U.S. patent application number 11/716616 was filed with the patent office on 2007-07-26 for optical head, optical information storage apparatus, and their fabrication method.
Invention is credited to Masaya Horino, Masatoshi Kanamaru, Takeshi Shimano.
Application Number | 20070171557 11/716616 |
Document ID | / |
Family ID | 36931824 |
Filed Date | 2007-07-26 |
United States Patent
Application |
20070171557 |
Kind Code |
A1 |
Shimano; Takeshi ; et
al. |
July 26, 2007 |
Optical head, optical information storage apparatus, and their
fabrication method
Abstract
A method of manufacturing an optical head includes joining of a
first substrate on which a plurality of lenses are formed in an
array, a second substrate on which a plurality of prisms and
mirrors are formed in an array, and a third substrate on which a
plurality of detectors and light sources are formed in an array,
after positioning of the individual substrates, and cutting the
joined substrates along the rows and columns of the array.
Inventors: |
Shimano; Takeshi; (Yokohama,
JP) ; Kanamaru; Masatoshi; (Miho, JP) ;
Horino; Masaya; (Yasato, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
36931824 |
Appl. No.: |
11/716616 |
Filed: |
March 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11211438 |
Aug 26, 2005 |
|
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11716616 |
Mar 12, 2007 |
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Current U.S.
Class: |
359/850 ;
G9B/7.055; G9B/7.108; G9B/7.121 |
Current CPC
Class: |
G11B 7/123 20130101;
G11B 7/08576 20130101; G11B 7/1374 20130101 |
Class at
Publication: |
359/850 |
International
Class: |
G02B 5/08 20060101
G02B005/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 28, 2005 |
JP |
2005-052252 |
Claims
1. A method of manufacturing an optical head, comprising the steps
of: joining a first substrate on which a plurality of lenses are
formed in an array, a second substrate on which a plurality of
prisms and mirrors are formed in an array, and a third substrate on
which a plurality of detectors and light sources are formed in an
array after positioning the individual substrates; and cutting the
joined substrates along the rows and columns of the array.
2. The method of manufacturing an optical head according to claim
1, wherein said second substrate includes cavities formed
therethrough and arranged in an array, wherein said second
substrate is positioned such that said light sources can be
disposed in said cavities.
3. The method of manufacturing an optical head according to claim
1, wherein said first, said second, and said third substrates are
each provided with an alignment mark, and wherein said first, said
second, and said third substrates are joined together by aligning
said alignment marks with one another.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional application of U.S.
application Ser. No. 11/211,438, filed Aug. 26, 2005, the contents
of which are incorporated herein by reference.
CLAIM OF PRIORITY
[0002] The present application claims priority from Japanese
application JP 2005-052252 filed on Feb. 28, 2005, the content of
which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to an optical head for
reproducing or recording information on optical information storage
media, an apparatus for reproducing and/or recording optical
information, and their manufacturing method.
[0005] 2. Background Art
[0006] Optical disc units for CDs and DVDs are widely available
examples of optical information reproducing apparatuses. In CDs,
light with the wavelength of 780 nm is focused beyond a 1.2-mm
thick substrate by an optical head with the numerical aperture (NA)
of 0.45. In DVDs, the wavelength is reduced to 650 nm, and the NA
is increased to 0.6 so as to achieve higher capacities than CDs. As
a result, the thickness of the substrate of the DVD is set to be
0.6 mm in order to reduce the influence of coma aberration that is
produced when the disc is inclined. In recent years, large-capacity
optical discs referred to as Blu-ray Discs (BDs) have also been put
on the market, in which a blue-violet laser diode is used. In BDs,
the NA of the objective lens is increased to 0.85 for even greater
capacities. At the same time, in order to reduce the influence of
the tilting of the disc, the thickness of the substrate is reduced
to 0.1 mm. In practice, however, a 0.1-mm thick substrate is unable
to carry a 120-mm disc. Therefore, a 0.1-mm thick cover layer is
provided on a 1.1-mm thick substrate, and light is focused beyond
the cover layer.
[0007] An example of an optical head used in such disc systems is
disclosed in Patent Document 1 (JP Patent Publication (Kokai) No.
11-144297 A (1999). In this example, a semiconductor laser chip is
integrally formed with a prism, a photodetector, and a substrate,
and one such unit is stacked on top of the other in two stages so
as to handle the two kinds of optical discs, namely CDs and DVDs,
for example. Light emitted from such a module is focused on a disc
by an objective lens separately mounted on an actuator. The light
is then reflected back to the same module, where it is reflected in
the prism and then received by the photodetector.
[0008] Another conventional example of an optical head is disclosed
in Patent Document 2 (JP Patent Publication (Kokai) No. 2004-103241
A), in which a semiconductor laser, a prism, and a photodetector
are also combined into a module. Light emitted by the module is
also focused by an externally disposed objective lens onto an
optical disc and then returned back to the module. This example
differs from that of Patent Document 1 in that a diffraction
grating is added to the module, whereby the reflected light from
the optical disc is guided to the photodetector.
[0009] In yet another example of an optical head, Patent Document 3
(JP Patent Publication (Kokai) No. 2004-272951 A) discloses a
module consisting of a semiconductor laser and a photodetector. The
module is further integrated with an objective lens as well as a
diffraction grating, which is disposed in an upright manner inside
the module such that it can act on the light from the semiconductor
laser before it is reflected by a mirror. In an optical disc unit
based on this technology, the optical head is mounted on a
swing-arm actuator so that the entire optical head can be actuated
for reproducing a signal from the optical disc.
[0010] Patent Document 4 (JP Patent Publication (Kokai) No.
6-251410 A (1994), corresponding to U.S. Pat. No. 5,481,386)
discloses yet another example of an optical head in which a
surface-emission laser, a photodetector, a diffraction lens, and a
diffraction grating are integrally fabricated in a module. The
light emitted from the surface-emission laser is focused on an
optical disc by a diffractive lens, and the reflected light is
guided to the photodetector by the diffraction grating. In an
optical disc unit based on this technology, the optical head is
disposed on a swing arm so that the entire optical head can be
actuated for positioning a light spot on a particular information
track.
[0011] Patent Document 1: JP Patent Publication (Kokai) No.
11-144297 A (1999)
[0012] Patent Document 2: JP Patent Publication (Kokai) No.
2004-103241A
[0013] Patent Document 3: JP Patent Publication (Kokai) No.
2004-272951A
[0014] Patent Document 4: JP Patent Publication (Kokai) No.
6-251410A (1994)
SUMMARY OF THE INVENTION
[0015] As the capacity of optical discs increases, a transparent
substrate or a cover layer with which a recording film on the
optical disc is covered is gradually becoming thinner. As a result,
not only has the size of the optical spot on the recording film
become smaller, but also the size of the optical spot on the
surface of the substrate or the cover layer has become smaller.
Specifically, when the refraction index of the substrate is
approximately 1.6 regardless of the wavelength, the size of the
optical spot on the surface of the substrate or cover layer is
0.45/1.6.times.1.2.times.2=0.68 mm for CDs;
0.6/1.6.times.0.6.times.2=0.45 mm for DVDs; and
0.85/1.6.times.0.1.times.2=0.11 mm for BDs. The beam size at a
position spaced apart from the surface of the substrate or cover
layer by approximately 0.1 mm is 0.68+0.45.times.0.1.times.2=0.77
mm for CDs; 0.45+0.6.times.0.1.times.2=0.57 mm for DVDs; and
0.11+0.85.times.0.1.times.2=0.28 mm for BDs. Thus, as the thickness
of the substrate or cover layer decreases, the beam size can be
further reduced, whereby it becomes possible in principle to reduce
the size of lenses to such an extent that they can be integrated
with a light source and a photodetector. However, as the lens
becomes smaller in size, other optical components must also be
reduced in size, which would make it very difficult to handle such
components for assembly or adjustment purposes.
[0016] In Patent Document 1, although the semiconductor laser,
photodetector, and prism are combined, the objective lens is not,
which is not quite advantageous in terms of minimization of the
optical system. Further, the prism must be individually affixed,
resulting in a difficulty in handling and an increased time for
adjustment, thereby making it difficult to achieve reduction in
manufacturing cost.
[0017] In Patent Document 2, the objective lens is not integrated,
as in Patent Document 1, and therefore this prior art is not
suitable for the minimization of the optical system as a whole.
Further, with regard to the prism, complex laminated prisms must be
individually adjusted and affixed to a semiconductor
laser/photodetector module, resulting in an increased adjustment
time and manufacturing cost.
[0018] In Patent Document 3, although the objective lens is
integrated, the number of components is large and adjustment is
difficult, such that reduction of manufacturing cost is difficult
to achieve. Particularly, it is difficult to secure sufficient
positioning accuracy for the diffraction grating because it is
disposed in an upright manner in the optical system.
[0019] In Patent Document 4, the objective lens is integrated and
the entire manufacturing process can be performed through a
semiconductor process. However, if the full-width at half maximum
of emission angle is narrow, the magnification of the optical
system that is required for obtaining a sufficiently small focused
spot must be increased, which would result in an increase in the
thickness between the laser and the objective lens. For example,
when the full-width at half value of the emission angle of laser is
10.degree., the effective pupil diameter of the objective lens is
0.5 mm, and the ratio of the intensity of light beam at the
outer-most edge of the effective light flux through the objective
lens to the intensity of the light beam at the center of the
optical axis (RIM intensity) is 0.2, the distance between the laser
and the lens that is required would be approximately 1.9 mm. When
the thickness for laser and that of the lens are further added, the
total required thickness could exceed 3 mm.
[0020] In view of these problems of the prior art, it is an object
of the invention to allow the objective lens to be integrated so
that a thin and ultra-small sized optical head that is easy to
assemble and adjust can be provided.
[0021] In order to overcome the aforementioned problems, in
accordance with the invention, microlenses are fabricated on a
transparent wafer in an array. Cavities each with an inclined plane
providing a prism and a reflecting mirror are fabricated on another
transparent substrate in an array. And photodetectors are
fabricated on a semiconductor substrate, such as that of silicon,
in an array. Light sources are also affixed to the semiconductor
substrate. The prism/mirror substrate, the lens substrate, and the
semiconductor substrate are then joined together and the joined
substrates are thereafter cut so as to produce optical heads. The
light source comprises a semiconductor laser of the Fabry-Perot
type, which is currently easily convertible for higher outputs. The
emitted light is directed vertically upwards using a mirror before
it is focused on the disc. Reflected light is incident on the
mirror surface via the lens, transmitted and refracted by the
mirror surface, reflected by the bottom and top surfaces of the
prism substrate, and then guided to the photodetector.
[0022] For size reduction purposes, the effective light flux
diameter of the objective lens is set to be not more than 0.5 mm,
and the thickness of a cover layer of the optical information
storage medium is set to be not more than 0.1 mm. In this way, the
thickness of the integrated optical head can be reduced to be 2 mm
or smaller. In the recent laptop computers, a slot capable of
accommodating a name-card sized card called a PC card with a
thickness of approximately 5 mm is mounted as a virtually standard
component. If an optical head with a thickness of 2 mm or smaller
can be realized, it would be possible to achieve a thickness of a
notebook computer of 5 mm or smaller, providing for 0.6 mm for the
thickness of the medium, 0.2 mm for the thickness of the casing,
both at the top and bottom, 1 mm for the thickness of the circuit
substrate or the like, 0.4 mm for the spacing between the head and
the disc in consideration of the disc plane fluctuations, and 0.8
mm for the thickness of the substrate for the stator of the spindle
motor in addition to the thickness of the optical head.
[0023] For ease of assembly and better dissipation of heat from the
laser, the cavities in the prism/mirror substrate are provided by
throughholes, and the semiconductor lasers are mounted on the
semiconductor substrate before the prism/mirror substrate and the
lens substrate are joined together.
[0024] Because the semiconductor substrate, prism/mirror substrate,
and lens substrate are cut only after they have been joined
together, each of the sides of the individual substrates or the
optical heads is placed in the same plane.
[0025] The lenses are fabricated by joining a collimator lens and
an objective lens for achieving higher NA.
[0026] The refracted ray transmitted through the prism has an
extended optical path if the magnification of the optical system is
to be ensured. Therefore, the refracted ray is caused to enter the
detector after being reflected by the bottom and top surfaces of
the prism/mirror substrate once or more.
[0027] The thus produced optical head is disposed on an actuator,
which is driven as a whole so as to position the head on a
particular information track on the optical information storage
medium.
[0028] The thus produced optical head comprises a first substrate
with a lens for focusing light on an information storage medium, a
second substrate with a detector disposed on the surface thereof,
and a layer disposed between the first and second substrates and
having a prism and a mirror. The layer also includes a cavity in
which a light source is disposed. The light emitted by the light
source is reflected by the mirror, passes through the lens, and is
then focused on an external information storage medium. Reflected
light from the information storage medium then passes through the
lens and the prism and is then detected by the detector.
Preferably, the light is reflected by the bottom and top surfaces
of the prism/mirror substrate once or more before it is incident on
the detector.
[0029] Because the optical head of the invention is manufactured
with the lenses, mirrors, prisms, light sources, and photodetectors
already disposed on the wafers, and adjustments are made with
reference to alignment marks or the like, ultra-small optical heads
can be manufactured accurately in large quantities at low cost
without requiring the handling of small components for adjustment
purposes.
[0030] By causing the light beam to be incident on the detector
after being reflected by the bottom and top surfaces of the
prism/mirror substrate once or more, the magnification of the
optical system can be increased while the thickness of the
prism/mirror substrate is reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 shows the basic configuration of an optical head
according to the invention.
[0032] FIG. 2 shows a lateral cross section of FIG. 1.
[0033] FIG. 3 shows an exploded view of the components of the
optical head shown in FIGS. 1 and 2.
[0034] FIG. 4 shows a conceptual chart of a manufacturing
process.
[0035] FIG. 5 shows individual wafers as joined together.
[0036] FIG. 6 shows a conceptual chart of how the joined wafers are
cut so as to produce optical heads.
[0037] FIG. 7 shows a process of producing a lens substrate and a
prism substrate.
[0038] FIG. 8 shows a second embodiment of the optical head
according to the invention.
[0039] FIG. 9 shows a top view of FIG. 8.
[0040] FIG. 10 shows a table of optical constants for the
embodiment shown in FIG. 8.
[0041] FIG. 11 shows a table of aspheric coefficients for the
aspherical planes shown in FIG. 10.
[0042] FIG. 12 shows spot aberration characteristics on the disc
surface in the optical system shown in FIG. 8.
[0043] FIG. 13 shows the on-disc defocus dependency of an optical
spot between detectors.
[0044] FIG. 14 shows detector patterns and signal computation
formulae.
[0045] FIG. 15 shows the on-disc defocus dependency of an optical
spot distribution on a detector.
[0046] FIG. 16 shows a focus error signal.
[0047] FIGS. 17A and 17B show an embodiment of a ultra-small
optical disc unit employing an optical head of the invention.
[0048] FIG. 18 shows an optical head according to another
embodiment of the invention.
[0049] FIG. 19 shows a top view of the embodiment shown in FIG.
18.
[0050] FIG. 20 shows detector patterns and signal computation
formulae for the embodiment shown in FIG. 18.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0051] Preferred embodiments of the invention will be hereafter
described with reference to the attached drawings.
Embodiment 1
[0052] FIG. 1 shows the basic structure of an optical head
according to the invention. The optical head comprises a silicon
substrate 101 on which photodetectors 102 and 103 are fabricated
and further a semiconductor laser 104 of the Fabry-Perot type is
mounted. On top of the silicon substrate, a prism/mirror substrate
107 with a cavity 106 having a reflecting mirror 105 is bonded. On
top of the prism/mirror substrate, there is further bonded a lens
substrate 109 with an objective lens 108 fabricated therein. The
four sides of the individual substrates are aligned such that each
side is substantially in the same plane. By "substantially in the
same plane" herein is meant that the plane may include some surface
irregularities that are produced in practice when such layered
substrate wafers are diced in a manufacturing process, as will be
described later.
[0053] FIG. 2 shows a lateral cross section of the optical head
shown in FIG. 1, additionally showing a light flux 201 and an
optical information storage medium 202. The light flux 201, which
is emitted by the semiconductor laser 104, is reflected by the
reflecting mirror 105 and is then focused on a recording film 203
on the optical information storage medium 202 through a cover layer
204. Reflected light is again incident on the objective lens 108
and then transmitted and refracted by the reflecting plane of the
reflecting mirror 105. Some of the light is incident on the
photodetector 102 as a detection signal, and the rest is further
reflected at the plane of junction between the lens substrate 109
and the prism/mirror substrate 107 before it is received by the
photodetector 103.
[0054] FIG. 3 shows an exploded view of the optical head shown in
FIGS. 1 and 2. The semiconductor laser 104 is mounted on the
silicon substrate 101, and the silicon substrate 101 and the
prism/mirror substrate 107 are bonded to each other such that the
semiconductor laser 104 is completely housed within the cavity 106
of the prism/mirror substrate 107. On top of this, the lens
substrate 109 is bonded. When mounting the semiconductor laser 104,
a transmitted image of an active laser stripe (not shown) of the
semiconductor laser or an alignment mark (not shown) patterned on
the surface of the semiconductor laser is aligned with an alignment
mark (not shown) on the silicon substrate, before the semiconductor
laser 104 is fixed in place using solder, which is patterned on the
silicon substrate in advance.
[0055] By thus causing the light beam to be reflected by the top
and bottom surfaces of the prism/mirror substrate at least once
before the light beam is incident on the detectors, the
magnification of the optical system can be increased while the
thickness of the prism/mirror substrate is reduced. The
"magnification of the optical system" herein refers to the ratio of
the effective NA on the light source side to the NA on the image
side. Particularly when an infinitive objective lens is combined
with an infinitive collimator lens, the magnification would be
equal to the ratio of the focal distance of the collimator lens to
the focal distance of the objective lens. In other words, when the
distance between the light source and the lens is increased so as
to increase the RIM intensity with a narrow laser emission angle,
the distance required for the refracted light beam within the prism
to converge also increases. And the increase is accommodated by
increasing the number of reflections using the prism/mirror
substrate.
[0056] FIG. 4 shows the basic concept of an actual manufacturing
process. On a silicon wafer 401, photodetectors 102 and 103 are
prepared in an array, and semiconductor lasers 104 are mounted for
individual photodetectors. The silicon substrate is provided with a
plurality of alignment marks 404, with which alignment marks 405 on
a prism/mirror substrate wafer 402 and alignment marks 406 on a
prism/mirror substrate wafer 403 are aligned when the wafers are
bonded together. When bonding the lens substrate wafer 403 and the
prism/mirror substrate wafer 402, a UV resin may be placed between
them and irradiated with UV light, for example. In this case, the
amount of resin must be carefully measured so that the resin is
evenly applied to each cell without overflowing into the cavity.
For the bonding of the prism/mirror substrate 402 and the silicon
wafer 401, methods other than the aforementioned method involving a
UV-cured resin may be employed, such as anodic bonding.
[0057] FIG. 5 shows the result of bonding the substrates of FIG. 4.
FIG. 6 shows a schematic diagram of a number of optical heads
prepared by cutting the bonded substrates. In FIG. 6, because the
cutting is performed after the three substrates have been bonded
together, each of the four sides of the optical head after cutting
is substantially disposed in the same plane.
[0058] FIG. 7 shows a process of manufacturing the lens wafer 403
and the prism/mirror substrate wafer 402. A glass substrate 701 is
coated with a photoresist 702 and then exposed with a gray scale
photomask 703, which has a light and shade pattern on it, placed
closely on the substrate. When the exposed substrate is developed,
a lens shape and a prism shape are formed on the resist. By
dry-etching these shapes using C.sub.4F.sub.8 gas, for example, the
shapes can be transferred onto the glass. Alternatively, for the
prism substrate, for example, a mold may be prepared by machining
and a pattern formed on it may be transferred to a glass or plastic
substrate.
[0059] Thus, the prism/mirror substrate with cavities provided
therethrough is bonded after the semiconductor lasers are mounted
on the semiconductor substrate. As a result, heat can be readily
dissipated from the semiconductor lasers through the semiconductor
substrate, which has better heat conductance than glass or plastic.
Further, mounting the semiconductor laser chips without there being
any blocking parts in surrounding areas makes it easier to handle
the semiconductor laser chips than if the semiconductor laser chips
are placed in the cavities and then adjusted.
[0060] Because the semiconductor substrate, prism/mirror substrate,
and lens substrate are bonded together before they are cut, the
sides of the individual substrates can be each placed in the same
plane. As a result, stress concentration does not easily occur and
the resultant shape of the optical head facilitates its mounting on
an actuator.
[0061] Furthermore, because the lens substrate is prepared by
bonding an objective lens and a collimator lens together, the NA of
the objective lens can be easily increased.
Embodiment 2
[0062] FIG. 8 shows a second embodiment of the invention. Light
emitted by a semiconductor laser 104 is reflected by a reflecting
mirror 105 and then turned into parallel beams by a collimator lens
802. The beams are then focused by an infinity objective lens 804
on a recording film on an optical information storage medium 202
through a cover layer 204 with a thickness of 0.1 mm. The
semiconductor laser 104 is mounted on a radiating stem 801 made of
SiC. The collimator lens 802 is comprised of an aspherical surface
formed on either side of a collimator lens substrate 803. In order
to reduce the distance between the end of the semiconductor laser
104 and the collimator lens 802 as much as possible while
maintaining a constant focal distance, the collimator lens 802 has
a meniscus shape. The objective lens 804 is an aspherical lens
formed on either side of the objective lens substrate 805 with an
effective pupil diameter of 0.5 mm and NA of 0.85. The side of the
objective lens 804 towards the recording medium is raised from the
surrounding areas of the substrate by approximately 0.1 mm so as to
reduce the possibility of collision with the cover layer 204. The
reflected light is then again incident on the reflecting surface of
the reflecting mirror 105 and then transmitted and refracted by the
reflecting surface. The light is then reflected by the bottom and
top surfaces of the prism/mirror substrate 107, and some of the
light is then received by the photodetector 102. The rest is
reflected by the photodetector 102 and again reflected by the top
surface before it is received by the photodetector 103. Thus, the
light emitted by the light source 104 is reflected a plurality of
times between the plane of junction with the second substrate 101
and the plane of junction with the first substrate 803 before it is
received by the detectors. Therefore, the magnification of the
optical system can be increased while reducing the thickness of the
prism/mirror substrate. Electric wires are connected to the
semiconductor laser 104 and photodetectors 102 and 103 via
throughholes 806, 807, 808, and 809 in the bottom surface of the
silicon substrate 101. Electric inputs and outputs to the optical
head are provided via flexible plastic cables (FPCs), which are not
shown, through the bottom surface of the silicon substrate 101. The
thickness of the optical head is 2 mm in total, and its length is
4.2 mm.
[0063] FIG. 9 shows a top plan view of the optical head of FIG. 8.
The optical head has a width of approximately 2 mm.
[0064] FIG. 10 shows a table of optical constants of the optical
head shown in FIG. 8. "TYPE" indicates the type of plane, such as S
for spherical or planar plane, A for an aspherical plane, SDM for a
planar or spherical mirror with the center of plane displaced from
optical axis, and SD for a planar or spherical plane with the
center of plane displaced from the optical axis. "RADIUS" indicates
the radius of curvature of the plane in millimeter units.
"Infinity" indicates that the radius of curvature is infinite and
therefore the plane is planar. "DISTANCE" indicates the distance
from the plane that is located immediately behind in millimeter
units; negative values show that the distance is that between the
planes after an odd number of times of reflection. "STO" indicates
that the plane has an aperture. Glasses are all M-LAF81, the
wavelength is 405 nm, and "INDEX" indicates the refraction index
under these conditions. The refraction index values assume negative
values after an odd number of times of refractions. "APE-Y"
indicates the radius of each plane shown in a light-beam tracing
chart, which is indicated in millimeter units. Because the radius
of aperture in the STO plane is 0.25 mm, it is seen that the
effective pupil diameter of the objective lens is 0.5 mm. "AP"
indicates the shape of the aperture in each plane, such as C for
circular and R for rectangular. "ADE" indicates the inclination of
the plane. It is indicated that the seventh plane is the recording
film of the information storage medium and that the same planes as
those of the incoming path are tracked in the opposite direction
until the 13.sup.th plane. The 14.sup.th plane and subsequent
planes are the reflecting planes within the prism.
[0065] FIG. 11 shows the aspherical coefficients of the aspherical
planes of FIG. 10. The second and third planes of FIG. 10, which
are the both sides of the collimator lens 802, are each indicated
to be an aspherical plane given by the aspherical coefficients
shown in the table. The 4.sup.th plane (aperture plane) and the
5.sup.th plane are the both sides of the objective lens 805. The
aspherical equation is given by: z .function. ( r ) = r 2 R + R 2 -
( k + 1 ) .times. r 2 + Ar 4 + Br 6 + Cr 8 + Dr 10 ( 1 )
##EQU1##
[0066] FIG. 12 shows the field-angle characteristics of wave
aberration of optical spot on the recording film shown in FIG. 8.
The characteristics are plotted while varying the wavelength by
.+-.5 nm from 405 nm, each plot representing the wave aberration at
the best focus. The result shows that generally favorable focusing
characteristics are obtained.
[0067] FIG. 13 shows the spot distribution on the upper plane of
the prism/mirror substrate between the photodetectors 102 and 103
that was obtained while varying the amount of defocus on the disc,
namely, the distance between the objective lens 804 and the cover
layer 204. It can be seen that astigmatism is produced on the
detectors. This is due to the fact that the convergent light is
incident on an inclined refractive plane, which cannot be avoided
in the optical system of FIG. 8. The astigmatism, however, does not
have any influence on the optical spot on the disc surface and does
not pose any problems as long as focal point detection and tracking
detection can be carried out.
[0068] FIG. 14 shows detector patterns for detecting a focal error
signal and a tracking error signal from a spot with astigmatism, as
in the case of the optical spot shown in FIG. 13. The figure also
shows signal computation formulae. A band-shaped photodetector
consisting of three sections and another consisting of six sections
are disposed in front of and behind, respectively, the focal point.
Individual output signals are calculated in accordance with the
computation formulae shown so as to obtain an FES (focus error
signal), a TES (tracking error signal), and an RFS (radio frequency
signal). k is a computational gain for the correction of imbalance
in the total amount of light in front of and behind the focal
point.
[0069] FIG. 15 shows the results of computing the distribution of
light incident on the photodetectors shown in FIG. 14 while varying
the amount of defocus on the information storage medium. The
numbers on the left of the drawing indicate the amount of defocus.
It can be seen from these results that the focus error signal can
be obtained by the calculation formulae of FIG. 14. It can also be
seen that the imbalance in intensity in the direction perpendicular
to the tracks of the recording medium can be detected by the
photodetector located on the further side, and that the tracking
signal based on the push-pull system can also be calculated using
the calculation formulae of FIG. 14.
[0070] FIG. 16 schematically shows a resultant focus error signal.
It can be seen that there is a range of approximately .+-.2 .mu.m
for focus error detection.
Embodiment 3
[0071] FIGS. 17A and 17B show an embodiment of a small-sized
optical disc unit utilizing a ultra-small optical head 1701
according to the invention. FIG. 17A is a plan view, and FIG. 17B
is a side view. The small optical head 1701 is mounted on an
actuator arm 1708, which can be moved finely by a focus actuator
1707 in the direction of the optical axis of the objective lens in
the optical head. The actuator arm 1708 and the focus actuator 1707
are fixed to a swing arm 1703, together with a counter balance
1705. The swing arm 1703 is driven by a swing motor 1704 so as to
move the small optical head 1701 in the radius direction of an
optical disc 1709. The optical disc 1709 is rotated by a spindle
motor 1702. Input and output of signal to the optical head are
enabled by flexible plastic cables (not shown) connected to a
control circuit 1706.
[0072] When the thus prepared optical head is mounted and driven on
an actuator, a large amount of disc eccentricity can be dealt with
even when the effective pupil diameter of the lens is reduced. In
conventional optical discs, the disc eccentricity is handled
through the actuation of only the lens mounted on the actuator. In
this case, however, the axis of the lens with respect to the fixed
optical system moves. As a result, when the push-pull method is
employed where the tracking signal is detected on the basis of the
distribution of the reflected light, an offset is produced by the
shifting of the lens, whose influence becomes greater as the
diameter of the effective light flux becomes smaller. To avoid
this, a differential push-pull method is employed in DVDs, for
example, whereby three beams of light are collected on the disc,
and the offset is canceled by a differential computation of
push-pull signals from sub-spots on either side and a push-pull
signal from the main spot. However, the optical system in which
three spots are collected requires a diffraction grating or the
like. Such system also has disadvantages such as a reduction in the
efficiency of optical utilization of the main spot due to the
presence of the sub-spots, or the generation of unwanted stray
light. In accordance with the above-described embodiment of the
invention, however, the optical head can be easily reduced in size
and integrated, so that the optical head can be driven integrally
and therefore the generation of offset can be prevented.
Embodiment 4
[0073] FIG. 18 shows another embodiment of the invention, in which
a quarter-wave plate 1801 with a thickness of approximately 0.1 mm
is inserted between an objective lens 804 and a collimator lens
802. A plane is formed in the peripheral areas of each lens that is
flat and protruding from the lens planes, and the quarter-wave
plate 1801 is sandwiched between these planes. In this way, the
quarter-wave plate 1801 can be fixed between the objective lens
substrate 805 and the collimator lens substrate 803, using an
adhesive agent (not shown) or the like, without the plate coming
into contact with the lens faces. The crystal axis direction of the
quarter-wave plate 1801 is adjusted such that the transmitted light
of the linearly polarized light incident on the quarter-wave plate
1801 becomes circularly polarized light. When the reflected light
from the recording film 203 passes through the objective lens 804
again and further passes through the quarter-wave plate 1801 again,
the light is converted into linearly polarized light with the
direction of polarization rotated by 90.degree. with respect to the
polarization of the light that was initially incident on the
quarter-wave plate 1801. When the surface of the reflecting mirror
105 is coated with a multilayered film (not shown) by vapor
deposition, for example, such that the s-polarized light is
reflected and the p-polarized light is transmitted by the
reflecting mirror, the reflected light from the disc can be again
reflected by the reflecting mirror and prevented from returning to
the semiconductor laser 104. In this way, noise components in the
intensity of laser oscillation induced by the returning light to
the semiconductor laser 104 can be reduced. In the present
embodiment, the thickness of the optical head as a whole would have
to increase in principle due to the addition of the quarter-wave
plate, as compared with the embodiment of FIG. 8. However, the
thickness is controlled to be the same as that of FIG. 8 through a
review of the design of the collimator lens 802 and an enhancement
of the beam enlarging effect. Further, in the present embodiment,
the position within the prism/mirror substrate 107 where the light
is most focused in the plane of the drawing sheet is the fifth
point of reflection, and the photodetectors 102 and 103 for the
detection of focal point error are disposed at the third and
seventh points of reflection, respectively. At the fifth point of
reflection, there is also newly disposed a photodetector 1802, the
output of which is used as a reproduction signal. Thus, the need to
use a separate-type photodetector for detecting a reproduction
signal is eliminated, thereby improving the signal-to-noise ratio
of the RF signal.
[0074] FIG. 19 shows a top plan view of the optical head of FIG.
18.
[0075] FIG. 20 shows the arrangement of photodetectors for signal
detection and formulae for signal calculation in the present
embodiment. As mentioned above, by using the output signal from the
photo-detecting region at the center as the RF signal, the SIN
ratio of the reproduction signal can be improved. Generally,
signals from separate light-receiving portions are once subjected
to current-to-voltage conversion and amplification in an amplifier,
before they are subtracted or summed. In this process,
amplification noise is added from the amplifier into the calculated
signal, the amount of such noise corresponding to the number of
contributing amplifiers. This is why it is desirable to detect the
RF signal, whose S/N ratio is particularly necessary to be
improved, using a single light-receiving portion and perform
current-to-voltage conversion and amplification in a single
amplifier.
[0076] In accordance with the invention, the optical head in an
optical information reproduction apparatus can be minimized to a
very high degree, adjusted easily, and manufactured at low cost.
The invention allows the optical disc units with large capacities
to be greatly reduced in size such that they can be mounted on
cellular phones, for example. As a result, a greater variety of
applications can be utilized on cellular phones. Furthermore, by
utilizing the technology of the invention on video cameras, it
becomes possible to realize video cameras with sizes comparable to
those of digital cameras. When a plurality of such ultra-small
optical heads are mounted on a single optical disc unit so as to
allow information to be recorded or reproduced in parallel, the
transfer rate can be effectively enhanced.
* * * * *