laser 181 partially
transmits through the hologram optical element 183, and progresses
toward a disk. Reflected light from the disk is partially
diffracted by the hologram optical element 183, and is received by
the optical detector 182.
[0025] FIG. 5 is a plane view of the hologram optical element 183.
The hologram optical element 183 is divided into four of a region
184 through a region 187 by two dividing lines respectively in
parallel with a radial direction and a tangential direction of the
disk.
[0026] FIG. 6 shows a pattern of the optical detector 182 and light
spots on the optical detector 182.
[0027] The optical detector 182 is provided with a light receiving
portion 188 through a light receiving portion 193. +1st-order
diffracted light from the region 184 of the hologram optical
element 183 forms a light spot 195 on a boundary line between the
light receiving portion 189 and the light receiving portion
190.
[0028] +1st-order diffracted light from the region 185 of the
hologram optical element 183 forms a light spot 194 on the light
receiving portion 188.
[0029] +1st-order diffracted light from the region 186 of the
hologram optical element 183 forms a light spot 197 on the light
receiving portion 193.
[0030] +1st-order diffracted light from the region 187 of the
hologram optical element 183 forms a light spot 196 on a boundary
line between the light receiving portion 191 and the light
receiving portion 192.
[0031] When outputs from the light receiving portion 188 through
the light receiving portion 193 are designated by notations V188
through V193, the focusing error signal by the Foucault method is
obtained by calculation of (V189+V192)-(V190+V191). The tracking
error signal by the differential phase method is obtained by a
phase difference between V189+V190+V191+V192 and V188+V193.
[0032] The tracking error signal by the push-pull method is
obtained from calculation of (V189+V190+V193)-(V188+V191+V192).
Further, a data signal recorded on the disk is obtained from
calculation of V188+V189+V190+V191+V192+V193.
[0033] FIG. 7 is a sectional view of the hologram optical element
183. The hologram optical element 183 is constituted so that a
dielectric film 198 is formed on a glass substrate 14.
[0034] With such a structure, emitted light from the semiconductor
laser 181 is incident as incident light 199 on the hologram optical
element 183, transmits therethrough as transmitted light 200, and
progresses toward the disk.
[0035] Reflected light from the disk is incident as incident light
201 onto the hologram optical element 183, is diffracted as
+1st-order diffracted light 202, and is received by the optical
detector 182.
[0036] By forming a sectional shape of the dielectric film 198 in a
sawtooth-like shape, the diffraction efficiency of the +1st-order
diffracted light is enhanced, and almost no -1st-order diffracted
light is generated.
[0037] In the first conventional optical head apparatus, the data
signal recorded on the disk 6 is obtained by the calculation of
V168+V169+V170+V171 or V164+V165+V166+V167+V168+V169+V170+V171.
[0038] In the latter case, the light spot 173 formed on the
boundary between the light receiving portion 164 and the light
receiving portion 165 and the light spot 174 formed on the boundary
line between the light receiving portion 166 and the light
receiving portion 167, are used for detecting the data signal.
[0039] However, the frequency characteristic as an optical detector
on the boundary line is lower than that on the light receiving
portion. Therefore, the optical spot formed on the boundary line
does not substantially contribute to detecting the data signal
which is a high frequency signal.
[0040] Hence, consider only a case in that the data signal recorded
on the disk 6 is obtained from calculation of
V168+V169+V170+V171.
[0041] High S/N is requested to the data signal recorded in the
disk 6 and the tracking error signal by the differential phase
method because both of them are high frequency signals.
[0042] In order to achieve high S/N, a ratio A of an optical amount
used in detecting these signals to an optical amount of the
reflected light from the disk 6 is needed to be as large as
possible.
[0043] A sectional shape of the hologram optical element 158 is
rectangular. Therefore, the diffraction efficiency of the
+1st-order diffracted light and the diffraction efficiency of the
-1st-order diffracted light are equal to each other.
[0044] In this case, maximum values of the diffraction efficiencies
of the .+-.1st-order diffracted light are about 40.5%,
respectively. Namely, the maximum value of the above-mentioned A is
equal to 0.405. The value is not necessarily regarded as
sufficiently large.
[0045] In the second conventional optical head apparatus, the data
signal recorded on the disk is obtained by the calculation of
V188+V189+V190+V191+V192+V193. In this case, the optical spot 195
formed on the boundary line of the light receiving portion 189 and
the light receiving portion 190 and the optical spot 196 formed on
the boundary line of the light receiving portion 191 and the light
receiving portion 192, are used for detecting the data signal.
[0046] However, the frequency characteristic as an optical detector
on the boundary line is lower than that on the light receiving
portion. Accordingly, the optical spot formed on the boundary line
does not substantially contribute to detecting the data signal that
is a high frequency signal.
[0047] That is, this is equivalent to detecting the data signal by
only using the optical spot 194 and the optical spot 197 in
correspondence with a half of the section for the reflected light
from the disk. Consequently, the resolution of the data signal and
crosstalk between contiguous tracks are deteriorated, and the data
signal cannot be correctly detected.
[0048] Further, the focusing error signal is detected by only using
the light spot 195 and the light spot 196 corresponding to a half
in the section of the reflected light from the disk. In
consequence, noise of the focusing error signal in traversing
tracks becomes large, and the focusing error signal cannot be
correctly detected.
[0049] There is conceivable a structure in which in place of the
hologram optical element 158 in the first conventional optical head
apparatus, the hologram optical element 183 in the second
conventional optical head apparatus is used, the focusing error
signal is detected from the -1st-order diffracted light and the
tracking error signal by the differential phase method and the
tracking error signal by the push-pull method and the data signal
recorded on the disk 6 are detected from the +1st-order diffracted
light.
[0050] However, in this case, the diffraction efficiency of the
+1st-order diffracted light is high. Therefore, the above-mentioned
value A can be increased. But, almost no -1st-order diffracted
light is generated. In consequence, the focusing error signal
cannot be actually detected.
SUMMARY OF THE INVENTION
[0051] It is therefore an object of the invention to provide an
optical head apparatus which has a large ratio A of an optical
amount used in detecting a data signal recorded on a disk and a
tracking error signal by a differential phase method to an optical
amount of reflected light from the disk.
[0052] It is another object of the invention to provide an optical
head apparatus which is capable of realizing high S/N with respect
to these signals.
[0053] According to a first aspect of the invention, there is
provided an optical head apparatus including a light source, an
object lens for focusing emitted light from the light source onto
an optical recording medium, first optical separating means
provided between the light source and the object lens for
separating an optical path of reflected light from the optical
recording medium, from an optical path of the emitted light from
the light source, second optical separating means further
separating the reflected light from the optical recording medium
via the first optical separating means into a first group of light
and a second group of light, and an optical detector for receiving
the first group of light and the second group of light.
[0054] With this structure, an optical amount of the first group of
light is larger than an optical amount of the second group of
light.
[0055] When the reflected light from the disk is divided into the
first group of light and the second group of light in this way, the
value of the ratio A of optical amounts is large.
[0056] Further, high S/N can be achieved in respect of the data
signal recorded on the disk and the tracking error signal by the
differential phase method. This is because the optical amount of
the first group of light is larger than the optical amount of the
second group of light.
[0057] According to a second aspect of the invention, a tracking
error signal by a differential phase method, a tracking error
signal by a push-pull method and a data signal recorded on the
optical recording medium are detected from the first group of light
while a focusing error signal is detected from the second group of
light.
[0058] According to a third aspect of the invention, the second
optical separating means may be a hologram optical element. The
first group of light is +1st-order diffracted light of the hologram
optical element while the second group of light is -1st-order
diffracted light of the hologram optical element.
[0059] According to a fourth aspect of the invention, the hologram
optical element is divided into four regions by two divided lines
respectively in parallel with a radial direction and a tangential
direction of the optical recording medium. Directions of lattices
or pitches of the lattices of the four regions differ from each
other.
[0060] According to a fifth aspect of the invention, a phase
distribution of the lattices in the hologram optical element is
formed in a step-like shape of four levels.
[0061] When a phase difference of light transmitting through the
two contiguous levels is designated by a notation .phi. and widths
of the lattices at a 1-st stage through a 4-th stage are
respectively designated by notations p/2-w, w, p/2-w and w, .phi.
is substantially .pi./2, and w/p falls within the range of
0<w/p<0.25 or 0.25<w/p<0.5.
[0062] According to a sixth aspect of the invention, the first
optical separating means and the second optical separating means
may be an integrated polarizing hologram optical element. The
polarizing hologram optical element transmits the emitted light
from the light source and diffracts the reflected light from the
optical recording medium.
[0063] The first group of light is +1st-order diffracted light of
the polarizing hologram optical element while the second group of
light is -1st-order diffracted light of the polarizing hologram
optical element.
[0064] According to a seventh aspect of the invention, the
polarizing hologram optical element is divided into four regions by
two divided lines respectively in parallel with a radial direction
and a tangential direction of the optical recording medium.
Directions of lattices or pitches of the lattices of the four
regions differ from each other.
[0065] According to an eighth aspect of the invention, a phase
distribution of lattices in the polarizing hologram optical element
is formed in a step-like shape of four levels.
[0066] When phase differences of light transmitting through the two
contiguous levels in respect of ordinary light and extraordinary
light are designated respectively by notations .phi.o and .phi.e
and widths of the lattices of a 1-st stage through a 4-th stage are
respectively designated by notations p/2-w, w, p/2-w and w, .phi.o
is substantially 0, .phi.e is substantially .pi./2, and w/p falls
in a range of 0<w/p<0.25 or 0.25<w/p<0.5.
[0067] The emitted light from the light source is incident on the
polarizing hologram optical element as the ordinary light. The
reflected light from the optical recording medium is incident on
the polarizing hologram optical element as the extraordinary
light.
[0068] According to a ninth aspect of the invention, a phase
distribution of lattices in the polarizing hologram optical element
is formed in a step-like shape of four levels.
[0069] When phase differences of light transmitting through the two
contiguous levels in respect of ordinary light and extraordinary
light are designated respectively by notations .phi.o and .phi.e
and widths of the lattices of a 1-st stage through a 4-th stage are
respectively designated by notations p/2-w, w, p/2-w and w, .phi. o
is substantially .pi./2, .phi.e is substantially 0, and w/p falls
within the range of 0<w/p<0.25 or 0.25<w/p<0.5.
[0070] The emitted light from the light source is incident on the
polarizing hologram optical element as the extraordinary light. The
reflected light from the optical recording medium is incident on
the polarizing hologram optical element as the ordinary light.
[0071] According to a tenth aspect of the invention, the second
optical separating means may be a Wollaston prism. With this
structure, the first group of light is one of two refracted lights
of the Wollaston prism while the second group of light is the other
of two refracted lights of the Wollaston prism.
[0072] According to an eleventh aspect of the invention, the
Wollaston prism has a first prism disposed on an incident side of
the reflected light from the optical recording medium and a second
prism disposed on an emitting side of the reflected light from the
optical recording medium.
[0073] In this event, an optical axis of the first prism is
inclined by an angle .theta. to a direction in parallel with a
polarizing direction of the reflected light from the optical
recording medium.
[0074] An optical axis of the second prism is inclined by the angle
.theta. to a direction orthogonal to the polarizing direction of
the reflected light from the optical recording medium.
[0075] The first group of light is a refracted light constituting
extraordinary light in the first prism and constituting ordinary
light in the second prism in the reflected light from the optical
recording medium.
[0076] The second group of light is refracted light constituting
the ordinary light in the first prism and constituting the
extraordinary light in the second prism in the reflected light from
the optical recording medium. In this condition, .theta. falls
within the range of -45.degree.<.theta.<0.degree. or
0.degree.<.theta.<45.degree.- .
[0077] According to a twelfth aspect of the invention, the
Wollaston prism has a first prism disposed on an incident side of
the reflected light from the optical recording medium and a second
prism disposed on an emitting side of the reflected light from the
optical recording medium.
[0078] An optical axis of the first prism is inclined by an angle
.theta. to a direction in parallel with a polarizing direction of
the reflected light from the optical recording medium.
[0079] An optical axis of the second prism is inclined by the angle
.theta. to a direction orthogonal to the polarizing direction of
the reflected light from the optical recording medium.
[0080] The first group of light is refracted light constituting
ordinary light in the first prism and constituting extraordinary
light in the second prism in the reflected light from the optical
recording medium.
[0081] The second group of light is refracted light constituting
the extraordinary light in the first prism and constituting the
ordinary light in the second prism in the reflected light from the
optical recording medium. In this case, .theta. falls within the
range of -90.degree.<.theta.<-45.degree. or
45.degree.<.theta.<90.degr- ee..
[0082] According to a thirteenth aspect of the invention, a four
division prism for refracting the reflected light from the optical
recording medium is provided between the Wollaston prism and the
optical detector or between the first optical separating means and
the Wollaston prism.
[0083] The four division prism is divided into four regions by two
dividing lines respectively in parallel with a radial direction and
a tangential direction of the optical recording medium.
[0084] Directions of inclination of emitting faces in respect of an
incident face or angles made by the emitting faces and the incident
face of the four regions differ from each other.
[0085] According to a fourteenth aspect of the invention, a
hologram optical element for diffracting the reflected light from
the optical recording medium as +1st-order diffracted light is
provided between the Wollaston prism and the optical detector or
between the first optical separating means and the Wollaston
prism.
[0086] The hologram optical element is divided into four regions by
two dividing lines respectively in parallel with a radial direction
and a tangential direction of the optical recording medium.
Directions of lattices, pitches of the lattices or phase
distributions of the lattices differ from each other.
[0087] According to a fifteenth aspect of the invention, the phase
distribution of the lattices in the hologram optical element is
formed in a step-like shape of N levels (N is an integer equal to
or larger than 3).
[0088] When a phase difference of light transmitting through the
two contiguous levels is designated by a notation .phi. and all of
widths of the lattices of a 1-st stage through an N-th stage are
designated by a notation p/N, .phi. is substantially 2.pi./N.
BRIEF DESCRIPTION OF THE DRAWINGS
[0089] FIG. 1 is a view showing a first conventional optical head
apparatus;
[0090] FIG. 2 is a plane view of a hologram optical element
according to the first conventional optical head apparatus;
[0091] FIG. 3 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the first
conventional optical head apparatus;
[0092] FIG. 4 is a view showing a module which is a principal
portion of a second conventional optical head apparatus;
[0093] FIG. 5 is a plane view of a hologram optical element
according to the second conventional optical head apparatus;
[0094] FIG. 6 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the second
conventional optical head apparatus;
[0095] FIG. 7 is a sectional view of a hologram optical element
according to the second conventional optical head apparatus;
[0096] FIG. 8 is a view showing a first embodiment of an optical
head apparatus according to the invention;
[0097] FIG. 9 is a plane view of a hologram optical element
according to the first embodiment of the optical head apparatus of
the invention;
[0098] FIG. 10 is a sectional view of the hologram optical element
according to the first embodiment of the optical head apparatus of
the invention;
[0099] FIG. 11 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the first
embodiment of the optical head apparatus of the invention;
[0100] FIG. 12 is a plane view of a hologram optical element
according to a second embodiment of an optical head apparatus of
the invention;
[0101] FIGS. 13A and 13B are sectional views of the hologram
optical element according to the second embodiment of the optical
head apparatus of the invention;
[0102] FIG. 14 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the second
embodiment of the optical head apparatus of the invention;
[0103] FIG. 15 is a view showing a third embodiment of an optical
head apparatus according to the invention;
[0104] FIG. 16 is a sectional view of a polarizing hologram optical
element according to the third embodiment of the optical head
apparatus of the invention;
[0105] FIG. 17 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the third
embodiment of the optical head apparatus of the invention;
[0106] FIGS. 18A and 18B are sectional views of a polarizing
hologram optical element according to a fourth embodiment of an
optical head apparatus of the invention, and FIG. 18A corresponds
to regions 36 and 37 while FIG. 18B corresponds to regions 38 and
39;
[0107] FIG. 19 is a view showing a pattern of an optical detector
and light spots on the optical detector according to the fourth
embodiment of the optical head apparatus of the invention;
[0108] FIG. 20 is a view showing a fifth embodiment of an optical
head apparatus according to the invention;
[0109] FIGS. 21A and 21B are views showing a constitution of a
Wollaston prism according to the fifth embodiment of the optical
head apparatus of the invention;
[0110] FIGS. 22A, 22B and 22C are views showing a constitution of a
four division prism according to the fifth embodiment of the
optical head apparatus of the invention;
[0111] FIG. 23 is a plane view of a hologram optical element
according to a sixth embodiment of an optical head apparatus of the
invention; and
[0112] FIGS. 24A and 24B are sectional views of the hologram
optical element according to the sixth embodiment of the optical
head apparatus of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0113] Referring to drawings, description will be hereinafter made
about embodiments of this invention.
First Embodiment
[0114] Referring FIG. 8, description will be made about a first
embodiment of this invention.
[0115] In FIG. 8, emitted light from the semiconductor laser 1 is
formed into a parallel ray by the collimator lens 2, is incident on
the polarizing beam splitter 3 as p-polarized light, transmits
therethrough substantially by 100%, is converted from linearly
polarized light to circularly polarized light by the quarter-wave
plate 4, and is focused onto the disk 6 through the object lens
5.
[0116] Reflected light from the disk 6 transmits through the object
lens 5 in a reverse direction, is converted from circularly
polarized light into linearly polarized light by the quarter-wave
plate 4, is incident on the polarizing beam splitter 3 as
s-polarized light, is reflected substantially by 100%, is
diffracted by the hologram optical element 7, transmits through the
lens 8, and is received by the optical detector 9.
[0117] In FIG. 9, the hologram optical element 7 is divided into
four of a region 10 through a region 13 by two dividing lines
respectively in parallel with a radial direction and a tangential
direction of the disk 6. It is to be noted that a direction of a
lattice is in parallel with the tangential direction of the disk 6
in any of the region 10 through the region 13. Further, pitches of
the lattices become wider in the order of the region 10, the region
11, the region 12 and the region 13.
[0118] In FIG. 10, the hologram optical element 7 is constituted so
that a dielectric film 15 is formed on the glass substrate 14. The
reflected light from the disk 6 is incident on the hologram optical
element 7 as incident light 16, is diffracted as -1st-order
diffracted light 17 and +1st-order diffracted light 18, and is
received by the optical detector 9.
[0119] The sectional shape of the dielectric film 15 is formed in a
step-like shape of four levels. All of differences in heights of
the two contiguous levels are equal to each other. When a phase
difference of light transmitting through the two contiguous levels
is designated by notation .phi. and widths of the lattices of a
first stage through a fourth stage are respectively designated by
notations p/2-w, w, p/2-w and w, the diffraction efficiency
.eta..sub.-1 of the -1st-order diffracted light and the diffraction
efficiency .eta..sub.+1 of the +1st-order diffracted light are
respectively given by Equation (1) and Equation (2).
.eta..sub.-1=(2/.pi..sup.2)(1-cos 2.phi.){1-sin(2.pi.w/p)sin .phi.}
(1)
.eta..sub.+1=(2/.pi..sup.2)(1-cos 2.phi.){1+sin(2.pi.w/p)sin .phi.}
(2)
[0120] When .phi.=.pi./2, w/p=0.135 or w/p=0.365, .eta..sub.-1=0.10
and .eta..sub.+1=0.71. Accordingly, when the focusing error signal
is detected from the -1st-order diffracted light 17 and the
tracking error signal by the differential phase method, the
tracking error signal by the push-pull method and the data signal
recorded on the disk 6 are detected by the +1st-order diffracted
light 18, the value of the above-mentioned A becomes 0.71 which is
larger than the value in the first conventional optical head
apparatus. In this event, w/p satisfying
.eta..sub.-1<.eta..sub.+1 and .eta..sub.-1.noteq.0 falls within
the range of 0<w/p<0.25 or 0.25<w/p<0.5.
[0121] When the difference in the heights of the two contiguous
levels of the dielectric film 15 is designated by notation h/4, the
refractive index of the dielectric film 15 is designated by
notation n and the wavelength of the incident light 16 is
designated by notation .lambda., .phi. is given by Equation (3) as
follows.
.phi.=(2.pi./.lambda.)(n-1)h/4 (3)
[0122] In the case of .lambda.=660 nm, when SiO.sub.2 is used for
the dielectric film 15, since n=1.46, in order to set .phi. as
.phi.=.pi./2, h may be as h=1.43 .mu.m.
[0123] In FIG. 11, the optical detector 9 has a light receiving
portion 19 through a light receiving portion 26. -1st-order
diffracted light from the region 10 of the hologram optical element
7 forms a light spot 27 on a boundary line between the light
receiving portion 19 and the light receiving portion 20 while
+1st-order diffracted light therefrom forms a light spot 34 on the
light receiving portion 26.
[0124] -1st-order diffracted light from the region 11 of the
hologram optical element 7 forms a light spot 28 on the boundary
line between the light receiving portion 19 and the light receiving
portion 20 while +1st-order diffracted light therefrom forms a
light spot 33 on the light receiving portion 25.
[0125] -1st-order diffracted light from the region 12 of the
hologram optical element 7 forms a light spot 29 on a boundary line
between the light receiving portion 21 and the light receiving
portion 22 while +1st-order diffracted light therefrom forms a
light spot 32 on the light receiving portion 24.
[0126] -1st-order diffracted light from the region 13 of the
hologram optical element 7 forms a light spot 30 on the boundary
line between the light receiving portion 21 and the light receiving
portion 22 while +1st-order diffracted light forms a light spot 31
on the light receiving portion 23.
[0127] When outputs from the light receiving portion 19 through the
light receiving portion 26 are respectively designated by notations
V19 through V26, the focusing error signal by the Foucault method
is obtained by calculation of (V19+V22)-(V20+V21). The tracking
error signal by the differential phase method is obtained from the
phase difference between V23+V26 and V24+V25.
[0128] The tracking error signal by the push-pull method is
obtained by calculation of (V23+V25)-(V24+V26). Further, the data
signal recorded on the disk 6 is obtained by calculation of
V23+V24+V25+V26.
Second Embodiment
[0129] Subsequently, description will be made about a second
embodiment of this invention. In the second embodiment, the
hologram optical element 7 and the optical detector 9 in the first
embodiment illustrated in FIG. 8 are replaced by a hologram optical
element 35 and an optical detector 48, respectively.
[0130] In FIG. 12, the hologram optical element 35 is divided into
four of a region 36 through a region 39 by two dividing lines
respectively in parallel with the radial direction and the
tangential direction of the disk 6. Directions of lattices are
inclined by a predetermined angle in - direction relative to the
tangential direction of the disk 6 in the region 36 and the region
37, and are inclined by a predetermined angle in + direction
relative to the tangential direction of the disk 6 in the region 38
and the region 39.
[0131] Further, pitches of the lattices are equal respectively in
the region 36 and the region 39 and in the region 37 and the region
38, and the latter is wider than the former.
[0132] In FIG. 13A, the hologram optical element 35 is constituted
so that a dielectric film 40 is formed on the glass substrate 14.
Reflected light from the disk 6 is incident on the hologram optical
element 35 as incident light 42, is diffracted as -1st-order
diffracted light 43 and +1st-order diffracted light 44, and is
received by the optical detector 48.
[0133] In the meanwhile, in FIG. 13B, the hologram optical element
35 is constituted so that a dielectric film 41 is formed on the
glass substrate 14. Reflected light from the disk 6 is incident on
the hologram optical element 35 as incident light 45, is diffracted
as -1st-order diffracted light 46 and +1st-order diffracted light
47, and is received by the optical detector 48.
[0134] FIG. 13A is a sectional view of portions of the region 36
and the region 37 while FIG. 13B is a sectional view of portions of
the region 38 and the region 39.
[0135] The sectional shapes of the dielectric film 40 and the
dielectric film 41 are the same as the sectional shape of the
dielectric film 15 in the hologram optical element 7 illustrated in
FIG. 10.
[0136] Accordingly, when the focusing error signal is detected from
the
[0137] -1st-order diffracted light 43 and the -1st-order diffracted
light 46, and the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from the +1st-order
diffracted light 44 and the +1st-order diffracted light 47, the
value of the above-mentioned A becomes 0.71 that is larger than the
value in the first conventional optical head apparatus.
[0138] In FIG. 14, the optical detector 48 has a light receiving
portion 49 through a light receiving portion 56. The -1st-order
diffracted light from the region 36 of the hologram optical element
35 forms a light spot 57 on a boundary line between the light
receiving portion 49 and the light receiving portion 50 while the
+1st-order diffracted light therefrom forms a light spot 64 on the
light receiving portion 56.
[0139] The -1st-order diffracted light from the region 37 of the
hologram optical element 35 forms a light spot 58 on the boundary
line between the light receiving portion 49 and the light receiving
portion 50 while the +1st-order diffracted light therefrom forms a
light spot 63 on the light receiving portion 55.
[0140] The -1st-order diffracted light from the region 38 of the
hologram optical element 35 forms a light spot 59 on a boundary
line between the light receiving portion 51 and the light receiving
portion 52 while the +1st-order diffracted light therefrom forms a
light spot 62 on the light receiving portion 54.
[0141] The -1st-order diffracted light from the region 39 of the
hologram optical element 35 forms a light spot 60 on the boundary
line between the light receiving portion 51 and the light receiving
portion 52 while the +1st-order diffracted light therefrom forms a
light spot 61 on the light receiving portion 53.
[0142] When outputs from the light receiving portion 49 through the
light receiving portion 56 are respectively designated by notations
V49 through V56, the focusing error signal by the Foucault method
is obtained by calculation of (V49+V52)-(V50+V51). The tracking
error signal by the differential phase method is obtained by the
phase difference between V53+V56 and V54+V55. The tracking error
signal by the push-pull method is obtained by calculation of
(V53+V55)-(V54+V56). Further, the data signal recorded on the disk
6 is obtained by calculation of V53+V54+V55+V56.
Third Embodiment
[0143] Subsequently, description will be made about a third
embodiment of this invention with reference to FIG. 15.
[0144] In FIG. 15, a semiconductor laser 66 and an optical detector
67 are installed inside a module 65. Emitted light from the
semiconductor laser 66 is formed into a parallel ray by the
collimator lens 2, transmits through a polarizing hologram optical
element 68 substantially completely, is converted from linearly
polarized light into circularly polarized light by the quarter-wave
plate 4, and is focused onto the disk 6 by the object lens 5.
[0145] Reflected light from the disk 6 transmits through the object
lens 5 in a reverse direction, is converted from circularly
polarized light into linearly polarized light by the quarter-wave
plate 4, is diffracted by the polarizing hologram optical element
68 substantially completely, transmits through the collimator lens
2, and is received by the optical detector 67.
[0146] Herein, it is to be noted that a plane view of the
polarizing hologram optical element 68 is the same as the plane
view of the hologram optical element 7 illustrated in FIG. 9. The
polarizing hologram optical element 68 is divided into four of the
region 10 through the region 13 by two dividing lines respectively
in parallel with the radial direction and the tangential direction
of the disk 6.
[0147] Directions of lattices are in parallel with the tangential
direction of the disk 6 in any of the region 10 through the region
13. Further, pitches of the lattices become wider in the order of
the region 10, the region 11, the region 12 and the region 13.
[0148] In FIG. 16, the polarizing hologram optical element 68 is
constructed so that a proton exchange region 70 is formed in a
substrate of a lithium niobate substrate 69 having birefringence
and a dielectric film 71 is formed on the substrate,
respectively.
[0149] Emitted light from the semiconductor laser 66 is incident on
the polarizing hologram optical element 68 as incident light 72,
transmits therethrough as transmitting light 73, and progresses
toward the disk 6. Reflected light from the disk 6 is incident on
the polarizing hologram optical element 68 as incident light 74, is
diffracted as -1st-order diffracted light 75 and +1st-order
diffracted light 76, and is received by the optical detector
67.
[0150] In the meanwhile, it is to be noted that the sectional
shapes of the proton exchange region 70 and the dielectric film 71
are formed in a step-like shape of four levels. All of differences
in depths of the two contiguous levels in the proton exchange
region 70 are equal to each other, and all of differences in
heights of the two contiguous levels in the dielectric film 71 are
equal to each other.
[0151] When phase differences of light transmitting through the two
contiguous levels in respect of ordinary light and extraordinary
light are designated by notations .phi.o and .phi.e and widths of
lattices at a first stage through a fourth stage are respectively
designated by notations p/2-w, w, p/2-w and w, transmittances
.eta.o.sub.O and .eta.e.sub.O for the ordinary and the
extraordinary light, diffraction efficiencies .eta.o.sub.-1 and
.eta.e.sub.-1 of the -1st-order diffracted light for the ordinary
light and the extraordinary light and diffraction efficiencies
.eta.o.sub.+1 and .eta.e.sub.+1 of the +1st-order diffracted light
for the ordinary light and the extraordinary light are respectively
given by Equation (4) through Equation (9).
.eta.o.sub.0=(1/2)(1+cos 2.phi.o).times.{1-4w/p(1-2w/p)(1-cos
.phi.o)} (4)
.eta.e.sub.0=(1/2)(1+cos 2.phi.e).times.{1-4w/p(1-2w/p)(1-cos
.phi.e)} (5)
.eta.o.sub.-1=(2/.pi..sup.2)(1-cos 2.phi.o){1-sin(2.pi.w/p)sin
.phi.o} (6)
.eta.e.sub.-1=(2/.pi..sup.2)(1-cos 2.phi.e){1-sin(2.pi.w/p)sin
.phi.e} (7)
.eta.o.sub.+1=(2/.pi..sup.2)(1-cos 2.phi.o){1+sin(2.pi.w/p)sin
.phi.o} (8)
.eta.e.sub.+1=(2/.pi..sup.2)(1-cos 2.phi.e){1+sin (2.pi.w/p)sin
.phi.e} (9
[0152] When .phi.o=0, .phi.e=.pi./2, w/p=0.135 or w/p=0.356, then,
.eta.o.sub.0=1,
[0153] .eta.o.sub.-1=0, .eta.o.sub.+1=0, .eta.e.sub.O=0,
.eta.e.sub.-1=0.10 and .eta.e.sub.+1=0.71. That is, when the
emitted light from the semiconductor laser 66 is incident on the
polarizing hologram optical element 68 as the ordinary light, the
emitted light transmits therethrough by 100% as transmitting light.
When reflected light from the disk 6 is incident on the polarizing
hologram optical element 68 as the extraordinary light, the
transmitted light is diffracted by 10% as the -1st-order diffracted
light and by 71% as the +1st-order diffracted light.
[0154] Therefore, when the focusing error signal is detected from
the -1st-order diffracted light 75, and the tracking error signal
by the differential phase method, the tracking error signal by the
push-pull method and the data signal recorded on the disk 6 are
detected from the +1st-order diffracted light 76, then the value of
the above-mentioned A becomes 0.71 which is larger than the value
in the first conventional optical head apparatus. In this event,
w/p satisfying .eta.e.sub.-1<.eta.e.sub.+1 and
.eta.e.sub.-1.noteq.0 falls within the range of 0<w/p<0.25 or
0.25<w/p<0.5.
[0155] When a difference in depths of the two contiguous levels of
the proton exchange region 70 is designated by notation d/4, a
difference in heights of the two contiguous levels of the
dielectric film 71 is designated by notation h/4, changes in
refractive indices for the ordinary light and the extraordinary
light by proton exchange are designated by .DELTA.no and .DELTA.ne,
the refractive index of the dielectric film 71 is designated by
notation n and wavelengths of the incident light 72 and the
incident light 74 are designated by notation .lambda., .phi.o and
.phi.e are given by Equation (10) and Equation (11),
respectively.
.phi.o=(2.pi./.lambda.){.DELTA.nod/4+(n-1)h/4} (10)
.phi.e=(2.pi./.lambda.){.DELTA.ned/4+(n-1)h/4} (11)
[0156] In the case of .lambda.=660 nm, .DELTA.no=-0.04 and
.DELTA.ne=0.12 and when Nb.sub.2O.sub.5 is used for the dielectric
film 71, since n=2.2, in order to set .phi.o and .phi.e as .phi.o=0
and .phi.e=.pi./2, d and h may be d=4.13 .mu.m and h=138 nm.
[0157] In FIG. 17, the semiconductor laser 66 and a mirror 77 are
arranged on the optical detector 67. The optical detector 67 has a
light receiving portion 78 through a light receiving portion 85.
Emitted light from the semiconductor laser 66 is reflected by the
mirror 77, and progresses toward the disk 6.
[0158] The -1st-order diffracted light from the region 10 of the
polarizing hologram optical element 68 forms a light spot 86 on a
boundary line between the light receiving portion 78 and the light
receiving portion 79 while the +1st-order diffracted light
therefrom forms a light spot 93 on the light receiving portion
85.
[0159] The -1st-order diffracted light from the region 1 1 of the
polarizing hologram optical element 68 forms a light spot 87 on the
boundary between the light receiving portion 78 and the light
receiving portion 79 while the +1st-order diffracted light
therefrom forms a light spot 92 on the light receiving portion
84.
[0160] The -1st-order diffracted light from the region 12 of the
polarizing hologram optical element 68 forms a light spot 88 on a
boundary line between the light receiving portion 80 and the light
receiving portion 81 while the +1st-order diffracted light
therefrom forms a light spot 91 on the light receiving portion
83.
[0161] The -1st-order diffracted light from the region 13 of the
polarizing hologram optical element 68 forms a light spot 89 on the
boundary line between the light receiving portion 80 and the light
receiving portion 81 while the +1st-order diffracted light
therefrom forms a light spot 90 on the light receiving portion
82.
[0162] When outputs from the light receiving portion 78 through the
light receiving portion 85 are represented respectively by
notations V78 through V85, the focusing error signal by the
Foucault method is obtained by calculation of
(V78+V81)-(V79+V80).
[0163] The tracking error signal by the differential phase method
is obtained by the phase difference between V82+V85 and V83+V84.
The tracking error signal by the push-pull method is obtained by
calculation of (V82+V84)-(V83+V85). Further, the data signal
recorded on the disk 6 is obtained by calculation of
V82+V83+V84+V85.
Fourth Embodiment
[0164] Subsequently, description will be made about a fourth
embodiment of this invention. In the fourth embodiment, the
polarizing hologram optical element 68 and the optical detector 67
in the third embodiment of the optical head apparatus illustrated
in FIG. 15 are replaced by a polarizing hologram optical element 94
and an optical detector 109, respectively.
[0165] Herein, it is to be noted that the plane view of the
polarizing hologram optical element 94 is the same as the plane
view of the hologram optical element 35 illustrated in FIG. 12.
[0166] The polarizing hologram optical element 94 is divided into
four of the region 36 through the region 39 by two dividing lines
in parallel respectively with the radial direction and the
tangential direction of the disk 6.
[0167] Directions of lattices are inclined by a predetermined angle
in
[0168] - direction relative to the tangential direction of the disk
6 in the region 36 and the region 37, and are inclined by a
predetermined angle in + direction relative to the tangential
direction of the disk 6 in the region 38 and the region 39.
[0169] Further, pitches of the lattices are equal to each other in
the region 36 and the region 39 and in the region 37 and the region
38, and the latter is wider than the former.
[0170] In FIG. 18A, the polarizing hologram optical element 94 is
constituted so that a proton exchange region 95 is formed in the
substrate of the lithium niobate substrate 69 having birefringence,
and a dielectric film 96 is formed on the substrate, respectively.
Emitted light from the semiconductor laser 66 is incident on the
polarizing hologram optical element 94 as incident light 99,
transmits therethrough as transmitting light 100 and progresses
toward the disk 6.
[0171] Reflected light from the disk 6 is incident on the
polarizing hologram optical element 94 as incident light 101, is
diffracted as -1st-order diffracted light 102 and +1st-order
diffracted light 103, and is received by the optical detector
109.
[0172] Meanwhile, in FIG. 18B, the polarizing hologram optical
element 94 is constituted so that a proton exchange region 97 is
formed in a substrate of the lithium niobate substrate 69 having
birefringence and a dielectric film 98 is formed on the substrate,
respectively. Emitted light from the semiconductor laser 66 is
incident on the polarizing hologram optical element 94 as incident
light 104, transmits therethrough as transmitting light 105 and
progresses toward the disk 6.
[0173] Reflected light from the disk 6 is incident on the
polarizing hologram optical element 94 as incident light 106, is
diffracted as 1st-order diffracted light 107 and +1st-order
diffracted light 108, and is received by the optical detector
109.
[0174] FIG. 18A is a sectional view of portions of the region 36
and the region 37 while FIG. 18B is a sectional view of portions of
the region 38 and the region 39.
[0175] Sectional shapes of the proton exchange region 95 and the
proton exchange region 97 are the same as the sectional shape of
the proton exchange region 70 in the polarizing hologram optical
element 68 illustrated in FIG. 16 while sectional shapes of the
dielectric film 96 and the dielectric film 98 are the same as the
sectional shape of the dielectric film 71 in the polarizing
hologram optical element 68 illustrated in FIG. 16.
[0176] That is, when emitted light from the semiconductor laser 66
is incident on the polarizing hologram optical element 94 as
ordinary light, the emitted light transmits therethrough by 100% as
transmitting light. When reflected light from the disk 6 is
incident on the polarizing hologram optical element 94 as
extraordinary light, the reflected light is diffracted by 10% as
the -1st-order diffracted light and by 71% as the +1st-order
diffracted light, respectively.
[0177] Accordingly, the focusing error signal is detected from the
-1st-order diffracted light 102 and -1st-order diffracted light
107, and the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from the +1st-order
diffracted light 103 and the +1st-order diffracted light 108. Under
this circumstance, value of the above-mentioned A becomes 0.71
which is larger than the value in the conventional first optical
head apparatus.
[0178] In FIG. 19, the semiconductor laser 66 and the mirror 77 are
arranged on the optical detector 109. The optical detector 109 has
a light receiving portion 110 through a light receiving portion
117. Emitted light from the semiconductor laser 66 is reflected by
the mirror 77, and progresses toward the disk 6.
[0179] The -1st-order diffracted light from the region 36 of the
polarizing hologram optical element 94 forms a light spot 118 on a
boundary between the light receiving portion 110 and the light
receiving portion 111 while the +1st-order diffracted light
therefrom forms a light spot 125 on the light receiving portion
117.
[0180] The -1st-order diffracted light from the region 37 of the
polarizing hologram optical element 94 forms a light spot 119 on
the boundary between the light receiving portion 110 and the light
receiving portion 111 while the +1st-order diffracted light
therefrom forms a light spot 124 on the light receiving portion
116.
[0181] The -1st-order diffracted light from the region 38 of the
polarizing hologram optical element 94 forms a light spot 120 on a
boundary line between the light receiving portion 112 and the light
receiving portion 113 while the +1st-order diffracted light
therefrom forms a light spot 123 on the light receiving portion
115.
[0182] The -1st-order diffracted light from the region 39 of the
polarizing hologram optical element 94 forms a light spot 121 on
the boundary line between the light receiving portion 112 and the
light receiving portion 113 while the +1st-order diffracted light
therefrom forms a light spot 122 on the light receiving portion
114.
[0183] When outputs from the light receiving portion 110 through
the light receiving portion 117 are respectively designated by
notations V110 through V117, the focusing error signal by the
Foucault method is obtained by calculation of
(V110+V113)-(V111+V112). The tracking error signal by the
differential phase method is obtained by the phase difference
between V114+V117 and V115+V116. The tracking error signal by the
push-pull method is obtained by calculation of
(V114+V116)-(V115+V117). Further, the data signal recorded on the
disk 6 is obtained by a calculation of V114+V115+V116+V117.
[0184] In the third and the fourth embodiments, when phase
differences of light transmitting through the two contiguous levels
for ordinary light and extraordinary light in the polarizing
hologram optical element 68 and the polarizing hologram optical
element 94 are designated by notations (.phi.o and .phi.e, then
.phi.o=0 and .phi.e=.pi./2.
[0185] Further, emitted light from the semiconductor laser 66 is
incident on the polarizing hologram optical element 68 or the
polarizing hologram optical element 94 as the ordinary light.
[0186] On the other hand, reflected light from the disk 6 is
incident on the polarizing hologram optical element 68 or the
polarizing hologram optical element 94 as the extraordinary
light.
[0187] Alternatively, when phase differences of light transmitting
through the two contiguous levels for ordinary light and
extraordinary light in the polarizing hologram optical element 68
and the polarizing hologram optical element 94 are designated by
notation .phi.o and .phi.e, then .phi.o=.pi./2 and
[0188] .phi.e=0. Further, the emitted light from the semiconductor
laser 66 may be incident on the polarizing hologram optical element
68 or the polarizing hologram optical element 94 as the
extraordinary light. The reflected light from the disk 6 may be
incident on the polarizing hologram optical element 68 or the
polarizing hologram optical element 94 as the ordinary light.
Fifth Embodiment
[0189] Subsequently, description will be made about a fifth
embodiment of this invention with reference to FIG. 20.
[0190] In FIG. 20, emitted light from the semiconductor laser 1 is
formed into a parallel ray by the collimator lens 2, is incident on
the polarization beam splitter 3 as p-polarized light, transmits
therethrough substantially by 100%, is converted from linearly
polarized light into circularly polarized light by the quarter-wave
plate 4, and is focused onto the disk 6 by the object lens 5.
[0191] Reflected light from the disk 6 transmits through the object
lens 5 in a reverse direction, is converted from circularly
polarized light into linearly polarized light by the quarter-wave
plate 4, is incident on the polarization beam splitter 3 as
s-polarized light, reflected thereby substantially by 100%,
refracted by a Wollaston prism 126 and a four division prism 127,
transmits through the lens 8, and is received by the optical
detector 9.
[0192] In FIGS. 21A and 21B, the structure of the Wollaston prism
126 is illustrated. Herein, it is to be noted that FIG. 21A is a
side view and FIG. 21B is a plane view.
[0193] The Wollaston prism 126 is constituted by pasting together a
prism 128 and a prism 129 made of lithium niobate having
birefringence. Reflected light from the disk 6 is incident on the
Wollaston prism 126 as incident light 130, refracted as refracted
light 131 and refracted light 132, and progresses toward the four
division prism 127.
[0194] The polarized light direction of the incident light 130
relative to a pasted face of the prism 128 and the prism 129 is
that of s-polarized light. An optical axis 133 of the prism 128 is
inclined to the s-polarized light direction by .theta. while an
optical axis 134 of the prism 129 is inclined to a p-polarized
light direction by .theta..
[0195] In the case of lithium niobate, the refractive index for
ordinary light is larger than the refractive index for
extraordinary light. As a result, a component of the incident light
130 constituting ordinary light in the prism 128 and extraordinary
light in the prism 129 becomes the refracted light 131. Further, a
component thereof constituting extraordinary light in the prism 128
and ordinary light in the prism 129 becomes the refracted light
132.
[0196] In this case, ratios of intensities of the refracted light
131 and the refracted light 132 to an intensity of the incident
light 130 are given by sin.sup.2.theta. and cos.sup.2.theta.,
respectively. When .theta.=-22.degree. or .theta.=22.degree., then,
sin.sup.2.theta.=0.14 and cos.sup.2.theta.=0.86. Further,
.theta.satisfying sin.sup.2.theta.<cos.sup.2.theta. and
sin.sup.2.theta..noteq.0 falls within the range of
-45.degree.<.theta.<0.degree. or
0.degree.<.theta.<45.degree..
[0197] In FIGS. 22A, 22B and 22C, the constitution of the four
division prism 127 is illustrated. In this event, FIG. 22A and FIG.
22B are sectional views while FIG. 22C is a plane view.
[0198] The four division prism 127 is made of plastic as its
material, and is divided into four of a region 135 through a region
138 by two dividing lines respectively in parallel with the radial
direction and the tangential direction of the disk 6.
[0199] FIG. 22A is a sectional view of portions of the region 135
and the region 136 while FIG. 22B is a sectional view of portions
of the region 137 and the region 138.
[0200] An emitting face thereof is inclined to an incident face
thereof in
[0201] + direction around the tangential direction of the disk 6 in
the region 135 and the region 136, and inclined in - direction
around the tangential direction of the disk 6 in the region 137 and
the region 138. Angles made by the emitting faces and the incident
face are respectively equal in the region 135 and the region 138
and in the region 136 and the region 137, and the former is larger
than the latter.
[0202] In FIG. 22A, refracted light from the Wollaston prism 126 is
incident on the region 135 and the region 136 of the four division
prism 127 respectively as incident light 139 and incident light
141, refracted respectively as refracted light 140 and refracted
light 142, and received by the optical detector 9.
[0203] In the meanwhile, in FIG. 22B, the refracted light from the
Wollaston prism 126 is incident on the region 137 and the region
138 of the four division prism 127 respectively as incident light
143 and incident light 145, refracted respectively as refracted
light 144 and refracted light 146, and received by the optical
detector 9.
[0204] With such a structure, the focusing error signal is detected
from light refracted as the refracted light 131 by the Wollaston
prism 126 and refracted by the four division prism 127 as the
refracted light 140, the refracted light 142, the refracted light
144 and the refracted light 146.
[0205] Further, the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from light
refracted by the Wollaston prism 126 as the refracted light 132 and
refracted by the four division prism 127 as the refracted light
140, the refracted light 142, the refracted light 144 and the
refracted 146.
[0206] Under this circumstance, the value of the above-mentioned A
becomes 0.86 which is larger than the value in the conventional
first optical head apparatus.
[0207] A pattern of the optical detector 9 and light spots on the
optical detector 9 are illustrated in FIG. 11. Among the lights
refracted as the refracted light 131 by the Wollaston prism 126,
light refracted as the refracted light 140 at the region 135 of the
four division prism 127 forms the light spot 27 on the boundary
line between the light receiving portion 19 and the light receiving
portion 20.
[0208] Further, light refracted as the refracted light 142 by the
region 136 of the four division prism 127 forms the optical spot 28
on the boundary line between the light receiving portion 19 and the
light receiving portion 20.
[0209] Moreover, light refracted as the refracted light 144 by the
region 137 of the four division prism 127 forms the light spot 29
on the boundary line between the light receiving portion 21 and the
light receiving portion 22.
[0210] In addition, light refracted as the refracted light 146 by
the region 138 of the four division prism 127 forms the light spot
30 on the boundary line between the light receiving portion 21 and
the light receiving portion 22.
[0211] Among the lights refracted as the refracted light 132 by the
Wollaston prism 126, light refracted as the refracted light 140 by
the region 135 of the four division prism 127 forms the light spot
31 on the light receiving portion 23.
[0212] Further, light refracted as the refracted light 142 by the
region 136 of the four division prism 127 forms the light spot 32
on the light receiving portion 24.
[0213] Moreover, light refracted as the refracted light 144 by the
region 137 of the four division prism 127 forms the light spot 33
on the light receiving portion 25.
[0214] In addition, light refracted as the refracted light 146 by
the region 138 of the four division prism 127 forms the light spot
34 on the light receiving portion 26.
[0215] The focusing error signal by the Foucault method, the
tracking error signal by the differential phase method, the
tracking error signal by the push-pull method and the data signal
recorded on the disk 6 are obtained by the same calculation as that
in the first embodiment.
[0216] In the fifth embodiment, the four division prism 127 is
arranged between the Wollaston prism 126 and the lens 8.
Alternatively, the four division prism 127 may be arranged between
the polarization beam splitter 3 and the Wollaston prism 126.
Sixth Embodiment
[0217] Subsequently, description will be made about a sixth
embodiment of this invention. In the sixth embodiment, the four
division prism 127 in the fifth embodiment illustrated in FIG. 20
is replaced by a hologram optical element 147.
[0218] With this structure, reflected light from the disk 6 is
refracted by the Wollaston prism 126, is diffracted by the hologram
optical element 147, and is received by the optical detector 9.
[0219] In FIG. 23, the hologram optical element 147 is divided into
four of a region 148 through a region 151 by two dividing lines
respectively in parallel with the radial direction and the
tangential direction of the disk 6.
[0220] Directions of lattices are in parallel with the tangential
direction of the disk 6 in any of the region 148 through the region
151. Further, pitches of the lattices are equal respectively in the
region 148 and the region 151 and in the region 149 and the region
150, and the latter is wider than the former.
[0221] FIG. 24A is a sectional view of portions of the region 148
and the region 149 while FIG. 24B is a sectional view of portions
of the region 150 and the region 151.
[0222] In FIG. 24A, the hologram optical element 147 is constructed
so that a dielectric film 152 is formed on the glass substrate 14.
With this structure, refracted light from the Wollaston prism 126
is incident on the hologram optical element 147 as incident light
154, diffracted as +1st-order diffracted light 155 and is received
by the optical detector 9.
[0223] Meanwhile, in FIG. 24B, the hologram optical element 147 is
constructed so that a dielectric film 153 is formed on the glass
substrate 14. With such a structure, refracted light from the
Wollaston prism 126 is incident on the hologram optical element 147
as incident light 156, diffracted as +1st-order diffracted light
157 and is received by the optical detector 9.
[0224] Herein, it is to be noted that sectional shapes of the
dielectric film 152 and the dielectric film 153 are formed in a
step-like shape of 8 levels. All of differences in heights of the
two contiguous levels are equal to each other.
[0225] When a phase difference of light transmitting through the
two contiguous levels is designated by notation .phi., and all of
widths of the lattices in a first stage through an eighth stage are
designated by notation p/8, the diffraction efficiency .eta..sub.+1
of the +1st-order diffracted light is given by Equation (12).
.eta..sub.+1=(4/.pi..sup.2)(1-1/{square
root}2){1+cos(.phi.-.pi./4)}{1+cos-
(2.phi.-.pi./2)}{1+cos(4.phi.-.pi.)} (12)
[0226] When .phi. is set as .phi.=.pi./4, then,
.eta..sub.+1=0.95.
[0227] Under this circumstance, the focusing error signal is
detected from light refracted as the refracted light 131 by the
Wollaston prism 126 and diffracted as the +1st-order diffracted
light 155 and the +1st-order diffracted light 157 by the hologram
optical element 147.
[0228] Further, the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from light
refracted as the refracted light 132 by the Wollaston prism 126 and
diffracted as the +1st-order diffracted light 155 and the
+1st-order diffracted light 157 by the hologram optical element
147.
[0229] In this condition, the value of the above-mentioned A
becomes 0.86.times.0.95=0.82 which is larger than the value in the
conventional first optical head apparatus.
[0230] When differences in heights of the two contiguous levels of
the dielectric film 152 and the dielectric film 153 are designated
by notation h/8, the refractive indices of the dielectric film 152
and the dielectric film 153 are designated by notation n, and
wavelengths of the incident light 154 and the incident light 156
are designated by notation .lambda., .phi. is given by Equation
(13).
.phi.=(2.pi./.lambda.)(n-1)h/8 (13)
[0231] In the case of .lambda.=660 nm, when SiO.sub.2 is used for
the dielectric film 152 and the dielectric film 153, since n=1.46,
in order to set .phi.=.pi./4, h may be h=1.43 .mu.m.
[0232] A pattern of the optical detector 9 and light spots on the
optical detector 9 are shown by FIG. 11. Among the lights refracted
as the refracted light 131 by the Wollaston prism 126, light
diffracted as the +1st-order diffracted light 155 by the region 148
of the hologram optical element 147 forms the light spot 27 on the
boundary line between the light receiving portion 19 and the light
receiving portion 20.
[0233] Further, light diffracted as +1st-order diffracted light 155
by the region 149 of the hologram optical element 147 forms the
light spot 28 on the boundary line between the light receiving
portion 19 and the light receiving portion 20.
[0234] Moreover, light diffracted as the +1st-order diffracted
light 157 by the region 150 of the hologram optical element 147
forms the light spot 29 on the boundary line between the light
receiving portion 21 and the light receiving portion 22.
[0235] In addition, light diffracted as the +1st-order diffracted
light 157 by the region 151 of the hologram optical element 147
forms the light spot 30 on the boundary line between the light
receiving portion 21 and the light receiving portion 22.
[0236] Among the lights refracted as the refracted light 132 by the
Wollaston prism 126, light diffracted as the +1st-order diffracted
light 155 by the region 148 of the hologram optical element 147
forms the light spot 31 on the light receiving portion 23.
[0237] Further, light diffracted as the +1st-order diffracted light
155 by the region 149 of the hologram optical element 147 forms the
light spot 32 on the light receiving portion 24.
[0238] Moreover, light diffracted as the +1st-order diffracted
light 157 by the region 150 of the hologram optical element 147
forms the light spot 33 on the light receiving portion 25.
[0239] In addition, light diffracted as the +1st-order diffracted
light 157 by the region 151 of the hologram optical element 147
forms the light spot 34 on the light receiving portion 26.
[0240] The focusing error signal by the Foucault method, the
tracking error signal by the differential phase method, the
tracking error signal by the push-pull method and the data signal
recorded on the disk 6 are obtained by the same calculation as that
in the first embodiment.
[0241] In the sixth embodiment, the hologram optical element 147 is
arranged between the Wollaston prism 126 and the lens 8.
Alternatively, the hologram optical element 147 may be arranged
between the polarization beam splitter 3 and the Wollaston prism
126.
[0242] Further, in the sixth embodiment, a phase distribution of
lattice in the hologram optical element 147 is formed in a
step-like shape of 8 levels and when the phase difference of light
transmitting through the two contiguous levels is designated by
notation .phi., all of widths of lattices of a first stage through
an eighth stage are designated by p/8, .phi.=.pi./4.
[0243] Alternatively, when a phase distribution of lattice in the
hologram optical element 147 is formed in a step-like shape of N
levels (N is an integer equal to or larger than 3), the phase
difference of light transmitting through the contiguous levels may
be generally designated by notation .phi. and all of widths of
lattices of a first stage through an N-th stage are designated by
notation p/N, then .phi.=2.pi./N.
[0244] In the fifth and the sixth embodiments, the optical axis 133
of the prism 128 is inclined to the s-polarized light direction by
.theta.. The optical axis 134 of the prism 129 is inclined to the
p-polarized light direction by .theta.. The focusing error signal
is detected from the refracted light 131 which is a component of
the incident light 130 constituting ordinary light in the prism 128
and extraordinary light in the prism 129.
[0245] Further, the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from the refracted
light 132 which is a component thereof constituting the
extraordinary light in the prism 128 and ordinary light in the
prism 129, and .theta. falls within the range of
-45.degree.<.theta.<0.degree. or
0.degree.<.theta.<45.degree..
[0246] In this case, the ratios of the intensities of the refracted
light 131 and the refracted light 132 to the intensity of the
incident light 130 are given respectively by sin.sup.2.theta. and
cos.sup.2.theta., and sin.sup.2.theta.<cos.sup.2.theta. and
sin.sup.2.theta..noteq.0.
[0247] In contrast, the optical axis 133 of the prism 128 is
inclined to the
[0248] s-polarized light direction by .theta.. The optical axis 134
of the prism 129 is inclined to the p-polarized light direction by
.theta.. The focusing error signal is detected from the refracted
light 132 which is a component of the incident light 130
constituting extraordinary light in the prism 128 and ordinary
light in the prism 129.
[0249] Further, the tracking error signal by the differential phase
method, the tracking error signal by the push-pull method and the
data signal recorded on the disk 6 are detected from the refracted
light 131 which is a component thereof constituting the ordinary
light in the prism 128 and the extraordinary light in the prism
129, and .theta. falls within the range of
-90.degree.<.theta.<-45.degree. or
45.degree.<.theta.<90.degree..
[0250] In this case, the ratios of the intensities of the refracted
light 131 and the refracted light 132 to the intensity of the
incident light 130 are respectively given by sin.sup.2.theta. and
cos.sup.2.theta., and sin.sup.2.theta.>cos.sup.2.theta. and
sin.sup.2.theta..noteq.0.
[0251] According to the above-mentioned optical head apparatus,
reflected light from the disk is divided into the first group of
light and the second group of light. The tracking error signal by
the differential phase method, the tracking error signal by the
push-pull method and the data signal recorded on the disk are
detected from the first group of light while the focusing error
signal is detected from the second group of light.
[0252] In this event, an optical amount of the first group of the
light is larger than an optical amount of the second group of the
light.
[0253] With this structure, the ratio A of the optical amount used
in detecting the data signal recorded on the disk and the tracking
error signal by the differential phase method to the optical amount
of the reflected light from the disk is large, and high S/N is
achieved with respect to these signals. This is because the optical
amount of the first group of light is larger.
* * * * *