Title: Optical information processor and optical element
Abstract: An optical disk device has an aperture of an objective lens in an incoming path of a beam from a semiconductor laser to an optical disk formed larger than an aperture in a return path from the optical disk or an aperture is varied in recording and in reproduction. This configuration improves recording/reproducing ability since light is focused on an optical disk with high numerical aperture. In addition, since reflected light from the optical disk is detected with low numerical aperture, margins for tilt and defocus are not reduced. Furthermore, since unnecessary signal components contained in the reflected light can be eliminated, a S/N (signal-to-noise ratio) of an information signal also increases. Thus, a high-performance optical disk device can be obtained. Alternatively, by varying the aperture of an objective lens in recording and in reproduction, an optical disk device in which recording density and recording quality are increased without deteriorating reproduction quality can be obtained.
Patent Number: 6,920,101 Issued on 07/19/2005 to Saitoh, et al.
| Inventors:
|
Saitoh; Youichi (Hirakata, JP);
Asada; Junichi (Kobe, JP);
Takahashi; Yuichi (Neyagawa, JP);
Nishiwaki; Seiji (Osaka, JP);
Nagashima; Kenji (Suita, JP);
Momoo; Kazuo (Hirakata, JP);
Nagaoka; Junji (Takatsuki, JP)
|
| Assignee:
|
Matsushita Electric Industrial, Co., Ltd. (JP)
|
| Appl. No.:
|
616859 |
| Filed:
|
July 10, 2003 |
Foreign Application Priority Data
| Nov 09, 1998[JP] | 10-317394 |
| Feb 24, 1999[JP] | 11-045748 |
| Jun 28, 1999[JP] | 11-181667 |
| Current U.S. Class: |
369/112.01; 369/110.04 |
| Intern'l Class: |
G11B 007/00 |
| Field of Search: |
369/11201,110.04,112.03,112.1,112.15,118
349/9,96,98
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Hindi; Nabil
Attorney, Agent or Firm: Osha Liang LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 09/436,847,
filed Nov. 9, 1999 now U.S. Pat. No. 6,618,343.
Claims
1. An optical element transmitting a beam with a wavelength λ
1 and
a beam with a wavelength λ
2 that is longer than the wavelength λ
1, comprising:
a polarization hologram portion that is formed by sandwiching a diffraction grating
made of a birefringent material and a wave film having an optical thickness of
(N+¼)λ
1 (wherein N indicates an arbitrary natural number) between
two glass substrates; and
a thin film structure that is attached to either one of the glass substrates
and varies an aperture area respectively for two lights with wavelengths λ
1
and λ
2 passing through the optical element.
2. The optical element according to claim 1,
wherein the other glass substrate, to which the thin film structure is not attached,
of the two glass substrates (with a refractive index ng) is provided with a structure
having a plurality of concentric stopped portions in which difference in height
between adjacent stepped portions is λ
1/(ng-;1).
3. The optical element according to claim 1,
wherein the wavelengths λ
1 and λ
2 of two kinds of lights
passing through the optical element satisfy a relationship of (N
1+¼)
λ
1≈N
2×λ
2, wherein N
1 and N
2
represent arbitrary natural numbers.
4. An optical element transmitting a beam with a wavelength λ
1 and
a beam with a wavelength λ
2 that is longer than the wavelength λ
1, comprising:
a polarization hologram portion that is formed by sandwiching a diffraction grating
made of a birefringent material and a wave film having an optical thickness of
(N+⅕) λ
1 (wherein N indicates an arbitrary natural number)
between two glass substrates; and
a thin film structure that is attached to either one of the glass substrates
and varies an aperture area respectively for two lights with wavelengths λ
1
and λ
2 passing through the optical element.
5. The optical element according to claim 4,
wherein the other glass substrate, to which the thin film structure is not attached,
of the two glass substrates (with a refractive index ng) is provided with a structure
having a plurality of concentric stepped portions in which difference in height
between adjacent stepped portions is λ
1/(ng-;1).
6. The optical element according to claim 4,
wherein the wavelengths λ
1 and λ
2 of two kinds of lights
passing through the optical element satisfy a relationship of (N
1+⅕)
λ
1≈N
2×λ
2, wherein N
1 and N
2
represent arbitrary natural numbers.
Description
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
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:
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:
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 λ1) and 19-2 (emitting
a beam with a wavelength λ2) that emit beams with different wavelengths
from each other. The laser beam 19-21 with a wavelength λ1
(in the case of DVD or the like, λ1=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 d in 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 n1). Generally, given the phase difference φ between a beam
passing through a groove portion and a beam passing between the groove portions,
transmittance is represented by cos
2(φ/2). When the substrate
has refractive indexes of n1 and n2 with respect to polarized lights
parallel and perpendicular to the grating grooves respectively, φ=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, φ=2π(n1-;n2) d/λ.
Therefore, when the depth d is set to obtain φ=π, 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 ¼ 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
19-10 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
λ2 (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 method. Light 19-31 reflected
from the signal surface 19-15 is diffracted by the hologram element
19-11 via the mirror 19-7, the collimator lens 19-6,
and the prism 19-5. Then, the diffracted light is incident onto detection
surfaces of photodetectors 19-16, thus detecting signals to reproduce
information. The objective lens 19-8 is designed to have a shape
that enables aberration to be minimum by optimally designing the aperture and the
optical system for respective disks having a substrate thickness of 0.6 mm for
the beam with the wavelength λ1 and having a substrate thickness of
1.2 mm for the beam with the wavelength λ2. In other words, with respect
to the beam with the wavelength λ2, the aperture is limited by the
aperture element 19-12 to form an optimum aperture.
With increase in density of the data information, further improvement in recording
and reproducing ability is required in optical disk devices. Generally, in order
to record and reproduce signals with higher density, a focusing spot on a disk
is reduced in size. That is, it is conceivable that the wavelength of light emitted
from a light source is shortened or NA of an objective lens is increased. However,
in general-purpose optical disk devices used in a general office or at home, an
available short-wavelength light source is a semiconductor laser emitting a red
beam with 660 nm at present. A semiconductor laser emitting a beam with a shorter
wavelength than that lacks in reliability and therefore it is difficult to use
it for recording purpose in the present situation. When the NA of the objective
lens is increased (i.e. when the aperture of an objective-lens holder is enlarged),
recording/reproducing characteristics are improved in part. However, margins for
tilt and defocus are reduced greatly, which has been a problem. In one or more
embodiments, it is a first object of the present invention to provide an optical
disk device in which excellent recording and reproduction can be performed on an
optical disk with higher density and the margins are not reduced at the same time.
On the other hand, there have been the following three problems in a conventional
optical disk device using the two laser beam sources shown in FIG. 19.
Firstly, when the lens is shifted in a track direction of an optical disk,
the relative position of the lens and the aperture element varies, thus causing
asymmetry in the aperture. Consequently, aberration (mainly spherical aberration
and coma aberration) is increased, thus deteriorating signal quality considerably.
Secondly, similarly when the lens is shifted, the relative position of
the lens and the hologram varies and therefore unbalance in quantity of lights
divided by the hologram and distributed to photodetectors occurs, thus causing
offset of a signal due to DC components, which is not preferable in tracking control.
Thirdly, generally due to refractive index variance of an objective lens
or a collimator lens, when the wavelength mode of a beam emitted from a laser beam
source is changed by power modulation for recording and reproduction, momentary
axial aberration (i.e. chromatic aberration) occurs. Consequently, a relative position
error (defocus) between a lens and a signal surface is caused. In order to prevent
this, any chromatic aberration compensation element is required. FIGS. 20(
a)
and 20(
b) show a cross-sectional structural view and a plane view
of a conventionally proposed chromatic-aberration compensation element 20-160
(see JP-A-6-82725 about the details). The chromatic-aberration compensation element
20-160 is formed of a glass plate having a refractive index n in
which a concentric stepped structure 20-150 is formed. In the figure,
the phase of a beam with a wavelength λ that passes through the concentric
stepped structural portion having a stepped depth t represented by:
is shifted for 2π between adjacent stepped portions. However, with respect
to undulation, the same wavefront is formed. On the other hand, when the wavelength
is shifted from λ, the phase of the light is shifted slightly between adjacent
stepped portions. However, since this stepped structure is formed in a concentric
shape, an almost spherical wave is generated in a direction canceling axial aberration
caused by the chromatic aberration. Thus, the aberration can be compensated by
combining this element with a lens.
In order to solve all the three problems of the optical disk device shown in
FIG.
19, it is preferable to mount all the components described above (the aperture
element, the hologram element, the chromatic-aberration compensation plate) on
a moving element. However, when all these elements are mounted, the moving element
becomes very heavy and in addition, it is difficult to keep the actuator in balance.
Further, as the weight of the actuator increases, more energy is required for its
operation, thus causing problem of high power consumption. In addition, since all
the elements must be positioned accurately with respect to the center of lenses
(the center of optical axes), highly accurate assembly processes are necessary,
thus decreasing mass-productiveness. In one or more embodiments, a second object
of the present invention is directed to solve these problems.
SUMMARY OF THE INVENTION
In order to attain the first object, one or more embodiments of the present invention
provide a configuration in which NA of an objective lens positioned in an incoming
optical path from a semiconductor laser to an optical disk is designed to be larger
than that in a return path from the optical disk or NA is varied in recording information
and in reproducing information.
This configuration enables recording/reproducing ability to be improved since
light is focused onto an optical disk with high NA. In addition, since reflected
light from the optical disk is detected with low NA, margins for tilt and defocus
are not reduced. Furthermore, since unnecessary signal components contained in
the reflected light can be eliminated, a SIN (signal-to-noise ratio) of an information
signal also increases. Thus, a high-performance optical disk device can be obtained.
Alternatively, by varying the aperture of an objective lens in recording information
and in reproducing information, an optical disk device in which recording density
and recording quality are increased without deteriorating reproduction quality
can be obtained.
Further, one or more embodiments of the present invention employ the following
means to attain the second object. Embodiments of the present invention are characterized
by an optical element in which a thin film for varying an aperture area corresponding
to each of two wavelengths (wavelengths λ1 and λ2; λ1<λ2)
of light is formed on one of two glass plates of a polarization hologram element.
The polarization hologram element is formed by sandwiching a diffraction grating
made of a birefringent material and a wave film with an optical thickness of (N1+¼)
λ1, wherein N represents a natural number, between the glass plates.
Embodiments of the present invention are characterized by an optical element having
a structure having a plurality of concentric stepped portions on the other glass
plate. In one or more embodiments, the present invention is characterized in that
wavelengths λ1 and λ2 of two types of lights passing
through an optical element satisfy the relationship of (N1+¼) λ1≈N2×λ2,
wherein N1 and N2 represent natural numbers. Embodiments of the present
invention are characterized in that an optical element using a wave film with an
optical thickness of (N1+⅕) λ1 instead of the wave film
with an optical thickness of (N1+¼) λ1 is mounted on an actuator.
According to the aforementioned configurations of the present invention,
the following excellent effects can be obtained. By varying the aperture in an
incoming path and in a return path in recording information on or reproducing information
from an optical disk, excellent recording and reproduction are performed by obtaining
a spot focused with high NA in the incoming path, and in the return path, crosstalk
compositions, intersymbol interference compositions, high aberration compositions
contained a lot in reflected light from the optical disk that passes through high
NA portions are eliminated by applying low NA, thus enabling high-quality signal
reproduction. In addition, the margins for defocus and tilt are not reduced.
Furthermore, by using a diffraction grating as an aperture element and
combining with another element to form one component, excellent effects such as
reduction in size, stabilization, reduction in cost, and the like can be obtained.
By leading at least a part of light outside the aperture in the return path to
second photodetectors and calculating with outputs from first photodetectors, intersymbol
interference compositions and crosstalk compositions can be canceled out, thus
providing an excellent effect in which further excellent information signals can
be obtained.
In addition, by designing the aperture of an aperture element to be variable,
an optimum aperture can be set for respective optical disks and therefore an excellent
effect enabling excellent signal reproduction continuously can be provided, thus
obtaining an optical disk device in which recording density and recording quality
are improved without deteriorating reproduction quality.
Moreover, according to the aforementioned configurations, a moving element
is not greatly increased in weight, thus suppressing the increase in power consumption.
In addition, precise positioning between respective elements is not required, thus
facilitating the assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(
a), 1(
b), and 1(
c) show structural
views showing an optical information processing method according to a first embodiment
of the present invention.
FIG. 2 is an analytical example according to the first embodiment of the present invention.
FIGS. 3(
a), 3(
b), 3(
c) and 3(
d)
show structural examples of an aperture element according to the first embodiment
of the present invention.
FIGS. 4(
a), 4(
b), and 4(
c) show structural
views showing an optical information processing method according to a second embodiment
of the present invention.
FIG. 5 is an analytical example according to the second embodiment of the present invention.
FIG. 6 is a structural example of an aperture element according to the second
embodiment of the present invention.
FIGS. 7(
a), 7(
b), and 7(
c) show structural
views showing an optical information processing method according to a third embodiment
of the present invention.
FIG. 8 is an analytical example according to the third embodiment of the present invention.
FIG. 9 is a structural example of an aperture element according to the third
embodiment of the present invention.
FIGS. 10(
a), 10(
b), and 10(
c) show
structural views showing another optical information processing method according
to the third embodiment of the present invention.
FIGS. 11(
a), 11(
b), and 11(
c) show
structural views showing an optical information processing method according to
a fourth embodiment of the present invention.
FIGS. 12(
a), 12(
b), and 12(
c) show
structural views showing an optical information processing method according to
a fifth embodiment of the present invention.
FIGS. 13(
a), 13(
b), and 13(
c) show
structural views showing an optical information processing method according to
a sixth embodiment of the present invention.
FIG. 14 is a structural example of an aperture element according to the sixth
embodiment of the present invention.
FIG. 15 is a cross-sectional structural view of an aperture element according
to a seventh embodiment of the present invention.
FIG. 16 shows a configuration of an optical disk device using an aperture element
according to the seventh embodiment of the present invention.
FIG. 17 is a cross-sectional structural view of an aperture element according
to an eighth embodiment of the present invention.
FIGS. 18(
a) and 18(
b) show structural views of
a conventional optical information processing method.
FIG. 19 shows a configuration of a conventional optical disk device.
FIGS. 20(
a) and 20(
b) show a cross sectional view
and a plane view of an element having a concentric stepped structure as a conventional
means for compensating chromatic aberration.
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment
FIGS.
1(
a),
1(
b), and
1(
c) show structural
examples of an optical information processing method according to a first embodiment
of the present invention. The description of the same points, such as a detection
principle of FE, TE, RF signals as those in the conventional example, shown in
FIGS.
18(
a) and
18(
b) is omitted. In the conventional
example, the NA in an incoming path and the NA in a return path are determined
by the objective lens holder
18-
6 and are equal. In the present embodiment,
an aperture is determined by an objective lens holder
1-
6, a λ/4
plate
1-
7 that is a component of an aperture element, and a diffraction
grating
1-
8. The diffracting grating
1-
8 is provided
with a grating at the portion indicated with hatching in the figure.
The operation of the aperture element is described as follows. The λ/4
plate
1-
7 has a function of providing a phase difference of λ/4
to incident light. The diffraction grating
1-
8 is a grating having
concave and convex portions made of an anisotropic material (with a refractive
index of, for example, n
1 and n
2) such as, for example, lithium niobate
and is formed by filling the concave portions with an isotropic material with a
refractive index (for example, n
1) equal to any one of the two refractive
indexes of the anisotropic material. When linearly polarized light is incident
onto the diffraction grating
1-
8, all the incident light passes through
the diffraction grating
1-
8, since with respect to light with a polarization
direction in which the refractive index of n
1 of the anisotropic material
is effective, the diffraction grating
1-
8 does not function (i.e.
this case is equal to the case where no diffraction grating is provided). On the
other hand, with respect to incident light with a polarization direction orthogonal
to the above-mentioned polarization direction, the diffraction grating
1-
8
functions to diffract the incident light. In FIGS.
1(
a),
1(
b),
and
1(
c), the polarization direction of a beam emitted from the semiconductor
laser
1-
1 corresponds to the polarization direction of a beam for
which the grating of the diffraction grating
1-
8 does not function.
Therefore, all the incident light onto the diffraction grating
1-
8
from the semiconductor laser
1-
1 passes through the diffraction grating
1-
8 regardless of the existence of the grating. Thus, the aperture
NA
1 in the incoming path is not determined by the aperture element, but
is determined by the objective lens holder
1-
6. On the other hand,
reflected light from the optical disk passes through the λ/4 plate
1-
7
twice, i.e. in the incoming and return paths and therefore its polarization direction
is orthogonal to that of a beam emitted from the semiconductor laser
1-
1.
Consequently, since the grating of the diffraction grating
1-
8 is
effective, light passes through the central part (the area without hatching in
the figure) of the diffraction grating
1-
8 to reach first photodetectors.
However, the grating portion (indicated with hatching in the figure) diffracts
light as shown in the figure and therefore the light does not reach the first photodetectors.
The aperture NA
2 in the return path is determined not by the objective lens
holder
1-
6 but by the diffraction grating
1-
8.
As can be seen from FIGS.
1(
a),
1(
b), and
1(
c),
NA
1 is greater than NA
2. For facilitating the description, suppose
that the NA
2 is equal to the NA of the aperture element in the conventional
example. In this case, when comparing the present embodiment with the conventional
example, the aperture in the return path is equal but the aperture in the incoming
path is larger in the present invention. This provides the following effects for
the present invention, which are not provided by the conventional example.
(1) A spot size on an optical disk is proportional to
wherein λ represents a wavelength. Therefore, in the present invention
in which the aperture NA1 for the incident light onto the optical disk is
greater than the NA in the conventional example, a small spot size can be obtain
on the optical disk, thus improving recording sensitivity, recording quality, and
resolution of reproduction signals and reducing crosstalk and intersymbol interference.
(2) The decrease in signal level caused by defocus is proportional to the
square of NA and that caused by tilt is proportional to the cube of NA. In the
present invention, since the aperture in the return path is NA2 that is
equal to the conventional NA, the margins for defocus and tilt are equivalent to
those in the conventional example. Therefore, although the aperture in the incoming
path is larger, the margins are not reduced in the present invention.
(3) The crosstalk components due to adjacent tracks, intersymbol interference
components due to sequential two signals, and aberration components due to defocus
and tilt are contained a lot in the area (surroundings of the aperture) with high
NA for the reflected light from the disk. In the present embodiment, the aperture
in the return path is set to be smaller than that in the incoming path. Consequently,
light containing a lot of crosstalk components, intersymbol interference components,
and aberration components can be eliminated, thus obtaining excellent reproduction characteristics.
FIG. 2 shows an analytical example of reproduction characteristics according
to the present embodiment. A wavelength λ of a beam emitted from an optical
head reproducing random data comprising marks between about 0.4 μm and 2
μm as information signals on an optical disk having a track pitch of 0.6
μm is supposed to be 660 nm. FIG. 2 shows jitter in reproduction signals
using NAs in the incoming and return paths as a parameter. In FIG. 2, the jitter
is shown by contour lines at intervals of 0.1%. In the case of a conventional example,
when both the NAs in the incoming and return paths are set to be 0.6 as a general
aperture, the reproduction-signal jitter is about 4.05%. On the contrary, when
NA
1 in the incoming path is 0.63 and NA
2 in the return path is 0.6
as an example of the present embodiment, the reproduction-signal jitter is 3.0%,
which shows 1.05% improvement compared to the conventional example. As a conventional
example, when both the NAs in the incoming and return paths are set to be 0.63,
the reproduction-signal jitter is about 3.07%, which falls 0.07% short of that
in the present invention. In addition, since the NA in the return path also is
0.63, the defocus margin and the tilt margin are reduced by about 9% and about
14% respectively compared to those in the present embodiment.
In the case of the supposed optical disk in the above-described analysis, when
the NA
2 in the return path is set to be a general value of 0.6, the jitter
decreases gradually as the NA
1 in the incoming path increases. However,
the variation in jitter becomes small, when the NA
1 is in the neighborhood
of 0.63 to 0.67. In order to reduce the influence of the variation in the NA
1,
it is desirable to set the NA
1 to be between 0.63 and 0.67. When priority
is given to the defocus- and tilt-margin expansion by reducing the NA
2 in
the return path, it is found that the jitter does not deteriorate even when the
NA
2 in the return path is 0.54 or less in the case where the NA
1
in the incoming path is 0.60. An optimum NA ratio varies depending on the purpose
such as, for example, giving priority to jitter, giving priority to margins, allowing
the jitter and margins to be compatible. However, the optimum condition can be
found when the NA ratio is set substantially in a range of
FIGS.
3(
a),
3(
b),
3(
c), and
3(
d)
show examples of an aperture element in the present embodiment. FIG.
3(
a)
is a view of the aperture element with the configuration shown in FIGS.
1(
a),
1(
b), and
1(
c) that is seen from the side of the objective
lens
1-
3. The portion indicated with hatching is the lens holder
1-
6 and its aperture corresponds to NA
1 in the incoming path.
Concentric portions inside the hatching portion denote the grating of the diffraction
grating
1-
8, and its aperture corresponds to NA
2 in the return
path. In FIG.
3(
a), both the apertures in the incoming and return
paths are formed in circular shapes. However, the shapes of the apertures are not
limited to the circular shapes. In the case of the circular shape, an advantage
of facilitating the processing and formation of apertures of the lens holder
1-
6
and the like can be obtained. Generally, however, an optimum NA in a radial direction
on an optical disk is different from that in a tangential direction in some cases.
In this case, the aperture with an elliptical shape is preferred to that with a
circular shape. FIG.
3(
b) shows the case where the aperture of the
objective lens holder
1-
6 has an elliptical shape with high NA in
the tangential direction. The direction in which the aperture has high NA is not
limited to the tangential direction and may be the radial direction. Further, there
may be the case where the aperture formed by the diffraction grating
1-
8
in the return path has an elliptical shape. The excellent effect of the present
embodiment can be obtained by a smaller aperture in the return path compared to
that in the incoming path. Therefore, generally the shape of the aperture is not
a problem. Even when the aperture in the return path has a square shape as shown
in FIG.
3(
c), the effect of the invention is not reduced. FIG.
3(
d)
shows an example of aperture in the return path formed of four circular apertures,
which has the excellent effect of the present invention. In addition, since light
passing through the central portion of the aperture in the incoming path also is
eliminated in the return path, light containing a lot of DC components is eliminated.
Therefore, the example also has an advantage of improving a modulation factor of
data information signal RF. As described above, in one or more embodiments, the
present invention is characterized by setting the aperture in the return path to
be smaller than that in the incoming path and therefore the shape of the aperture
is not particularly a problem.
Second Embodiment
FIGS.
4(
a),
4(
b), and
4(
c) show structural
examples according to a second embodiment of the present invention when the apertures
in the incoming and return paths are varied only in the radial direction. The description
of the same parts as those in the configuration shown in FIGS.
1(
a),
1(
b), and
1(
c) is omitted. As shown in FIGS.
4(
a),
4(
b), and
4(
c), the aperture formed by an objective
lens holder
4-
1 in the incoming path and the aperture formed by a
diffraction grating
4-
2 in the return path are set to be equal in
the tangential direction. On the contrary, the aperture NA
2 (R) in the return
path is set to be smaller than the aperture NA
1 (R) in the incoming path
in the radial direction shown in FIG.
4(
b). In an optical disk system
that is affected less by intersymbol interference in the tangential direction,
the following advantages are obtained compared to an optical disk system having
the configuration shown in FIGS.
1(
a),
1(
b), and
1(
c).
(1) Since the aperture NA in the return path is smaller than that in the
incoming path in the radial direction, not only the decrease in crosstalk in adjacent
tracks and the improvement in recording sensitivity are achieved, but also crosstalk
compositions and high aberration portions contained a lot in reflected light from
a disk that passes through high NA portions can be eliminated. Consequently, excellent
reproduction signals can be obtained, and in addition the margins are not reduced
by defocus and tilt.
(2) Since the aperture in the incoming path is equal to that in the return
path in the tangential direction, the loss in quantity of light due to reduction
in NA in the return path can be reduced, thus obtaining reproduction signals with
a high S/N.
FIG. 5 shows an analytical example of reproduction characteristics when the
apertures in the incoming and return paths are set to be 0.60 in the tangential
direction and the apertures in the incoming and return paths are varied only in
the radial direction. The parameter of the optical disk is the same as in the embodiment
1. When both the apertures in the incoming and return paths are 0.63 in the radial
direction, jitter is about 3.54%. However, it can be found that when the aperture
in the incoming path is fixed to 0.63 and the aperture in the return path is decreased
from 0.63, the jitter is improved to 3.5% or less in the case where the aperture
in the return path is between 0.585 and 0.62. On the contrary, when the aperture
in the return path is fixed to 0.60 and the aperture in the incoming path is increased
from 0.60, the minimum jitter is obtained in the case where the aperture in the
incoming path is about 0.64 and the jitter is improved in the case where the aperture
in the incoming path is between about 0.60 and 0.72. An optimum NA ratio varies
depending on the purpose such as, for example, giving priority to jitter, giving
priority to margins, allowing the jitter and margins to be compatible. However,
the optimum condition can be found when the NA ratio is set substantially in the
range of
FIG. 6 shows a structural example of an aperture element in the present embodiment.
The hatching portion shows an objective lens holder
4-
1 with an aperture
in the incoming path having an elliptical shape with its major axis in the radial
direction. The circular aperture inscribed in the elliptical aperture is an aperture
formed by a diffraction grating
4-
2 in the return path, and the portion
indicated with horizontal lines in the figure is a grating. In the present configuration,
since the circular aperture is inscribed in the elliptical aperture in the tangential
direction, the aperture in the incoming path is equal to that in the return path
in the tangential direction, and the aperture NA
1(R) in the incoming path
is larger than the aperture NA
2(R) in the return path only in the radial
direction. FIG. 6 illustrates the combination of an ellipse and a circle as shapes
of apertures. However, the shapes of the apertures are not particularly limited
as described in the first embodiment and as shown in FIGS.
3(
a),
3(
b),
3(
c), and
3(
d).
Third Embodiment
FIGS.
7(
a),
7(
b), and
7(
c) show structural
examples according to a third embodiment of the present invention when apertures
in incoming and return paths are varied only in the tangential direction. The description
of the same parts as those in the configurations shown in FIGS.
1(
a),
1(
b), and
1(
c) and
4(
a),
4(
b),
and
4(
c) is omitted. As shown in FIG.
7(
b), the aperture
formed by an objective lens holder
7-
1 in the incoming path and the
aperture formed by a diffraction grating
7-
2 in the return path are
set to be equal to each other in the radial direction. On the contrary, in the
tangential direction, an aperture NA
2(T) in the return path is set to be
smaller than an aperture NA
1(T) in the incoming path. In an optical disk
system that is affected less by crosstalk in the radial direction, the following
advantages are obtained compared to the configuration shown in FIGS.
1(
a),
1(
b), and
1(
c).
(1) Since the aperture NA in the return path is smaller than that in the
incoming path in the tangential direction, not only the decrease in intersymbol
interference and the improvement in resolution and recording sensitivity are achieved,
but also intersymbol interference compositions and high aberration portions contained
a lot in reflected light from a disk that passes through high NA portions can be
eliminated. Consequently, excellent reproduction signals can be obtained, and in
addition the margins are not reduced by defocus and tilt.
(2) Since the aperture in the incoming path is equal to that in the return
path in the radial direction, the loss in quantity of light due to reduction in
the NA in the return path can be reduced, thus obtaining reproduction signals with
a high S/N.
FIG. 8 shows an analytical example of reproduction characteristics when both
the apertures in the incoming and return paths are set to be 0.60 in the radial
direction and the apertures in the incoming and return paths are varied only in
the tangential direction. The parameter of the optical disk is the same as in the
first and second embodiments. When both the apertures in the incoming and return
paths are 0.66 in the tangential direction, jitter is about 3.2%. However, it can
be found that when the aperture in the incoming path is fixed to 0.66 and the aperture
in the return path is decreased from 0.66, the jitter is improved to 3.2% or less
in the case where the aperture in the return path is between 0.57 and 0.66. An
optimum NA ratio varies depending on the purpose such as, for example, giving priority
to jitter, giving priority to margins, allowing the jitter and margins to be compatible.
However, the optimum condition can be found when the NA ratio is set substantially
in the range of
FIG. 9 shows a structural example of an aperture element according to the third
embodiment of the present invention. The hatching portion shows an objective lens
holder
7-
1 with an aperture in the incoming path having an elliptical
shape with its major axis in the tangential direction. A circular aperture inscribed
in the elliptical aperture is the aperture formed by a diffraction grating
7-
2
in the return path, and the portion indicated with vertical lines in the figure
is a grating. In the present configuration, since the circular aperture is inscribed
in the elliptical aperture in the radial direction, the aperture in the incoming
path is equal to that in the return path in the radial direction, and the aperture
NA
1(T) in the incoming path is larger than the aperture NA
2(T) in
the return path only in the tangential direction. FIG. 9 illustrates the combination
of an ellipse and a circle as shapes of apertures. However, the shapes of the apertures
are not particularly limited as described in the first embodiment and as shown
in FIGS.
3(
a),
3(
b),
3(
c), and
3(
d).
FIGS.
10(
a),
10(
b), and
10(
c) show
structural examples of the present embodiment using a λ/4 plate
1-
7
and a polarized beam splitter (hereinafter referred to as "PBS")
10-
1
as aperture elements. In a polarization film of the PBS
10-
1, incident
light is transmitted or reflected depending on its polarization direction. Therefore,
the PBS
10-
1 has the same function as that of the diffraction grating
7-
2. As an aperture element, the PBS is desirable for eliminating
light passing through the high NA portions with a high quenching ratio, and the
diffraction grating is desirable for achieving the reduction in size and thickness.
As another configuration of the aperture element, it is possible to combine, for
example, a λ/4 plate and liquid crystal. An optimum configuration may be
selected depending on the intended use.
Fourth Embodiment
A configuration according to a fourth embodiment of the present invention is
described
with reference to FIGS.
11(
a),
11(
b), and
11(
c).
The description of the same parts as in the first to third embodiments is omitted.
A semiconductor laser
11-
1 as a light source and first photodetectors
11-
2-
1 and
11-
2-
2 are combined with a
base
11-
3 to form one component. A beam emitted from the semiconductor
laser
11-
1 is incident onto an optical disk
1-
2 via
a diffraction grating
11-
5, a λ/4 plate
1-
7,
and an objective lens holder
11-
4 that form one component with the
objective lens
1-
3. Reflected light from the optical disk
1-
2
is diffracted by the diffraction grating and a part of the diffracted light is
incident onto the first photodetectors. In the present embodiment of the invention,
the diffraction grating
11-
5 has both the functions of the diffracting
grating
1-
8 and the hologram
1-
5 as a separation element
in FIGS.
1(
a),
1(
b), and
1(
c). The portion
indicated with hatching in the diffraction grating
11-
5 has a function
of diffracting unwanted light outside the first photodetectors, and the central
portion of the diffraction grating
11-
5 has a function of the hologram
1-
5. The present embodiment provides the following excellent effects.
(1) The diffraction grating also has the function of a hologram as a separation
element, thus enabling the reduction in number of components, in size, and in cost.
(2) The aperture elements and the objective lens are combined to form one
component, thus reducing the influence by the movement of the objective lens and
the like.
(3) The light source and the first photodetectors are combined to form one
component, thus achieving the size reduction and stabilization of an optical system.
It is not necessary to satisfy the above-mentioned effects (1), (2), and (3)
at
the same time. Even when one of the above-mentioned effects is satisfied individually
according to the convenience in the configuration of the optical system or the
like, the same individual effect can be obtained.
In FIG.
11(
c), exactly speaking, only the base
11-
3
can be seen. In order to facilitate the description, the view is shown in a manner
of seeing through the semiconductor laser and the photodetectors.
FIGS.
11(
a),
11(
b), and
11(
c) show
a configuration in which the apertures in incoming and return paths are varied
from each other both in the radial and tangential directions. However, needless
to mention, the same effect can be obtained even when the apertures in the incoming
and return paths are equal either in the radial or tangential direction.
Fifth Embodiment
A configuration according to a fifth embodiment of the present invention is described
with reference to FIGS.
12(
a),
12(
b), and
12(
c).
The description of the same parts as in the fourth embodiment is omitted. In the
present embodiment, four elements as, second photodetectors are provided on a base
12-
1. Incident light onto a grating portion of a diffraction grating
12-
3 out of reflected light from an optical disk is diffracted to
be led to the second photodetectors. The light led to the second photodetectors
contains a lot of information about intersymbol interference in the tangential
direction and information about crosstalk in the radial direction. In the configurations
of the first to fourth embodiments, excellent information signals were detected
by eliminating this light. In the present embodiment, information signals are obtained
by calculating outputs from the first photodetectors and outputs from the second
photodetectors. Even when light passing through the high NA portions in an aperture
is eliminated, the intersymbol interference components and crosstalk components
cannot be eliminated from the light incident onto the first photodetectors completely.
By subtracting the outputs from the second photodetectors containing a lot of intersymbol
interference components and crosstalk components from the outputs from the first
photodetectors, the intersymbol interference components and crosstalk components
contained in the outputs from the first photodetectors can be cancelled out, thus
further improving information signal quality. In FIG. 12, the light passing through
the high NA portions in both the tangential and radial directions is led to the
second photodetectors. However, the light only in either one of the directions
may be led to the second photodetectors as required. In a system affected slightly
by the intersymbol interference, even in the configuration in which the light passing
through the high NA portion only in the radial direction is led to the second photodetectors,
information signal quality can be improved. The same is applied to the case where
the light passing through the high NA portions only in the tangential direction
is led to the second photodetectors.
Sixth Embodiment
A configuration according to a sixth embodiment of the present invention is described
with reference to FIGS.
13(
a),
13(
b), and
13(
c).
The description of the same parts as in the first embodiment is omitted. In this
embodiment of the present invention, an aperture element is formed of a λ/4
plate
1-
7, a liquid crystal element
13-
1, and a driving
circuit
13-
2 of the liquid crystal element
13-
1. The
liquid crystal element
13-
1 functions to diffract a predetermined
portion of reflected light from an optical disk outside first photodetectors as
in the diffraction grating
1-
8. However, in the liquid crystal element
of the present embodiment, the region in which light is diffracted can be varied.
FIG. 14 shows a structural example of the liquid crystal element. In FIG. 14, the
portion indicated with hatching is the liquid crystal
14-
1 that is
sandwiched between transparent electrodes
14-
2 and
14-
3
in the vertical direction. The respective transparent electrodes are provided with
translucent substrates
14-
4 and
14-
5 formed of glass
or the like. A side face of the liquid crystal
14-
1 is sealed with
a sealing material
14-
6. When voltage is applied to the upper and
lower transparent electrodes by a driving circuit
14-
7, portions
of the liquid crystal that are sandwiched between respective periodic structure
portions of the upper transparent electrode
14-
2 show anisotropy
corresponding to the period of the transparent electrode and thus functions as
an anisotropic diffraction grating. In the present embodiment, the upper transparent
electrode
14-
2 is divided into three regions and is formed so that
voltage can be applied to respective three regions individually by the driving
circuit
14-
7 through switching a switching circuit
14-
8.
In FIG. 14, only (a) of the switching circuit
14-
8 is ON, and therefore
only one region in an outer side of the transparent electrode
14-
2
functions as a diffraction grating. Depending on the state of the switching circuit
14-
8, the region functioning as a diffraction grating can be changed.
In the present embodiment, the transparent electrode
14-
2 was divided
into three regions, but of course the number of regions to be divided is changed
as required. Since the region functioning as a diffraction grating in an aperture
element is changed by using liquid crystal to make the aperture in the return path
variable, the present embodiment can provide an excellent effect in which an optimum
aperture can be set according to an optical disk. When performing information recording
on and information reproduction from a plurality of optical disks having different
track pitches and bit pitches from one another, it also is possible to learn an
optimum aperture for each optical disk automatically.
In the present embodiment, the aperture in the return path is allowed to be variable
by the liquid crystal element, but of course the same can apply to the aperture
in the incoming path. It also is possible to allow both the apertures in the incoming
and return paths to be variable to perform recording and reproduction as required.
In the present embodiment, only the configuration corresponding to the first embodiment
is shown. However, the configuration is not limited to this, and the aperture elements
in any configurations of the second to fifth embodiments are allowed to be variable.
Seventh Embodiment
FIG. 15 shows a cross-sectional configuration of an aperture element
15-
41
according to a seventh embodiment of the present invention. In the present embodiment,
an aperture element is formed of a polarization hologram formed of a ¼ wave
plate and a diffraction grating made of a birefringent material.
A film
15-
45 made of birefringent resin provided adjacently to a
adhesion layer
15-
44 has an optical thickness corresponding to 5/4
wavelengths with respect to a beam with a wavelength λ
1 (in this case,
660 nm) with its refractive index, thickness, birefringence orientation with respect
to a polarization direction being optimized. The optical thickness mentioned above
corresponds to almost 1 wavelength with respect to a beam with a wavelength λ
2
(in this case, 790 nm) emitted from a light source. Therefore, with respect to
the beam with the wavelength λ
1, linearly polarized light passes through
the above-mentioned polarization hologram layer without being diffracted and the
light that has been reflected by a reflection surface and is incident from the
opposite direction is totally diffracted by the polarization hologram layer. On
the other hand, with respect to the beam with the wavelength λ
2, a
plane of polarization is not varied and therefore the beam is not diffracted even
when passing through the element both in the incoming and return paths.
In the present embodiment, the optical thickness of the film
45 corresponds
to 5/4 wavelengths with respect to the beam with a wavelength of 660 nm. However,
when the wavelengths are λ
1 (nm) and λ
2 (nm), generally
the wave plate is designed according to the following condition,
wherein N
1 and N
2 represent arbitrary natural numbers.
On the other glass plate
15-
47, a color separation film
15-
48
that transmits the beam with the wavelength λ
1 and shields the beam
with the wavelength λ
2 is formed. Furthermore, a phase adjustment
film
15-
49 for compensating the phase difference between lights passing
through regions A and B is formed on the glass plate
15-
47. Thus,
the beam with the wavele
