U.S. patent application number 09/899204 was filed with the patent office on 2001-12-27 for optical head.
Invention is credited to Arai, Akihiro, Hayashi, Takao, Nagata, Takayuki, Nakamura, Toru.
Application Number | 20010055248 09/899204 |
Document ID | / |
Family ID | 17624102 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010055248 |
Kind Code |
A1 |
Nagata, Takayuki ; et
al. |
December 27, 2001 |
Optical head
Abstract
An optical head comprises a photodetector 8 having divided
photosensitive areas to detect light 13 reflected from an optical
disk 7, means 10, 11, 12 for obtaining tracking error signal by
operating signals from the photosensitive areas. The photodetector
8 has a first division line 9a parallel to an information track on
the optical disk and second and third division lines 9b, 9c
perpendicular to the first division line 9f and symmetrical to the
optical axis. Further, a light-shielding area 8i is provided for
shielding a part of the reflected light between the division lines
9b and 9c. The signals are operated to reduce offset of the
tracking error signal due to a shift of object lens and a tilt of
the optical disk. Accordingly, the optical head of the present
invention has a small offset of tracking error signal with a simple
structure.
Inventors: |
Nagata, Takayuki;
(Hirakata-shi, JP) ; Arai, Akihiro; (Soraku-gun,
JP) ; Nakamura, Toru; (Katano-shi, JP) ;
Hayashi, Takao; (Toyonaka-shi, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 "K" Street N.W., Suite 800
Washington
DC
20006-1021
US
|
Family ID: |
17624102 |
Appl. No.: |
09/899204 |
Filed: |
July 6, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09899204 |
Jul 6, 2001 |
|
|
|
09064847 |
Apr 23, 1998 |
|
|
|
Current U.S.
Class: |
369/44.41 ;
369/112.17; G9B/7.063; G9B/7.067; G9B/7.111; G9B/7.124 |
Current CPC
Class: |
G11B 7/0903 20130101;
G11B 7/13 20130101; G11B 7/095 20130101; G11B 7/1381 20130101 |
Class at
Publication: |
369/44.41 ;
369/112.17 |
International
Class: |
G11B 007/095; G11B
007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 1995 |
JP |
7-280372 |
Claims
1. An optical head comprising: a light source (2); an optical
system which condenses a light emitted from said light source; a
focus (14) controller for controlling said optical system to form a
light spot on an information recording medium; a photodetecting
means (208) for detecting a plurality of light beam portions in a
light beam (13) reflected from the information recording medium; a
processing means which operates signals of the plurality of light
beam portions detected by said photodetecting means to supply a
tracking error signal; and a tracking controller (15) for
controlling said optical system according to the tracking error
signal to make the light spot follow an information track formed on
the information recording medium; characterized in that a boundary
between the plurality of light beam portions to be detected by said
photodetecting means is defined by a first division line (209)
extending along a direction of information track and that at least
one light-shielding area (208e, 208i', 208i") is arranged
symmetrically along a second line extending through a center of the
light beam (13) and being perpendicular to the first division
line.
2. The optical head according to claim 1, wherein said
photodetecting means comprises a photodetector having a plurality
of photosensitive areas (208a, 208b).
3. The optical head according to claim 1, wherein said
photodetecting means comprises a diffraction element having a
grating divided into a plurality of parts, wherein said diffraction
element divides the light beam to the plurality of light beam
portions.
4. The optical head according to claim 1, wherein said
photodetecting means comprises an element comprising a plurality of
prisms provided, on a plane for dividing the light beam to the
plurality of light beam portions.
5. The optical head according to claim 1, wherein said at least one
light-shielding area is rectangular.
6. The optical head according to claim 1, wherein said at least one
light-shielding area has a shape shielding a part (13a, 13b) of the
light beam wherein zeroth and first order diffraction light beams
diffracted from the information track overlap with each other.
7. The optical head according to claim 1, wherein said
photodetecting means receives two light beam portions defined by
the first division line (209) and said at least one light-shielding
area (208i', 208i") extends perpendicularly to the first division
line (209) so as not to divide each of two areas divided by the
first division line into two.
8. The optical head according to claim 1, wherein said
photodetecting means receives four light beam portions defined by
the first division line (209) and said at least one light-shielding
area (208e) extending perpendicularly to the first division
line.
9. The optical head according to claim 1, wherein a width V of said
at least one light-shielding area along the same direction as said
first division line satisfies a following relation: 9 0.1 1 - ( 1 4
) ( NAd ) 2 < ( V D ) < 0.5 1 - ( 1 4 ) ( NAd ) 2 ,wherein D
is a diameter of light beam in a plane where the plurality of light
beam portions are detected by said photodetecting means, NA is a
numerical aperture of said optical system, .lambda. is a wavelength
and d is a track pitch of the information track.
10. The optical head according to claim 1, wherein said at least
one light-shielding area comprises a first light-shielding portion
(108j) extending in two sides of the first division line (9a), and
second and third light-shielding portions (108k, 108l, 108k',
108l', 108k", 108l") provided at outer sides of the first
light-shielding portion with respect to the first division
line.
11. The optical head according to claim 10 wherein a distance W
between inner boundaries of said second and third light-shielding
portions satisfies a following relation: 10 1 - ( 3 a D ) < W D
< 1 - ( a D ) ,wherein "a" is maximum shift of light beam in a
plane where the plurality of light beam portions are detected by
said photodetecting means and D is diameter of light beam in the
plane including the plurality of light beam portions detected by
said photodetecting means.
12. The optical head according to claim 1, wherein said
photodetecting means receives the light beam portions defined by
said first division line (9a) and second and third division lines
(9b, 9c) crossing said first division line, and said at least one
light-shielding area is arranged between said second and third
division lines.
13. The optical head according to claim 12, wherein said at least
one light-shielding area comprises a first light-shielding portion
(108j) at two sides of the first division line (109a), and second
and third light-shielding portions (108k, 108l) provided at outer
sides of the first light-shielding portion with respect to the
first division line.
14. The optical head according to claim 13, wherein a distance U
between said second and third division lines satisfies a following
relation: 11 0.8 1 - ( 1 4 ) ( NAd ) 2 < ( U D ) < 1.1 1 - (
1 4 ) ( NAd ) 2 ,wherein D is a diameter of light beam in a plane
where the plurality of light beam portions are detected by said
photodetecting means, NA is a numerical aperture of said optical
system, .lambda. is a wavelength and d is a track pitch of the
information track.
15. The optical head according to claim 14 wherein a distance U
between the second and third light-shielding portions satisfies a
following relation: 12 1 - ( 3 a D ) < W D < 1 - ( a D )
,wherein D is diameter of light beam in the plane where the
plurality of light beam portions are detected by said
photodetecting means and "a" is maximum shift of light beam in the
plane where the plurality of light beam portions are detected by
said photodetecting means.
16. The optical head according to claim 12, wherein said
photodetecting means receives six light beam portions defined by
the first, second and third division lines (109a, 109b, 109c) and
said at least one light-shielding area (108j, 108k, 108l, 108k',
108l') extends between said second and third division lines so as
not to divide each of two light beam portions surrounded by the
first to third lines into two.
17. The optical head according to claim 12, wherein said
photodetecting means receives eight light beam portions defined by
the first, second and third division lines (9a, 9b, 9c) and said at
least one light-shielding area (8i, 108j, 108k", 108l"), said at
least one light-shielding area extending to divide each of two
light beam portions surrounded by the first to third lines into
two.
Description
[0001] This application is a divisional application of application
Ser. No. 09/064,847, filed Apr. 23, 1998.
TECHNICAL FIELD
[0002] The present invention relates to an optical head used for
optical recording, reproducing or erasing for information recording
media such as an optical disk.
BACKGROUND ART
[0003] Many reports have been published on techniques for detecting
a tracking error signal in an optical head. Push-pull technique is
well known as one of representative techniques and it is used
practically.
[0004] An optical head using push-pull technique is explained
below. In the optical head, a light emitted by a light source is
condensed by an object lens to form a light spot on a plane of an
optical disk for recording information on which a continuous groove
of information track is formed spirally. Two photosensitive areas
are provided by dividing a photosensitive area of the photodetector
with a division line. The photodetector is shown in FIGS. 1-3 at
the left side. The light reflected from the optical disk enters to
the photodetector. Two photo-detecting signals from the two
photosensitive areas are subjected to differential amplification to
generate a tracking error signal. Tracking control is performed by
controlling the position of the object lens in response to the
tracking error signal. When the light spot is subjected to focus
control, light intensity distribution of the reflected light is
affected by diffraction at the continuous groove due to a position
shift of the light spot from the groove. If the prior art optical
head is used for an optical disk, light intensity distribution of
the reflected light is known to be ascribed to interference of
diffracted light beams of 0th, +1st and -1st orders at the
continuous groove. Two hatched areas in circular light beam in
FIGS. 1-3 denote interference regions. The light intensities in the
two regions become asymmetrical according to the position shift of
the spot from the continuous groove, and the differential signal is
used as the tracking error signal.
[0005] The above-mentioned prior art optical head has a simple
structure to detect a tracking detection signal. However, it has a
problem that an offset of the tracking detection signal is
generated due to tracking movement of the object lens to the
information track or to the tilting of the optical disk. This
problem is explained below. FIGS. 1-3 show positions of the light
beam on the photodetector in three cases and tracking error signals
therefor. The abscissa of graphs of tracking error signal in the
three cases illustrated at the right side in FIGS. 1-3 denotes a
relative position X' of the center of the spot to the track. The
tracking error signal shows a waveform schematically when the light
spot crosses tracks. FIG. 1 shows a case where the object lens is
located above the reference point (X=0). Because the light beam
extends symmetrically relative to the division line between the two
photosensitive areas, the tracking error signal changes
symmetrically with no offset. On the other hand, in a second case
where the object lens moves along X direction (or +X direction in
the case shown in FIG. 2) , the position of the light beam is
shifted on the photodetector and the light beam distribution
becomes asymmetrical. Then, the tracking error signal has a
positive offset relative to the reference voltage. Tracking control
performance is deteriorated if a value of (A-B)/(A+B) exceeds 20%
where A and B denote the positive and negative maximum voltages of
the tracking error signal.
[0006] Further, when an optical disk is tilted relative to the
photodetector along .theta. direction, the light beam distribution
becomes asymmetrical. FIG. 3 shows a case where an optical disk is
tilted in -.theta. direction. The position of the light beam is
also shifted on the photodetector in this case and the light beam
distribution becomes asymmetrical relative to the division line.
Then, the tracking error signal has an offset. Therefore, if the
optical disk is tilted in +.theta. direction and the object lens is
shifted in +X direction, the offset of the tracking error signal
increases as a sum of the two causes. In an ordinary optical disk,
tolerance of off-track is about 0.1 .mu.m where off-track denotes a
shift of position of zero tracking error signal relative to the
track center. Tracking control for an optical disk is usually
needed in a range of about 200 .mu.m of the shift of the object
lens and in a range of about 1.degree. of tilt of the optical disk.
However, in the prior art push-pull optical head, if X is 100 .mu.m
and the tilt along .theta. direction is 0.5.degree., the value of
(A-B)/(A+B) is 35% and off-track is 0.12 .mu.m. Therefore, the two
values exceed the tolerances.
[0007] Because the prior art optical head using push-pull technique
has the above-mentioned characteristics, an apparatus for
reproducing an optical disk with the prior art optical head needs a
means for carrying the optical head at a fast speed precisely for
fast search to an object information track or for an optical disk
having a large eccentricity such as about 100 .mu.m. Then, though
the optical head of simple structure is installed, the optical disk
reproducing apparatus becomes expensive. Further, because the means
for carrying the optical head needs fast speed and high precision,
it is not easy to increase tolerance for external shock and
vibrations. Therefore, the optical head of push-pull technique is
difficult to be installed in a portable optical disk reproducing
apparatus.
[0008] A push-pull system is used in a focus and tracking error
detector apparatus described in U.S. Pat. No. 5,113,386 by
Whitehead et al. In order to make the tracking error output signal
insensitive on side areas in a detector array, the detector array
has a plurality of detectors, and masks or open areas are
positioned at side areas outside the central areas including
regions where the zeroth order diffraction beam overlaps the first
order beams. However, this push-pull system does not solve the
above-mentioned problem on the offset of the tracking error
signal.
[0009] The present invention intends to solve the aforementioned
problems, and its object is to provide an optical head which has a
simple structure as the optical head of push-pull technique and
reduces an offset of tracking error signal due to shift of the
object lens and tilt of optical disk.
DISCLOSURE OF THE INVENTION
[0010] In one aspect of the present invention, an optical head
comprises a light source which emits a light condensed by an
optical system. A focus controller controls the optical system to
form a light spot on an information recording medium. A
photodetecting means has a first division line dividing the light
beams and at least one light-shielding area arranged symmetrically
to the first division line. The photodetecting means divides a
light reflected from the information detection medium to a
plurality of light beams and detects the plurality of light beams.
An operator operates signals of the plurality of light beams
detected by the photodetecting means to supply a tracking error
signal. Then, a tracking controller controls the optical system
according to the tracking error signal to make the light spot
follow an information track formed on the information recording
medium. In the photodetecting means, a first division line divides
the light beam reflected from the information recording medium
into, for example, two, and at least one light-shielding area is
arranged symmetrically to the first division line and shields a
part of a region (preferably a region wherein first order
diffraction light beams reflected from the information recording
medium overlap with each other). Thus, the light beam is divided by
the first division line except the regions covered by the
light-shielding area where the light intensity distribution is
affected largely by tilt of optical disk. Thus, the offset of
tracking error signal due to tilt of an optical disk is
decreased.
[0011] Preferably, the width V of the light-shielding area along
the same direction as the first division line satisfies a following
relation: 1 0.1 1 - ( 1 4 ) ( NAd ) 2 < ( V D ) < 0.5 1 - ( 1
4 ) ( NAd ) 2 , ( 1 )
[0012] wherein D is diameter of light beam, NA is numerical
aperture, .lambda. is wavelength and d is a track pitch of the
information track. If a plurality of divided light beams is
detected by setting the width V as explained above, the optical
intensity can be sufficient for deriving the tracking error signal
while the regions which are liable to be affected by tilt of
optical disk are shielded. Then, the tracking error signal is not
largely affected by tilt of optical disk. Further, if the
light-shielding area extends in parallel to moving direction of the
object lens, even if the reflected light beam is shifted due to
movement of the object lens, a similar advantage is realized.
[0013] In another aspect of the present invention, in the
photodetecting means, second and third division lines perpendicular
to the first division line further divide the light beam besides
the first division line. Then, the light beam is divided into, for
example, six to detect the light. Further, at least one
light-shielding area is arranged to extend symmetrically to the
first division line. Thus, the photodetecting means divides the
light beam into regions affected largely due to overlapping of
zeroth and first order diffraction light beams in the light
reflected from the information recording medium. Thus, the offset
of tracking error signal due to shift of the reflected beam can be
corrected selectively. Preferably, the distance U between the
second and third division lines satisfies a following relation: 2
0.8 1 - ( 1 4 ) ( NAd ) 2 < ( U D ) < 1.1 1 - ( 1 4 ) ( NAd )
2 , ( 2 )
[0014] wherein D is diameter of light beam, NA is numerical
aperture, .lambda. is wavelength and "d" is a track pitch of the
information track. Then, the photosensitive areas outside the
second and third division lines do not include all or almost all
the interference regions. Therefore, signals which are affected
little by tilt of optical disk and corresponds to the shift of the
reflected beams can be taken out selectively. Thus, the offset due
to shift of the reflected beam can be corrected advantageously.
[0015] In a third aspect of the invention, the light-shielding area
comprises a first light-shielding portion extending symmetrically
from the first division line in two directions, and second and
third light-shielding portions provided outside the boundaries of
the first light-shielding portion (or peripheries of the light
beam). By providing the second and third light-shielding portions,
even if the beam reflected from an optical disk is shifted, areas
for detecting tracking error signal is limited to equal distance
from the first division line. Thus, the offset of tracking error
signal due to shift of the light beam becomes small, and the offset
is decreased. Preferably, the distance between the second and third
light-shielding portions satisfies a following relation: 3 1 - ( 3
a D ) < W D < 1 - ( a D ) , ( 3 )
[0016] wherein D is diameter of light beam and a is maximum shift
of light beam.
[0017] An advantage of the present invention is that the optical
head supplies a tracking error signal having small offset due to
shift of object lens along tracking direction.
[0018] An advantage of the present invention is that the optical
head supplies a tracking error signal having small offset due to
tilt of optical disk.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram of a position of the light beam on a
photodetector and of a tracking error signal in a case where an
object lens is located at a reference position and an optical disk
is not tilted;
[0020] FIG. 2 is a diagram of a position of the light beam on a
photodetector and of a tracking error signal in another case where
the object lens is shifted in +X direction;
[0021] FIG. 3 is a diagram of a position of the light beam on a
photodetector and of a tracking error signal in a further case
where the object lens is located at a reference position and the
optical disk is tilted in -.theta. direction;
[0022] FIG. 4 is a schematic sectional view of an optical head
according to a first embodiment of the invention;
[0023] FIG. 5 is a plan view of a photodetector according to a
first embodiment of the invention;
[0024] FIG. 6 is a diagram of a position of the light beam on a
photodetector in a case where an object lens is located at a
reference position and an optical disk is not tilted;
[0025] FIG. 7 is a diagram of a position of the light beam on a
photodetector in another case where the object lens is shifted in
+X direction;
[0026] FIG. 8 is a diagram of a position of the light beam on a
photodetector in a further case where the object lens is located at
a reference position and the optical disk is tilted in -.theta.
direction;
[0027] FIG. 9 is a graph of off-track of the optical head in
various conditions;
[0028] FIG. 10 is a graph of a value of (A-B)/(A+B) for the optical
head in various conditions;
[0029] FIG. 11 is a plan view of a photodetector according to a
second embodiment of the invention;
[0030] FIG. 12 is a plan view of a photodetector according to
another example of the second embodiment of the invention;
[0031] FIG. 13 is a plan view of a photodetector according to a
further example of the second embodiment of the invention;
[0032] FIG. 14 is a schematic sectional view of an optical head
according to a third embodiment of the invention;
[0033] FIG. 15 is a plan view of a photodetector according to the
third embodiment of the invention; and
[0034] FIG. 16 is a plan view of a photodetector according to a
modified example of the third embodiment of the invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] Referring now to the drawings, wherein like reference
characters designate like or corresponding parts throughout the
views, preferred embodiments of the invention are explained below
in detail.
Embodiment 1
[0036] FIG. 4 shows an optical head according to a first embodiment
of the invention. In the optical head, an object lens 5 opposes an
optical disk 7 as an example of a medium for recording information
optically. A continuous groove as an information track is formed
spirally on the optical disk, and the tangent direction of the
groove is vertical to this sheet of paper in FIG. 4. An actuator 6
moves the object lens 5 in X and Y directions under the control of
a focus controller 14 and a tracking controller 15, where the Y
direction is perpendicular to the disk surface and the X direction
is parallel with the disk surface and perpendicular to the tracks.
Two half-mirrors 3 and 4 are arranged between the object lens 5 and
a photodetector 8 along the optical axis 1 of the optical head at
an angle of 45.degree. relative to the optical axis 1. A light
source 2 is located off the optical axis 1, laterally to the first
half-mirror 3. A light beam emitted by the light source 2 is
reflected by the half-mirror 3 to enter to the object lens 5. The
light beam condensed by the optical system forms a light spot on
the optical disk 7. The light reflected from the optical disk 7
propagates through the object lens 5 and the first and half-mirror
3 and is split by the second half-mirror 4 into two light beams.
One of the two light beams reflected by the second halfmirror 4
enters into the focus controller 14 as focus error signal. The
focus controller 14 makes the light spot follow the information
track formed on the optical disk 7 by controlling the actuator 6 in
response to a focus error signal to move the object lens 5 along
the .+-.Y direction so that the light spot is formed on the plane
on which information is recorded. On the other hand, the other of
the two light beams transmitting the second half-mirror 4 enters
into the photodetector 8. As will be explained later, the
photodetector 8 divides the light beam and outputs a plurality of
photosensitive areas in correspondence to a plurality of the
divided light beams. The output signals are supplied through
differential amplifiers 10, 11 and 12 as a calculation device to
the tracking controller 15 as a means for controlling tracking. In
FIG. 4, the photodetector 8 is also illustrated as a partial plan
view as observed from the optical axis for illustrating connection
of the output signals to the differential amplifiers 10-12. The
tracking controller 15 controls the actuator 6 to move the object
lens 5 in .+-.X directions in order to guide the light spot to the
center of the continuous groove of information track in response to
tracking error signal.
[0037] FIG. 5 shows the photodetector 8 having a plurality of
divided photosensitive areas according to this embodiment. The
photodetector 8 has a rectangular photosensitive area wider than a
light beam 13 incident on the photodetector 8. In the
photosensitive area, in order to divide the reflected light beam
into a plurality of light beams, a division line 9a is provided for
bisecting the reflected light beam in the vertical direction, and
two division lines 9b and 9c perpendicular to the division line 9a
are further provided above and below symmetrically with respect to
the optical axis. The division line 9a is set to be parallel to a
direction corresponding to the information track optically.
Further, a light-shielding area 8i is provided between the division
lines 9b and 9c, perpendicular to the division line. The
light-shielding area 8i is arranged symmetrically relative to the
division line 9a to shield a part of a region where 0th, +1st and
-1st order diffraction light beams in the light beam reflected from
the information track overlap with each-other. Thus, the three
division lines 9a, 9b and 9c and the light-shielding area 8i divide
the photosensitive area of the photodetector 8 into eight divided
areas 8a, 8b, 8c, 8d, 8e, 8f, 8g and 8h. It is to be noted that the
areas 8c and 8e are not necessarily a single connected area, and
they may comprises two or more subareas in correspondence to the
shape of the light-shielding area 8i. In the example shown in FIG.
5, the areas 8c and 8e are separated. This holds also for the
fourth area 8d, 8f.
[0038] Thus, the reflected light beam from the optical disk is
divided into six regions by the first, second and third division
lines 9a, 9b and 9c except the light-shielding area 8i. If the
photosensitive area is observed vertically with respect to the
first division line 9a, the top left side comprises a first area
8a, the top right side comprises a second area 8b, the central left
side comprises a third area 8c, 8e, the central right side
comprises a fourth area 8d, 8f, the bottom left side comprises a
fifth area 8g and the bottom right side comprises a sixth area 8h.
Then, the reflected beam incident on the six areas are operated as
follows: A difference of a sum of detection signals of light beams
on the first and the fifth areas 8a, 8g from another sum of those
on the second and sixth areas 8b, 8h is calculated. Further, a
difference of a sum of detection signals of light beams on the
third area 8c, 8e from another sum of those on the fourth area 8d,
8f is calculated. Then, a differential signal of the two
differences is calculated as a tracking error signal.
[0039] The operation is explained further according to the
connections shown in FIG. 4. The first and fifth areas 8a, 8g are
connected to -terminal, while the second and sixth areas 8b, 8h are
conected to +terminal of the differential amplifier 10. Further,
the two areas 8c, 8e of areas 8d, 8f of the fourth area are
connected to +terminal of the differential amplifier 11. The output
signals of the two differential amplifiers 10 and 11 are connected
to + and - terminals of the third differential amplifier 12, and an
output signal (or tracking error signal) of the amplifier 12 is
supplied to the tracking controller 15. Therefore, by using the
connections shown in FIG. 4, a signal TE in proportional to the
error tracking signal is obtained as follows:
TE=G1 (8a+8g-8b-8h)-G2 (8c+8e-8d-8f) (5)
[0040] where 8a-8h denote detection signals of the corresponding
areas and G1 and G2 denotes gains of the first and second
differential amplifiers 10 and 11. That is, a sum of signals of the
areas 8a and 8g is subtracted from a sum of signals of the areas 8b
and 8h, and the difference is amplified by the differential
amplifier 10 of gain G1, while a sum of signals of the areas 8c and
8e is subtracted from a sum of signals of the areas 8d and 8f and
the difference is amplified by the differential amplifier 11 of
gain G2. Then, the third differential amplifier 12 amplifies the
signal TE to supply the error detection signal.
[0041] FIGS. 6-18 shows light beams on the photodetector 8 in three
cases. FIG. 6 shows a case where an object lens 5 is located at a
reference position and an optical disk 7 is not tilted, FIG. 7
shows another case where the object lens 5 is shifted in +X
direction, FIG. 8 shows a further case where the object lens is
located at a reference position and the optical disk 7 is tilted in
-.theta. direction. In FIGS. 6-8, two hatched areas 13a, 13b shows
regions where 0th, +1st and -1st order diffraction light beams
overlap with each other.
[0042] In the case shown in FIG. 6, the light beam is located
symmetrically with respect to the first division line 9a.
Therefore, the differential amplifier 12 outputs the tracking error
signal with no offset. On the other hand, in the case shown in FIG.
7, the symmetry is broken by shifting the light beam in the right
direction. Then, the areas of the light beam 13 included in the
areas 8b, 8d, 8f and 8h increases, while the light beam 13 included
in the areas 8a, 8c, 8e and 8g decreases. Because the change in
light beam distribution appears in the interference regions 13a and
13b, a difference in optical intensity between the areas 8c, 8e and
8d, 8f which includes the interference regions 13a, 13b mainly
becomes a problem. In the areas, besides the difference in optical
intensity of the interference regions 13a, 13b, an area of incident
light of 0th order is changed with a shift of the light beam 13,
and this causes an offset. On the other hand, the areas 8a, 8b, 8g
and 8h receives mainly the 0th order light, and they are affected
by a shift of the light beam. Therefore, if the gains G1 and G2 of
the differential amplifiers 10 and 11 are set to have appropriate
values, the offset due to the shift of light beam can be corrected
selectively by the operation of the above-mentioned signal TE in
proportional to the tracking error signal:
TE=G1 (8a+8g-8b-8h)-G2 (8c+8e-8d-8f) (4)
[0043] where 8a-8h denote detection signals of the corresponding
areas.
[0044] In the case shown in FIG. 8 where the optical disk 7 is
tilted, regions 13c and 13d where light intensity difference
appears are generated in the interference regions 13a and 13b.
However, these regions 13c and 13d are shielded by the
light-shielding regions 8i. Therefore, they have little affect on
the tracking error signal. Further, because the light-shielding
area 8i is provided in parallel to the tracking direction, they
have little affect on the tracking error signal even if the object
lens 5 is shifted.
[0045] FIG. 9 shows results of numerical calculation on the
off-track in various conditions for comparing the photodetector of
the embodiment with the prior art photodetector of push-pull
technique. The off-track is a shift of position of zero tracking
error signal relative to the track center when the optical head is
shifted from the track center. Further, FIG. 10 shows results of
numerical calculation on the value of
(A-B)/(A+B) (4)
[0046] in various conditions, where A and B are positive and
negative maximum voltages of the tracking error signal. The
abscissa in FIGS. 9 and 10 show four conditions as a combination of
shift of 0.33 mm of the object lens 5 in radial direction and tilt
of 1.degree. of the optical disk 7. In FIGS. 9 and 10, "OL shift"
denotes a shift of the object lens 5, "tilt" denotes a tilt of the
optical disk 7, "invention" denotes an optical head of the
embodiment and "PP" denotes an optical head of prior art push-pull
technique.
[0047] Calculation conditions are as follows: Width, pitch and
depth of the continuous groove of information track on the optical
head is 1.1 .mu.m, 100 nm and 1.5 .mu.m, focal length of the object
lens 5 is 4 mm, numerical aperture is 0.45, and wavelength of light
source 1 is 780 nm. The light beam 13 is divided as follows:
U/D=0.82, V/D=0.21, W/D=0.79, gain ratio G2/G1=2.8. It is found
that both the off-track and the value of Eq. (5) of the optical
head of the embodiment are much lower than the counterparts of the
prior art optical head. In other words, for a practical combination
of shift of 0.33 mm and tilt of 1.degree., the results satisfy the
conditions of off-track of 0.1 .mu.m or less and the value of Eq.
(5) of 20% or less with a sufficient margin.
[0048] In this embodiment, by providing the light-shielding region
8i to cover the regions 13c, 13d affected by the tilt of the
optical disk 7, the effect of the tilt is suppressed. Further, the
reflected beam is divided by the second and third division lines 9b
and 9c. Therefore, the offset of the tracking error signal due to
shift of the reflected beam 13 can be corrected by operating the
signals of the areas 8a, 8b, 8g and 8h multiplied with a specified
coefficient with the signals of the areas 8c, 8d, 8e and 8f.
[0049] Preferably, the second and third division lines 9b and 9c
are set as described below. The distance U between the second and
third division lines 9b and 9c is set to satisfy the following
equation: 4 0.8 1 - ( 1 4 ) ( NAd ) 2 < ( U D ) < 1.1 1 - ( 1
4 ) ( NAd ) 2 , ( 2 )
[0050] wherein D is a diameter of the beam, NA is a numerical
aperture, .lambda. is a wavelength and d is a pitch of information
tracks. This condition of Eq. (2) on the second and third division
lines 9b and 9c is determined so that the interference regions 13a,
13b (refer to FIGS. 6-8) in the reflected beam 13 is not included
or nearly included in the first, second, fifth and sixth areas 8a,
8b, 8g and 8h along the direction parallel to the division line 9a
or track. Therefore, only the signal in correspondence to the shift
of the reflected beam can be taken out selectively, and this is
advantageous for correcting the offset of the error correction
signal. If the distance U between the second and third division
lines 9b and 9c is set to be wider than the range shown in Eq. (2),
the signal used for correction becomes smaller. On the other hand,
if the distance U between the second and third division lines 9b
and 9c is set to be narrower than the range shown in Eq. (2), the
interference regions 13a, 13b in the reflected beam 13 affects
largely to the signal used for correction, and this deteriorates
the quality of tracking error signal.
[0051] Preferably, the light-shielding area 8i is set as described
below. The width V of the light-shielding area 8i along the same
direction as the first division line 9a is set to satisfy the
following equation: 5 0.1 1 - ( 1 4 ) ( NAd ) 2 < ( V D ) <
0.5 1 - ( 1 4 ) ( NAd ) 2 , ( 1 )
[0052] wherein D is a diameter of the beam, NA is a numerical
aperture, .lambda. is a wavelength and d is a pitch of information
tracks. The condition of Eq. (1) on the light-shielding area 8i is
determined so that the regions 13c and 13d which are liable to be
affected by tilt of optical disk 7 are shielded while a quantity of
signal light for obtaining the tracking error signal can be secured
sufficiently. Therefore, by providing the light-shielding area
having the width of Eq. (1), the tracking error signal is affected
less by the tilt of the optical disk 7. If the width V of the
light-shielding area 8i is wider than the range shown in Eq. (1),
the quantity of signal light used for correction becomes smaller.
On the other hand, if the width V. of the light-shielding area 8i
is narrower than the range shown in Eq. (1), the interference
regions 13c, 13d in the interference regions 13a, 13b affect
largely to the signal used for correction, and this deteriorates
the quality of tracking error signal. In a different way, the
light-shielding area 8i is set to be parallel to the shifting
direction of the object lens 5. Then, a similar advantage is
obtained even if the reflected beam 13 is moved due to the shift of
the object lens 5.
[0053] In this embodiment, the light-shielding area 8i is
rectangular. However, it may comprise two trapezoids put together
to form a substantially same size or it may be curved, as far as
substantially the same area is formed. Further, it may comprise a
plurality of separated areas.
Embodiment 2
[0054] FIG. 11 shows division of light beam 13 of a photodetector
108 according to a second embodiment of the invention. This
embodiment is different from the first embodiment only in the
division of light beam 13. The structure except this point and the
operation of the optical head are similar to the counterparts of
the first embodiment, and they are not explained here. The
photodetector 108 shown in FIG. 11 is different in that areas for
shielding the peripheries of the light beam are provided between
second and third division lines 109b and 109c.
[0055] In a rectangular photosensitive area of the photodetector
108, similarly to the photodetector 8 of the first embodiment, a
division line 109a is provided for bisecting the light beam in the
vertical direction, and two division lines 109b and 109c
perpendicular to the division line 109a are provided symmetrically
above and below with respect to the optical axis. The division line
109a is set to be parallel to a direction corresponding to the
information track optically. Further, a first light-shielding area
108i is provided between the division lines 109b and 109c,
perpendicular to the division line 109a, to shield the light beam,
and further to divide the light beam. The first light-shielding
area 108j is arranged symmetrically relative to the division line
109a to shield the region 13c and 13d in the interference regions
13a and 13b. Further, second and third areas 108k and 108l are
provided to shield the peripheries of the light beam 13 between the
second and third division lines 109b and 109c. The-second and third
light-shielding areas 108k and 108l are arranged symmetrically with
respect to the first division line 109a. Then, the three division
lines 109a, 109b and 109c and the light-shielding areas 108j, 108k
and 108l divide the photosensitive area of the photodetector 108
into eight divided areas 108a, 108b, 108c, 108d, 108e, 108f, 108g
and 108h. By providing the second and third light-shielding areas
108k and 108l, the areas for detecting tracking error signal are
limited within an equal distance from the first division line 109a
at the left and right sides. Then, the correction coefficient G1/G2
in Eq. (5) can be decreased. When the correction coefficient G1/G2
is small, tolerance for a shift from the optimum value becomes
large, and the correction coefficient can be set easily.
[0056] Preferably, if "a" is maximum shift of the light beam 13 in
one side, a width W between inner boundaries of the second and
third light-shielding areas 108k and 108l is set to satisfy the
following equation: 6 1 - ( 3 a D ) < W D < 1 - ( a D ) , ( 3
)
[0057] wherein D is a diameter of the light beam. Then, the
light-shielding areas 108k and 108l cover areas not affected
largely by the shift of the light beam. If the width W between the
second and third light-shielding areas 108k and 108l is set to be
wider than the range shown in Eq. (3), the signal used for
correction includes signals not affected by the shift of the light
beam. On the other hand, if the width W is set to be narrower than
the range shown in Eq. (3), the signal used for correction becomes
smaller, and this deteriorates the quality of tracking error
signal.
[0058] Similarly to the first embodiment mentioned above, the width
V of the light-shielding area 108j along the same direction as the
first division line 109a is set to satisfy the following equation:
7 0.1 1 - ( 1 4 ) ( NAd ) 2 < ( V D ) < 0.5 1 - ( 1 4 ) ( NAd
) 2 , ( 1 )
[0059] wherein D is a diameter of the beam, NA is a numerical
aperture, .lambda. is a wavelength and d is a pitch of information
tracks. The condition of Eq. (1) on the light-shielding area 108j
is determined so that the regions 13c and 13d which are liable to
be affected by tilt of optical disk 7 are shielded while a quantity
of signal light for obtaining the tracking error signal can be
secured sufficiently.
[0060] In this embodiment, the second and third light-shielding
areas 108k and 108l have linear boundaries and continuous to the
first one 108j. However, they may be curved as far as a
substantially same area is formed. Further, each of them may
comprise a plurality of separated areas.
[0061] FIG. 12 shows division of a light beam of a photodetector
according to another example of the second embodiment. Similar to
the example shown in FIG. 11, second and third light-shielding
areas 108k' and 108l' are provided besides the first one 108j in
order to shield the peripheries of the light beam between second
and third division lines 109b and 109c. However, the second and
third light-shielding areas 108k' and 108l' are not continuous to
the first one 108j. In this example, the reflected light beam is
divided into six. Because the peripheral areas of the light beam
are shielded, the offset of the signal due to shift of light beam
can be decreased, and the correction coefficient G1/G2 in Eq. (5)
can be easily set.
[0062] FIG. 13 shows division of a light beam of a photodetector
according to a further example of the second embodiment. Similar to
the example shown in FIG. 11, second and third light-shielding
areas 108k" and 108l" are provided besides the first one 108j in
order to shield the peripheries of the light beam between second
and third division lines 109b and 109c. The second and third
light-shielding areas 108k" and 108l" are continuous to the first
one 108j, but the width thereof is not constant. The width
increased from the width of the first light-shielding area 108j at
the inner side to the distance between the second and third
boundaries 109b and 109c of the light-receiving area. Similarly to
the above example, because the peripheral areas of the light beam
are shielded, the offset of the signal due to shift of light beam
can be decreased, and the correction coefficient G1/G2 in Eq. (5)
can be set easily.
Embodiment 3
[0063] FIG. 14 shows an optical head according to a third
embodiment of the invention and FIG. 15 shows division of a light
beam 13 of a photodetector 208 according to the third embodiment.
This embodiment is different from the first embodiment only in the
division of light beam 13 and the operation of signals. The
structure except this point and the operation of the optical head
are similar to the counterparts of the first embodiment, and they
are not explained here. The photodetector 208 shown in FIGS. 14 and
15 is different in that division of reflected beam is simple.
[0064] In a rectangular photosensitive area of the photodetector
208, similarly to the photodetector 8 of the first embodiment, a
division line 209 is provided for bisecting the light beam in the
vertical direction. The division line 209 is set to be parallel to
a direction corresponding to the information track optically.
Further, at least one light-shielding area 208e perpendicular to
the division line 209 is arranged symmetrically relative to the
division line 209 to shield the regions 13c and 13d to be affected
by tilt of the optical disk 7 in the interference regions 13a and
13b where zeroth, +1st and -1st order diffraction light beams
overlap with each other. Thus, the photosensitive area of the
photodetector 208 is divided by the division line 209 and the
light-shielding area 208e is further provided perpendicularly to
the division line 209 to form four photosensitive areas 208a, 208b,
208c and 208d.
[0065] Because the regions 13c and 13d are covered by the
light-shielding area 208e, the offset due to a tilt of the optical
disk 7 can be reduced. Then, the optical head can control tracking
stably in a condition where the shift of light beam is small so
that the offset is mainly caused by the tilt of the optical disk
7.
[0066] It is to be noted that the area 208a and 208c are not
necessarily separated, or it may comprise two or more subareas in
correspondence to the shape of the light-shielding area 208e. In
the example shown in FIG. 15, the areas 208a and 208c are
separated. This holds also for the second area 208b, 208d. In the
operation of signals detected in the areas, the two areas 8a, 8c of
a first area is connected to -terminal, while the two areas 8b, 8d
of a second area are connected to +terminal of the differential
amplifier 210. The output signal of the differential amplifiers 210
is supplied as a tracking error signal to the tracking controller
15.
[0067] Preferably, the light-shielding area 208e is set as
described below, similarly to the first embodiment. The width V of
the light-shielding area 208e along the same direction as the
division line 209 is set to satisfy the following equation: 8 0.1 1
- ( 1 4 ) ( NAd ) 2 < ( V D ) < 0.5 1 - ( 1 4 ) ( NAd ) 2 , (
1 )
[0068] wherein D is a diameter of the beam, NA is a numerical
aperture, .lambda. is a wavelength and d is a pitch of information
tracks. The condition of Eq. (1) on the light-shielding area 208e
is determined so that the regions 13c and 13d which are liable to
be affected by tilt of optical disk 7 are shielded while a quantity
of signal light for obtaining the tracking error signal can be
secured sufficiently. Therefore, by providing the light-shielding
area having the width of Eq. (1), the tracking error signal is
affected less by the tilt of the optical disk 7.
[0069] It is noted that instead of the light-shielding area 208c,
the light-shielding area may comprise three areas as in the second
embodiment. In this case, the width W between inner boundaries of
the second and third light-shielding areas is set preferably to
satisfy Eq. (3).
[0070] FIG. 16 shows division of light beam of a photodetector in a
modified example of the third embodiment. Two light-shielding areas
208i' and 208i" are provided at two side of the division line 209
symmetrically to the division line 209, but they are separated from
each other in contrast to the example shown in FIG. 15. The
light-shielding areas 208i' and 208i" are provided to shield areas
affected largely by tilt of optical disk 7 in the overlapping areas
13a and 13b. Then, the offset of tracking error signal due to tilt
of optical disk can be decreased. It is to be noted that the
light-shielding areas 8i and 108i in the examples of the first and
second embodiments may also consist of a plurality of parts,
similarly to that shown in FIG. 16.
[0071] In the above-mentioned embodiments, the reflected beam is
divided into a plurality of light beams in the photodetector 8,
108, 208. However, an optical head according to the invention may
comprise an optical component which splits a light beam provided in
the optical path, instead of the photodetector. For example, the
optical component may be a diffraction element having a grating
divided into a plurality of parts, or an element comprising a
plurality of prisms provided on a plane.
[0072] In the above-mentioned embodiments, the information track of
the optical disk comprises a continuous groove. However, if the
differential amplifiers 10, 11, 12, 210 and the tracking controller
15 have an appropriate characteristic of low-pass filter, tracking
error signal of small offset can be obtained also for an
information track comprising a pit array.
[0073] Although the present invention has been fully described in
connection with the preferred embodiments thereof with reference to
the accompanying drawings, it is to be noted that various changes
and modifications are apparent to those skilled in the art. Such
changes and modifications are to be understood as being included
within the scope of the present invention as defined by the
appended claims unless they depart therefrom.
* * * * *