U.S. patent application number 09/819794 was filed with the patent office on 2001-09-06 for optical pickup for discriminating between and reading different types of optical discs.
Invention is credited to Ohsato, Kiyoshi, Uemura, Kamon, Utsumi, Masamichi, Yamakawa, Akio.
Application Number | 20010019520 09/819794 |
Document ID | / |
Family ID | 27318863 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019520 |
Kind Code |
A1 |
Uemura, Kamon ; et
al. |
September 6, 2001 |
Optical pickup for discriminating between and reading different
types of optical discs
Abstract
An optical pickup including a light source for radiating a light
beam, a diffraction element for separating a light beam radiated
from the light source into at least three beams, namely a main beam
and two side beams, an objective lens for converging the light
beams separated by the diffraction element on a signal recording
surface of the optical recording medium, a light receiving unit
having a four-segment first light receiving portion for receiving
the main beam reflected by the recording surface of the optical
recording medium and second and third light receiving portions
arranged on both sides of the first light receiving portion for
receiving the side beams reflected by the recording surface of the
optical recording medium, and a calculation unit for generating a
first tracking signal based on respective outputs of the first
light receiving portion and for generating a second tracking signal
based on outputs of the second and third light receiving
portions.
Inventors: |
Uemura, Kamon; (Tokyo,
JP) ; Ohsato, Kiyoshi; (Chiba, JP) ; Yamakawa,
Akio; (Kanagawa, JP) ; Utsumi, Masamichi;
(Chiba, JP) |
Correspondence
Address: |
Ronald P. Kananen
RADER, FISHMAN & GRAUER, PLLC
1233 20th Street, Suite 501, N.W.,
Washington
DC
20036
US
|
Family ID: |
27318863 |
Appl. No.: |
09/819794 |
Filed: |
March 29, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09819794 |
Mar 29, 2001 |
|
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08661459 |
Jun 11, 1996 |
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6222803 |
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Current U.S.
Class: |
369/44.23 ;
369/112.26; 369/44.29; 369/44.37; 369/53.2; G9B/19.017; G9B/7.066;
G9B/7.102 |
Current CPC
Class: |
G11B 7/0901 20130101;
G11B 19/12 20130101; G11B 7/1381 20130101; G11B 19/128 20130101;
G11B 7/1372 20130101; G11B 2007/0006 20130101; G11B 2007/0013
20130101; G11B 7/139 20130101 |
Class at
Publication: |
369/44.23 ;
369/44.37; 369/53.2; 369/112.26; 369/44.29 |
International
Class: |
G11B 007/095 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 21, 1995 |
JP |
P07-154399 |
Jun 30, 1995 |
JP |
P07-166637 |
Jun 12, 1995 |
JP |
P07-144714 |
Claims
1. An optical pickup comprising: a light source for radiating a
light beam; a diffraction element for separating a light beam
radiated from said light source into at least three beams, namely a
main beam and two side beams; an objective lens for converging the
light beams separated by said diffraction element on a signal
recording surface of the optical recording medium; a light
receiving unit having at least two-segment first light receiving
portion for receiving said main beam reflected by the recording
surface of the optical recording medium and second and third light
receiving portions arranged on both sides of said first light
receiving portion for receiving the side beams reflected by the
recording surface of the optical recording medium; and calculation
means for generating a first tracking signal based on respective
outputs of said first light receiving portion and for generating a
second tracking signal based on outputs of said second and third
light receiving portions.
2. The optical pickup as claimed in claim 1 wherein said
calculation means calculates the first tracking error signal by
calculating phase differences between signals obtained on summing
desired two of said four outputs of said first light receiving
section, said calculation means calculating the second tracking
signal by calculating the differences of signals from said second
and third light receiving portions.
3. The optical pickup as claimed in claim 1 further comprising
means for varying the aperture ratio of said optical lens.
4. The optical pickup as claimed in claim 1 wherein said objective
lens has plural lens portions having different aperture ratios.
5. An optical pickup comprising: a light source for radiating a
light beam; a diffraction element for separating a light beam
radiated from said light source into at least three beams, namely a
main beam and two side beams; an objective lens for converging the
light beams separated by said diffraction element on a signal
recording surface of the optical recording medium; a light
receiving unit having a four-segment first light receiving portion
for receiving said main beam reflected by the recording surface of
the optical recording medium and second and third two-segment light
receiving portions arranged on both sides of said first light
receiving portion for receiving the side beams reflected by the
recording surface of the optical recording medium; and calculation
means for generating a first tracking signal based on respective
outputs of said first light receiving portion, a second tracking
signal based on differential outputs of said first, second and
third light receiving portions and a third tracking error signal by
a differential output between an output of said second light
receiving portion and an output of said third light receiving
portion.
6. The optical pickup as claimed in claim 5 further comprising
means for varying the aperture ratio of said objective lens.
7. A tracking servo system in an optical disc recording and/or
reproducing apparatus comprising: a light source for radiating a
light beam; a diffraction element for separating a light beam
radiated from said light source into at least three beams, namely a
main beam and two side beams; an objective lens for converging the
light beams separated by said diffraction element on a signal
recording surface of the optical recording medium; a light
receiving unit having a four-segment first light receiving portion
for receiving said main beam reflected by the recording surface of
the optical recording medium and second and third light receiving
portions arranged on both sides of said first light receiving
portion for receiving the side beams reflected by the recording
surface of the optical recording medium; calculation means for
finding a plurality of tracking error signals based on outputs of
said first, second and third light receiving portions; means for
discriminating the sorts of the optical discs; switching means for
selecting one of the tracking error signals calculated by said
calculation means based on a signal from said discrimination means;
and means for driving an objective lens based on the tracking error
signal selected by said switching means.
8. The tracking servo system in the optical disc recording and/or
reproducing apparatus as claimed in claim 7 wherein said
calculation means . produces a first tracking error signal by
calculating the phase difference between signals obtained on
addition of desired two each of the four outputs of the first light
receiving portion, said second calculation means producing a second
tracking error signal based on outputs of said second and third
light receiving portions.
9. The tracking servo system in the optical disc recording and/or
reproducing apparatus as claimed in claim 7 wherein said
calculation means calculates the phase differences of signals
produced on summing desired two each of four outputs of said first
light receiving portion for producing a first tracking error
signal, said calculation means also calculating the differences of
signals from the second and third light receivers for producing a
second tracking error signal.
10. The tracking servo system in the optical disc recording and/or
reproducing apparatus as claimed in claim 7 wherein each of said
second and third light receiving portions is a two-segment light
receiving portion, said calculation means producing a first
tracking error signal based on respective outputs of said first
light receiving portion and also producing a second tracking error
signal based on respective differential outputs of said first to
third light receiving portions, said calculation means also
producing a third tracking error from a differential output between
an output of the second light receiving portion and an output of
the third light receiving portion.
11. The tracking servo as claimed in claim 7 further comprising
means for varying the aperture ratio of said optical lens.
12. The tracking servo system as claimed in claim 7 wherein said
objective lens has plural lens portions having different aperture
ratios.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to an optical pickup for recording
and/or reproduction of an optical recording medium, a reproducing
apparatus and a recording apparatus for an optical recording
medium. More particularly, it relates to an apparatus capable of
recording and/or reproducing plural sorts of optical discs
different in track pitch by one and the same apparatus.
[0003] 2. Description of the Related Art
[0004] Heretofore, in an apparatus for reproducing an optical disc,
such as a compact disc, a three-beam method has been in use as a
system for detecting tracking error signals. This system splits a
light beam radiated by a semiconductor laser element by a
diffraction grating into three beams, namely a main beam and both
side beams. The main beam is radiated on a recording track of an
optical disc, while both side beams are radiated on the positions
of the disc offset by one-fourth of a track to both sides of the
recording track. The light beams radiated on the optical disc are
reflected by the recording surface of the recording medium so as to
be received by a photodetector. This photodetector is made up of a
first photodetector portion for receiving a main beam and second
and third photodetector portions for receiving both side beams. The
tracking error signal is detected by finding the difference between
the signals received by the second and third photodetector
portions.
[0005] Recently, such an optical disc has been investigated in
which data may be recorded to a high density for recording data of
high precision, such as still pictures or moving pictures. With
such optical disc, it may be contemplated to set the track pitch to
approximately 0.8 .mu.m instead of to 1.6 .mu.m conventionally
used, or to form a recording layer of a narrower track pitch as
multiple layers. The optical disc, recorded to a high density, is
not limited to a replay-only type, but a rewritable optical disc,
such as a phase change type disc, is also contemplated. Such a disc
having a guide groove may also been envisaged as such rewritable
optical disc.
[0006] However, it is difficult with the optical disc for high
density recording to detect tracking error signals by the
above-mentioned three-beam system. That is, since the track pitch
is of a narrow width, registration of the side spots of the three
spots radiated on the recording surface of the optical disc becomes
difficult. Also, if the high-density recording layer is formed as
multiple layers, there is raised a problem that an offset be
produced in the tracking error signal due to leakage of the
reflected light from the layer other than the layer being recorded
or reproduced. In addition, if the rewritable optical disc is a
phase change type disc, and recorded and unrecorded portions are
produced on the disc, a noise is produced with the three-beam
method due to differences in reflectance in the recorded and
unrecorded portions, thus making it difficult to detect correct
tracking error signals.
OBJECT AND SUMMARY OF THE INVENTION
[0007] In view of the above-depicted status of the art, it is an
object of the present invention to provide an apparatus capable of
performing selective recording and/or reproduction on or from
plural sorts of optical discs, such as optical discs with different
track pitches.
[0008] An optical pickup according to the present invention
includes a light source for radiating a light beam, a diffraction
element for separating a light beam radiated from the light source
into at least three beams, namely a main bean and two side beams,
an objective lens for converging the light beams separated by the
diffraction element on a signal recording surface of the optical
recording medium, a light receiving unit having a four-segment
first light receiving portion for receiving the main beam reflected
by the recording surface of the optical recording medium and second
and third light receiving portions arranged on both sides of the
first light receiving portion for receiving the side beams
reflected by the recording surface of the optical recording medium,
and a calculation unit for generating a first tracking signal based
on respective outputs of the first light receiving portion and for
generating a second tracking signal based on outputs of the second
and third light receiving portions.
[0009] A tracking servo system in an optical disc recording and/or
reproducing apparatus according to the present invention includes a
light source for radiating a light beam, a diffraction element for
separating a light beam radiated from the light source into at
least three beams, namely a main beam and two side beams, an
objective lens for converging the light beams separated by the
diffraction element on a signal recording surface of the optical
recording medium, a light receiving unit having a four-segment
first light receiving portion for receiving the main beam reflected
by the recording surface of the optical recording medium and second
and third light receiving portions arranged on both sides of the
first light receiving portion for receiving the side beams
reflected by the recording surface of the optical recording medium,
a calculation unit for finding a plurality of tracking error
signals based on outputs of the first, second and third light
receiving portions, a discrimination unit for discriminating the
sorts of the optical discs, a switching unit for selecting one of
the tracking error signals calculated by the calculation unit based
on a signal from the discrimination unit, and a driving unit for
driving an objective lens based on the tracking error signal
selected by the switching means.
[0010] The discrimination unit discriminates the sorts of at least
two sorts of the optical discs with different track pitches, and
the signal processing unit is responsive to the results of
discrimination to switch the calculation operations for obtaining a
tracking error signal from a detection signal of the photodetector
unit, so that at least two sorts of the optical disc can be
reproduced by simplified adjustment operations.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 illustrates a schematic structure of an optical disc
recording and/or reproducing apparatus embodying the present
invention.
[0012] FIG. 2 illustrates a schematic structure of an optical
pickup embodying the present invention.
[0013] FIGS. 3 and 4 illustrate a bi-axial mechanism of an
objective lens in the optical pickup.
[0014] FIG. 5 illustrates a first embodiment of a tracking servo
system according to the present invention.
[0015] FIGS. 6A and 6B illustrate the state of radiation of three
beams on signal pits of an optical disc.
[0016] FIG. 7 illustrates a second embodiment of a tracking servo
system according to the present invention.
[0017] FIG. 8 illustrates a third embodiment of a tracking servo
system according to the present invention.
[0018] FIG. 9 illustrates a schematic structure of an optical disc
recording and/or reproducing apparatus having an aperture ratio
varying unit according to an embodiment of the present
invention.
[0019] FIGS. 10A and 10B are perspective views showing a light
shielding ring used as the aperture ratio varying unit of FIG. 9
and a movement mechanism for the light shielding ring.
[0020] FIGS. 11A, 11B show the manner in which the aperture ratio
is varied by the light shielding ring shown in FIGS. 10A and
10B.
[0021] FIG. 12 is a graph showing the relation between the aperture
ratio of the objective lens and the spatial frequency.
[0022] FIG. 13 is a perspective view showing a light shielding
plate used as the aperture ratio varying unit of FIG. 9 and a
movement mechanism for the light shielding plate.
[0023] FIG. 14 illustrates an objective lens and a movement
mechanism for the objective lens, in which the objective lens has
lens portions having different aperture ratios and used as the
aperture ratio varying unit of FIG. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring to the drawings, preferred embodiments of an
optical pickup device according to the present invention will be
explained in detail.
[0025] FIG. 1 schematically shows an optical disc reproducing
apparatus according to the present invention.
[0026] An optical disc reproducing apparatus 10 is a so-called
compatible optical disc reproducing apparatus for reading out and
reproducing information signals from an optical disc having a track
pitch of 1.6 .mu.m and a substrate thickness of 1.2 .mu.m, such as
a compact disc, and a double-layer optical disc 11 having a track
pitch of approximately 0.8 .mu.m and having two information signal
layers oriented in the same read-out direction.
[0027] An optical pickup device 13 radiates a laser light beam to
these optical discs having different track pitches and different
substrate thicknesses for reproducing information signals from
tracks formed on the information signal layers.
[0028] Referring to FIG. 2, the optical pickup device 3 includes a
light source 21 radiating a laser beam, such as a laser diode, and
an objective lens 25 for focusing the laser beam on the information
signal layers of plural sorts of optical discs having different
track pitches, of which only the double-layer optical disc 11 is
shown in FIG. 2. The double-layer optical disc 1 shown in FIG. 2
has a first information signal layer 11a and a second information
signal layer 11b. The optical pickup device 13 also includes a
photodetector 24 for receiving the reflected light from the optical
disc for converting it into electrical signals, and a disc
discrimination unit 27 for discriminating the sort of the optical
disc. The optical pickup device 13 further includes a detection
signal processor 26 responsive to the optical disc sort
discriminated by the disc discrimination unit 27 to switch the
calculation operations for calculating tracking error signals from
the detected signals of the photodetector 24 for producing tracking
error signals, in addition to a focusing error signal and main
playback signals.
[0029] The tracking error signals and the focusing error signal,
obtained by the detection signal processor 26 of the optical pickup
device 13, are supplied to a servo circuit 16 of FIG. 1. The servo
circuit 16 manages tracking control and servo control responsive to
these signals. Specifically, a focusing driving signal is applied
in the optical pickup unit 13 to a bi-axial mechanism 20 holding
the objective lens 25 for driving the objective lens 25 in a
direction into and out of contact with, for example, the
double-layer optical disc 11 for managing focusing control. Also, a
tracking driving signal is applied to the bi-axial mechanism 20 for
driving the objective lens 25 radially of, for example, the
double-layer optical disc 11 for managing focusing control. A
thread driving signal is generated by extracting low-frequency
components of the tracking error signal for driving a thread
mechanism for moving the optical pickup device 13 in its entirety
radially of, for example, the double-layer optical disc 11. The
main playback signal, obtained by the detection signal processor
26, is processed with demodulation for EFM and CIRC decoding into
playback digital data which is then converted by a D/A converter 14
into an analog signal which is outputted at an output terminal 15.
The servo circuit 16 controls rotation of a spindle motor 18 based
on clocks obtained from the man playback signals.
[0030] The detailed structure and operation of the optical pickup
device 13 will now be explained. Referring to FIG. 2, a diffused
laser beam, radiated by the optical source 21, is diffracted by a
diffraction grating 22 and thereby separated into three beams,
namely a 0 order beam and .+-.1 order beams. The laser beams,
diffracted by the diffraction grating 22, are reflected by a beam
splitter 23 and collimated by a collimator lens 19 so as to enter
the objective lens 25. The objective lens 25 is tracking- and
focusing-controlled by the bi-axial mechanism 20 for converging the
laser beams on information signal layers of the optical disc, such
as the first information signal recording layer 11a and the second
information signal layers 11b of the double-layer optical disc 11
for forming three spots. The three reflected laser beams from the
first information signal recording layer 11a and the second
information signal layers 11b of the double-layer optical disc 11
reach a light receiving surface of the photodetector 24 via
objective lens 25 and beam splitter 23.
[0031] The bi-axial mechanism 20 is of an axial sliding type as
shown in FIGS. 3 to 5, wherein a movable part 30 is constituted by
a bobbin 30A formed of a non-magnetic material. At a mid position
of the bobbin 30A is formed an axially fitted tubular bearing 31.
On the outer peripheral surface of the bobbin 30A is wound a
focusing coil 32 for forming a ring about the bearing 31. The
focusing coil 32 is used for moving the movable part 30 in the
focusing direction, that is in a direction perpendicular to the
disc surface. On the surface of the focusing coil 32 are formed two
sets of tracking coils 33A, 33B in tight contact with the focusing
coil 32 for moving the movable part 30 in the tracking direction,
that is along the radius of the disc. These tracking coils 33A, 33B
are wound about an axis extending perpendicular to the winding axis
of the focusing coil 32 so that four rings are formed on the outer
peripheral surface of the bobbin 30A.
[0032] The mid portion of the bi-axial mechanism 20 is traversed by
a supporting shaft 39. A stepped hole 43 is formed parallel to the
center axis of the supporting shaft 39 in the bobbin 30A at an
offset position with respect to the supporting shaft 39. Within
this hole 43 is mounted a lens barrel 35 within which is secured
the objective lens 25.
[0033] The movable part 30, thus constructed, has the supporting
shaft 39 set upright at a mid portion of a stationary yoke 38 of a
magnetic material guided and introduced into a center hole of the
bearing 31, so that the movable part 30 is supported for sliding
along and for rotation about the supporting shaft 39. On the lower
surface of the stationary yoke 38 is secured an annular permanent
magnet 40 in intimate contact therewith about the supporting shaft
39 as center. On the lower end face of the permanent magnet 40 is
secured a first yoke 42 having a lug 41. On the stationary yoke 38
is protuberantly formed a second yoke 44 for facing the lug 41 of
the first yoke 42 on the inner side of the bobbin 30A. The
stationary yoke 38, permanent magnet 40, the first yoke 42 and the
second yoke 44 make up a magnetic circuit. The focusing coil 32 and
the tracking coils 33A, 33B are arranged in a magnetic gap defined
between the first yoke 42 and the second yoke 44. The stationary
yoke 38 has the hole 43 larger in diameter than the outer diameter
of the lens barrel 35 held by the bobbin 30A. Into this hole 43 is
guided and introduced an upper end of the lens barrel 35.
[0034] FIGS. 5 and 6 show a first embodiment of tracking servo for
reproducing plural discs having different track pitches.
[0035] With the present first embodiment of the optical disc
recording and/or reproducing apparatus, it is possible to reproduce
an optical disc having a substrate thickness of 1.2 mm and a track
pitch of 1.6 .mu.m, an optical disc having a substrate thickness of
1.2 mm and a track pitch of 0.8 .mu.m and an optical disc having a
substrate thickness of 0.6 mm and a track pitch of 0.8 .mu.m. It is
in addition possible to record a rewritable phase-change type
optical disc having a substrate thickness of 0.6 mm and a track
pitch of approximately 0.8 .mu.m.
[0036] Referring to FIG. 5, the optical disc recording and/or
reproducing apparatus of the first embodiment has a light receiver
24 for receiving the light beam radiated from the light source 21
and reflected from the signal surface of the optical disc 11 via
the objective lens 25, collimator lens 19 and the beam splitter 23
after irradiation of the optical disc 11 via the diffraction
grating 22, beam splitter 23, collimator lens 19 and the objective
lens 25, and a signal detector 26 for generating two sorts of
tracking error signals based on a light volume detection signal
from the receiver 24. The optical disc recording and/or reproducing
apparatus also includes a disc discrimination unit 27 for
discriminating the sorts of the optical discs recorded or
reproduced, a changeover switch 28 for selecting the tracking error
signals from the signal detector 26 and an objective lens driving
unit 29 for driving the objective lens 25 based on the selected
tracking error signal.
[0037] The light receiver 24 has first to third light receivers 51
to 53 for receiving the light beams split by the diffraction
grating 22 into three portions and reflected by the optical disc
11. The first light receivers 51 receives the main beam (0-order
light) of the three split light beams and is divided into at least
two areas of A1 and B1. The second and third light receivers 52, 53
receive two side beams (.+-.1 order light beams) of the three split
light beams and have two split portions each of which is divided
into areas E1, G1 and F1, H1.
[0038] Of the outputs of the receivers 51 to 53, the outputs E1, G1
of the receiver 52 and the outputs F1, H1 of the receiver 53 are
summed respectively at adders 54 and 58, outputs of which are
supplied to a comparator 61 for generating a first tracking error
signal. The outputs E1, G1 of the receiver 52 are supplied to a
comparator 55, while the outputs F1, H1 of the receiver 53 are
supplied to a comparator 57. A difference output of the comparator
55 and a difference output of the comparator 57 fed via a variable
gain amplifier 59 are summed together and a resulting sum output is
supplied further via a variable gain amplifier 60 to a comparator
62. A difference output of the comparator 62 gives a second
tracking error signal.
[0039] FIGS. 6A and 6B illustrate the state of irradiation of the
discs of different track pitches with the three split light
spots.
[0040] FIG. 6A illustrates an example of an optical disc having a
track pitch of approximately 0.84 .mu.m, in which side beams are
illuminated at the positions offset by one-half the track pitch
with respect to the main beam. FIG. 6B shows an example of an
optical disc having a track pitch of 1.6 .mu.m in which side beams
are illuminated at the positions offset by one-fourth the track
pitch with respect to the main beam. With the optical disc of FIG.
6A, since the track pitch is one-half that of the optical disc of
FIG. 6B, offset due to optical axis deviation of the objective lens
25 of offset due to tilt of the optical disc are produced
frequently.
[0041] If an optical disc is judged by the disc discrimination unit
27 to be an optical disc having a track pitch of 1.6 .mu.m, a first
tracking error signal, which is a differential output of the
comparator 61, is selected. On the other hand, if an optical disc
is judged by the disc discrimination unit 27 to be an optical disc
having a track pitch of 0.8 .mu.m, a second tracking error signal,
which is a differential output of the comparator 62, is
selected.
[0042] By detecting the tracking error signals as described above,
a three-beam method is applied to reproduction of a compact disc,
while a differential push-pull method, removing unneeded offset, is
applied to reproduction of a high recording density replay-only
disc or a recording/reproducing disc. Thus, a compatible optical
disc recording and/or reproducing apparatus may be realized by
employing a common optical system and by simply switching the
calculation operations.
[0043] FIG. 7 shows a second embodiment of tracking servo for
recording or reproducing plural discs having different track
pitches. The optical disc recording and/or reproducing apparatus in
the present second embodiment is similar to that of the first
embodiment except for the structures of the light receiver 24 and
the signal detector 26.
[0044] The light receiver 24 has first to third light receivers 71
to 73 for receiving the three light beams split by the diffraction
grating 22 and reflected by the optical disc 11. The first light
receiver 71 receives the main beam (0-order light) of the three
split light beams and is divided into four areas of A2, B2, C2 and
D2. The second and third light receivers 72, 73 receive two side
beams (.+-.1 order light beams) of the three split light beams and
have portions E2, F2 for receiving the .+-.1 order side beams of
the three split light beams.
[0045] Of the outputs of the light receivers 71 to 73, the outputs
of the light receivers 72 and 73, that is the outputs of the areas
E2 and F2, are supplied to a comparator 74 where a difference
output E-F, that is the first tracking error signal, is produced.
Of the outputs of the first light receiver 71, the outputs of the
areas A2 and C2 are summed by an adder 75, while the outputs of the
areas B2 and D2 are summed by an adder 76. The phase differences of
the outputs of the adders 75, 76 are compared by a phase comparator
77 to produce the second tracking error signal.
[0046] If a disc is judged by the disc discrimination portion 27 to
be an optical disc having a track pitch of 1.6 .mu.m, the first
tracking error signal, which is a difference output from the
comparator 74, is selected. If a disc is judged to be an optical
disc having a track pitch of 0.8 .mu.m, the second tracking error
signal, which is a difference output from the comparator 77, is
selected.
[0047] In the second embodiment, similarly to the first embodiment,
a three-beam method is applied for reproducing, for example, a
compact disc, while a phase difference based tracking error
detection system, advantageous for removing the offset, is applied
for recording or reproducing a high recording density optical
disc.
[0048] FIG. 8 shows a third embodiment for a tracking servo for
recording or reproducing plural discs with different track pitches.
The optical disc recording and/or reproducing apparatus in the
present third embodiment is similar to that of the first and second
embodiments except for the structures of the light receiver 24 and
the signal detector 26.
[0049] The light receiver 24 has first to third light receivers 81
to 83 for receiving the three light beams split by the diffraction
grating 22 and reflected by the optical disc 11. The first light
receiver 81 receives the main beam (0-order light) of the three
split light beams and is divided into four areas of A3, B3, C3 and
D3. The second and third light receivers 82, 83 receive two side
beams (.+-.1 order light beams) of the three split light beams and
have two portions each of which is divided into E3, G3 and F3,
H3.
[0050] Of the outputs of the light receivers 81 to 83, the outputs
E3 and G3 of the light receiver 82 and the outputs F3 and H3 of the
light receiver 83 are summed by adders 84, 92, respectively. The
outputs of the adders 84, 92 (E3+G3, F3+H3) are supplied to a
comparator 95 for producing a first tracking error signal.
[0051] Of the outputs of the first light receiver 81, the outputs
A3, C3 are summed by an adder 86, while outputs B3 and D3 are
summed by an adder 89. The phase differences of the outputs of the
adders 86 and 89 are compared by a comparator 96 for producing a
second tracking error signal.
[0052] The outputs E3, G3 of the light receiver 82 are sent to a
comparator 85, while the outputs F3, H3 of the light receiver 83
are supplied to a comparator 91. A difference output of the
comparator 85 is summed to a difference output of the comparator 91
passed through a variable gain amplifier 93 and the resulting sum
output is supplied via a variable gain amplifier 94 to a comparator
97. The outputs A3, D3 of the light receiver 81 are summed by an
adder 88, while the outputs B3, C3 of the light receiver 81 are
summed by an adder 87. Outputs of the adders 87 and 88 are supplied
to a comparator 90, an output of which is sent to a comparator 97.
With an output of the comparator 97, a third tracking error signal
is detected.
[0053] With the present third embodiment, the three-beam method is
applied for reproducing a compact disc, for example, while the
phase difference based tracking error signal detection method,
advantageous for removing the offset, is applied to recording or
reproduction of a high density optical disc. In addition, in the
present embodiment, tracking error signal detection of the
differential push-pull system is applied for recording or
reproduction of, for example, a phase change type rewritable
optical disc. Thus the same optical pickup may be used for
recording or reproducing plural sorts of optical discs.
[0054] Next, with the optical disc recording and/or reproducing
apparatus of the present invention, an optical disc with a
substrate thickness of 0.6 mm may be recorded or reproduced as a
first optical disc with, for example, a track pitch of
approximately 0.8 .mu.m. For such recording or reproduction, a
semiconductor laser radiating a laser beam with a wavelength of,
for example, 635 nm, is used as the light source 21. The objective
lens 25 has an aperture ratio of, for example, 0.52. Therefore, if
a second optical disc having a substrate thickness of 1.2 mm, such
as a compact disc, is used, spherical aberration is generated due
to errors in substrate thickness, so that correct reproduction of
the recorded data cannot be achieved. Consequently, with the
present embodiment of the optical disc recording and/or reproducing
apparatus, the disc discrimination unit 27 sends a detection output
of the optical disc both to the changeover switch 28 selecting the
desired tracking error signal and to an aperture ratio variable
control unit 100, as shown in FIG. 9.
[0055] If fed with a detection output specifying a first optical
disc with a substrate thickness of 1.2 mm, the aperture ratio
variable control unit 100 forms a corresponding motor driving pulse
and routes the pulse to a stepping motor 102 of a variable ratio
varying unit 101 shown in FIG. 10a. This rotates the stepping motor
102 in a direction of moving a light shielding ring 103 into a
light path of the laser beam, so that the rotary force of the
stepping motor 102 is transmitted via a gear portion 105a meshing
with a gear 104a of a rotary gear 104 to a ring slider 105. Thus
the light shielding ring 103 is controlled to be moved over the
objective lens 25 along with the ring slider 105 as shown in FIG.
10B. The light shielding ring 103 thus shields a portion of the
laser beam radiated from the objective lens 25 by its light
shielding portion 103b for varying the aperture ratio of the
objective lens 25 to 0.37 (corresponding to 70% of the aperture
ratio of 0.52) for the first optical disc. The shielded portion of
the laser beam is by the outer peripheral portion and corresponds
to 30% of the entire laser beam. Thus, during reproduction of the
second optical disc, the light shielding ring 103 is controlled to
be moved over the objective lens 25 so that a portion of the laser
beam from the objective lens 25 is shielded as the laser beam is
illuminated on the second optical disc, as shown in FIG. 11a. This
prohibits spherical aberration from being produced during
reproduction of the optical disc having a substrate of an increased
thickness due to errors in the substrate thicknesses.
[0056] Specifically, if the second optical disc having the
objective lens 25 with the aperture ratio kept at 0.52 is
reproduced, wavefront aberration of approximately 0.3 rms.lambda.
is produced due to substrate thickness error of 0.6 mm, thus
producing significant distortion in the spatial frequency
characteristics, as indicated by circle marks .oval-hollow. in the
graph of FIG. 12. Conversely, if the aperture ratio of the
objective lens 25 is controlled to 0.37 by the light shielding ring
103, the wavefront aberration is decreased to approximately 0.07
rms.lambda., thus eliminating the distortion in the spatial
frequency characteristics, as shown by .quadrature. marks in the
graph of FIG. 12. Meanwhile, marks .diamond. indicate spatial
frequency characteristics in case reproduction is performed using
the optical system dedicated to the second optical disc. Comparison
of the marks .diamond. and .quadrature. reveals that the two
characteristics are similar to each other at approximately 1100/mm.
If the aperture ratio of the objective lens 25 is controlled by the
light shielding ring 103 to 0.37, the spherical aberration may be
decreased to a fourth power of the aperture ratio, that is about
25%, as compared to the spherical aberration generated on
reproducing the second objective lens with the aperture ratio of
the objective lens 25 remaining unchanged at 0.52. Thus it becomes
possible to sufficiently reproduce the second optical disc having
the substrate thickness different from that of the first disc using
the optical system for the first optical disc.
[0057] If fed with a detection output specifying a first optical
disc with a substrate thickness of 0.6 mm, the aperture ratio
variable control unit 100 forms a corresponding motor driving pulse
and routes the pulse to the stepping motor 102 of a variable ratio
varying unit 101 shown in FIG. 10a. This rotates the stepping motor
102 in a direction of moving the light shielding ring 103 out of
the light path of the laser beam, so that the rotary force of the
stepping motor 102 is transmitted via the gear 105a meshing with
the gear portion 104a of the rotary gear 104 to the ring slider
105. Thus the light shielding ring 103 is moved away from the
objective lens 25 along with the ring slider 105. Thus the laser
beam from the objective lens 25 may be illuminated on the first
optical disc with the substrate thickness of 0.6 mm, without being
shielded, as shown in FIG. 11B. In this case, the wavelength of the
laser beam is 635 nm, and the aperture ration of the objective lens
25 is 0.52, so that the spatial frequency is equal to 1500/mm, as
shown by marks x in FIG. 12, and hence the first optical disc
having a small recording pit size can be reproduced
satisfactorily.
[0058] It is seen from above that, with the optical disc recording
and/or reproducing apparatus according to the present invention,
the light shielding ring 103, shielding a portion of a laser beam
from the objective lens 25, is provided in the optical system for
the first optical disc having the substrate thickness of 0.6 mm,
and is used only for reproduction of the second optical disc having
the substrate thickness of 1.2 mm for shielding a portion of the
laser beam radiated from the objective lens 25 for variably
controlling the aperture ratio of the objective lens 25 for
conformity to the second optical disc for enabling reproduction of
the two different sorts of the optical discs having different
substrate thicknesses. Since the two sorts of the optical discs
with different substrate thicknesses may be reproduced in this
manner, the optical disc reproducing apparatus may be improved in
universality in application.
[0059] A second embodiment of the present invention concerning the
varying of the aperture ratio in the optical disc recording and/or
reproduction according to the present invention is now explained.
In the previous first embodiment, the aperture ratio of the
objective lens 25 is variably controlled by the light shielding
ring 103 and the ring slider 105. In the present second embodiment
of the optical disc recording and/or reproducing apparatus, a pair
of light shielding plates 106, 109 as shown in FIG. 13 are used for
shielding a portion of the laser beam from the objective lens 25,
using a pair of light shielding plates 106, 109 as shown in FIG.
13, for variably controlling the aperture ratio of the objective
lens 25.
[0060] Except for the mechanism related with this construction, the
optical disc recording and/or reproducing apparatus of the preset
second embodiment is similar in structure to the optical disc
recording and/or reproducing apparatus of the previous first
embodiment, only the above mechanism is explained in connection
with the optical disc recording and/or reproducing apparatus of the
preset second embodiment, while detailed description of the
remaining portion is omitted for clarity.
[0061] The aperture ratio varying unit 101, provided in the present
second embodiment of the optical disc recording and/or reproducing
apparatus, is made up of stepping motors 107, 110 for controlling
the movement of the light shielding plates 106, 109, as shown in
FIG. 13.
[0062] The light shielding plates 106, 109 are arranged on a
straight line perpendicular to a laser beam radiated from the
objective lens 25 so that one ends of the plates face each other.
Part of the bottom surface portions consecutive to the facing ends
of the light shielding plates 106, 109 is designed as light
shielding portions 106b, 109b for shielding a portion of the laser
beam radiated from the objective lens 25. The bottom sides of the
light shielding plates 106, 109 are provided with rack gears 106a,
109a, respectively, so as not to contact with the light shielding
portions 106b, 109b, respectively. These rack gear portions 106a,
109a are designed to mesh with gear portions 108a, 111a of rotary
gears 108, 111 provided on rotary shafts 107a, 110a of the stepping
motors 107, 110, respectively.
[0063] The above-described structure of the aperture ratio varying
unit 101 is controlled to be driven by a motor driving pulse
supplied from the aperture ratio varying controlling unit 100
responsive to a detection output of the disc discrimination unit
27. That is, if fed from the disc discrimination circuit 27 with a
detection output specifying the reproduction of the first optical
disc having the substrate thickness of 1.2 mm, the aperture ratio
variable control unit 100 generates motor driving pulses for
rotating the stepping motors 107, 110 in a direction of reducing
the gap delimited between the facing ends of the light shielding
plates 106, 109. These motor driving pulses are supplied to the
stepping motors 107, 110. This drives the stepping motors 107, 110
into rotation. The rotational force of the stepping motors is
transmitted via the gear portions 108a, 111a of the gears 108, 111
to the rack gear portions 106a, 109a of the light shielding
portions 106, 109. The light shielding plates 106, 109 are
controlled to be moved for hiding part of the objective lens 25.
The range of hiding of the objective lens 25 by the light shielding
plates 106, 109 is set to a range which will give an aperture ratio
of the objective lens 25 of 0.37 equal to the aperture ratio for
the second objective lens 25. By controlling the movement of the
light shielding plates 106, 109 in this manner, part of the laser
beam radiated from the objective lens 25 is shielded by the light
shielding portions 106b, 109b of the light shielding plates 106,
109 for setting the aperture ratio of 0.37 of the objective lens
25. Thus, the second optical disc with the substrate thickness of
1.2 mm may be reproduced correctly, as in the first embodiment
described above.
[0064] Next, if fed from the disc discrimination circuit 27 with a
detection output specifying the reproduction of the first optical
disc having the substrate thickness of 0.6 mm, the aperture ratio
variable control unit 100 generates motor driving pulses for
rotating the stepping motors 107, 110 in a direction of enlarging
the gap delimited between the facing ends of the light shielding
plates 106, 109. The motor driving pulses are supplied to the
stepping motors 107, 110. This drives the stepping motors 107, 110
into rotation. The rotational force of the stepping motors is
transmitted via the gear portions 108a, 111a of the gears 108, 111
to the rack gear portions 106a, 109a of the light shielding
portions 106, 109. The light shielding plates 106, 109 are
controlled to be moved to positions not shielding the laser beam
radiated from the objective lens 25. This gives the aperture ratio
of the objective lens 25 of 0.52 which is equal to the aperture
ratio for the first optical disc. Thus the first optical disc may
be reproduced correctly.
[0065] A third embodiment of the present invention concerning the
varying of the aperture ratio in the optical disc recording and/or
reproduction according to the present invention is now explained.
With the optical disc recording and/or reproducing apparatus
according to the present third embodiment, an objective lenses
having two sorts of the aperture ratio as shown in FIG. 14 is
employed in place of the objective lens 25 and the aperture ratio
variable control unit 101 and the objective lens having the
aperture ratio corresponding to the substrate thickness of the
optical disc being reproduced is used by switching. Since the
optical disc recording and/or reproducing apparatus of the present
third embodiment is similar in structure to the optical disc
reproducing apparatus of the first and second embodiments except
the objective lens, the following description of the third
embodiment is centered about the objective lens, while explanation
of the remaining portions is not made for clarity.
[0066] That is, the above objective lens has a first light
condensing portion 112 having an aperture ratio (0.52) for the
first optical fisc having the substrate thickness of 0.6 mm and an
aperture ratio (0.37) for the second optical disc having the
substrate thickness of 1.2 mm. The objective lens has a slider 114
for moving the objective lens on an optical path of the laser beam.
The bottom portion of the slider 114 has a rack gear meshing with a
rotating gear formed on the stepping motor. The force of rotation
of the stepping motor is transmitted via the rotating gear and the
rack gear to the slider 114 for controlling movement of the
objective lens. If fed with a detection output from the disc
discrimination unit 27 with a detection output specifying the
reproduction of the first optical disc with the substrate thickness
of 0.6 mm, the aperture ratio variable control unit 100 supplies a
motor driving pulse to the stepping motor for controlling movement
of the first light condensing portion 112 onto the light path of
the laser beam. This drives the stepping motor into rotation so
that the first light condensing portion 112 of the objective lens
is moved by the slider 114 on the light path of the laser beam.
Since the first light condensing portion 112 has the aperture ratio
of 0.52 for the first optical disc, the first optical disc may be
correctly reproduced by controlling movement of the first light
condensing portion 112 on the light path of the laser beam.
[0067] If fed with a detection output from the disc discrimination
unit 27 with a detection output specifying the reproduction of the
second optical disc with the substrate thickness of 1.2 mm, the
aperture ratio variable control unit 100 supplies the motor driving
pulse to the stepping motor for controlling movement of the second
light condensing portion 113 onto the light path of the laser beam.
This drives the stepping motor into rotation so that the second
light condensing portion 113 of the objective lens is moved by the
slider 114 on the light path of the laser beam. Since the second
light condensing portion 113 has the aperture ratio of 0.37 for the
second optical disc, the second optical disc may be correctly
reproduced by controlling movement of the second light condensing
portion 113 on the light path of the laser beam.
[0068] In the above embodiments, the aperture ratio is variably
controlled for the first and second optical discs having different
substrate thicknesses. However, since it is sufficient in the case
of the light shielding plates 106, 109 shown in FIG. 13 to variably
control the laser beam light shielding range depending on the
substrate thickness of the optical disc, it is possible to variably
control three or more aperture ratios of the light shielding ranges
of the objective lens for enabling reproduction of three or more
optical discs having different substrate thicknesses. In the
embodiment of FIG. 14 of an objective lens having the light
condensing portions 112, 113 with two different aperture ratios,
light condensing portions with three or more different aperture
ratios may be provided for enabling reproduction of three or more
optical discs having different substrate thicknesses.
[0069] In the above description of the third embodiment, the light
condensing portions 112, 113 are controlled to be moved by the
slider 114. It is however possible to provide a rotary shaft
between the light condensing portions 112, 113, using the shaft
sliding type biaxial mechanism shown in FIG. 3, and to control the
objective lens to be rotated about the rotary shaft as the center
of rotation for controlling movement of the light condensing
portions 112, 113 on the light path of the laser beam.
* * * * *