U.S. patent application number 09/063946 was filed with the patent office on 2001-08-16 for robust and versatile focus/tracking method and system for optical pickup heads.
Invention is credited to FREEMAN, MARK O., SHIH, HSI-FU, WANG, JINN-KANG.
Application Number | 20010014060 09/063946 |
Document ID | / |
Family ID | 21627097 |
Filed Date | 2001-08-16 |
United States Patent
Application |
20010014060 |
Kind Code |
A1 |
FREEMAN, MARK O. ; et
al. |
August 16, 2001 |
ROBUST AND VERSATILE FOCUS/TRACKING METHOD AND SYSTEM FOR OPTICAL
PICKUP HEADS
Abstract
A focus/tracking method and system is provided for use in an
optical drive with a possibly multiple-wavelength laser source for
control of the focus/tracking of the pickup head of the optical
drive. The use of a multiple-wavelength laser source allows the
optical drive to read data from various types of optical discs. The
focus/tracking method and system combines the use of a
differential, dual optical channel method for focusing control of
the laser beam used to read data from the optical disc and the use
of the single-beam tracking method for tracking control of the
same. The focus error signal and the tracking error signal can be
obtained from the same set of multi-element photodetectors. The
structure of the system can therefore be simplified to include a
reduced number of constituent components, thus allowing a reduction
in manufacturing cost.
Inventors: |
FREEMAN, MARK O.; (SAN
MATEO, CA) ; WANG, JINN-KANG; (YONG-HO CITY, TW)
; SHIH, HSI-FU; (CHANG-HUA HSIEN, TW) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
7TH FLOOR
LOS ANGELES
CA
900251026
|
Family ID: |
21627097 |
Appl. No.: |
09/063946 |
Filed: |
April 21, 1998 |
Current U.S.
Class: |
369/44.23 ;
369/112.19; G9B/7.066; G9B/7.073; G9B/7.092; G9B/7.113; G9B/7.114;
G9B/7.134 |
Current CPC
Class: |
G11B 7/1365 20130101;
G11B 7/1384 20130101; G11B 2007/0006 20130101; G11B 7/127 20130101;
G11B 7/1356 20130101; G11B 7/0912 20130101; G11B 7/1353 20130101;
G11B 7/0943 20130101; G11B 7/0901 20130101; G11B 7/131
20130101 |
Class at
Publication: |
369/44.23 ;
369/112.19 |
International
Class: |
G11B 007/095; G11B
007/135 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 1997 |
TW |
86115108 |
Claims
What is claimed is:
1. A focus/tracking method for use on an optical drive to control
the focus/tracking of a pickup head, comprising the steps of (1)
generating a laser beam; (2) focusing the laser beam on the optical
disc; (3) splitting the returning light from the optical disc in
half along a line into a first half part and a second half part;
(4) guiding the first half part of the returning light to a first
optical axis while guiding the second half part of the returning
light to a second optical axis; (5) at a fixed position on said
first optical axis, detecting the first half part of the returning
light to thereby generating a first set of opto-electrical signals;
(6) at a fixed position on said second optical axis, detecting the
second half part of the returning light to thereby generating a
second set of opto-electrical signals; and (7) from said first and
second set of opto-electrical signals, obtaining a focus error
signal and a tracking error signal, said focus error signal being
used for feedback control of the focusing of the laser beam until
the laser beam is focused precisely on the optical disc, and said
tracking error signal being used for feedback control of the
tracking of the laser beam until the laser beam is spotted on the
target data track.
2. The method of claim 1, wherein in said step (1), a
multiple-wavelength laser source is used to generate the laser
beam.
3. The method of claim 1, wherein said laser means includes: a
plurality of laser sources, each being selected for use to
generating a laser beam of a specific wavelength; and a combinator
for guiding each one of the various laser beams generated by the
plurality of laser sources to the same optical axis.
4. The method of claim 1, wherein said laser means includes a
plurality of laser sources, each capable of generating a laser beam
of a unique wavelength, and which are connected via optical fibers
to a coaxial position, allowing each one of the laser beams
generated by the laser sources to be emitted along the same optical
axis in the same direction.
5. The method of claim 4, wherein said optical fibers are linked to
an optical coupler which allows each one of the laser beams from
the plurality of laser sources to be guided to a source fiber which
emits the selected laser beam along the same optical axis in the
same direction.
6. The method of claim 4, wherein said optical fibers are disposed
in parallel and close proximity with each other, allowing each one
of the laser beams to be emitted along the same optical axis in the
same direction.
7. The method of claim 1, wherein said laser means includes a laser
diode mounted on a substrate, said laser diode being capable of
generating various wavelengths of laser beams.
8. The method of claim 1, wherein said beam splitting means is a
holographic beamsplitter having a first holographic region and a
second holographic region which are disposed to receive and split
the returning light from the optical disc in half into said first
and second half parts.
9. The method of claim 1, wherein said beam splitting means is a
reflective-type beamsplitter having: a first reflective surface
arranged to receive and reflect a half part of the returning light
from the optical disc along the first optical axis toward a first
multi-element photodetector, and a second reflective surface
arranged to receive and reflect another half part of the returning
light from the optical disc along the second optical axis toward a
second multi-element photodetector.
10. The method of claim 1, wherein said beam splitting means is a
refractive-type beam splitting means including. a standard
beamsplitter coupled to receive the returning light from the
optical disc; and a refractive beamsplitter element coupled to said
standard beamsplitter, said refractive beamsplitter element having
a first refracting surface and a second refracting surface which
are disposed to receive and split the returning light from the
optical disc in half into said first and second half parts.
11. The method of claim 1, wherein said first multi-element
photodetector is disposed behind the converging point of the first
half part of the returning light, and said second multi-element
photodetector is disposed in front of the converging point of the
second half part of the returning light when the optical beam is
focused on the information surface of the disk.
12. The method of claim 11, wherein said first and second
multi-element photodetectors are disposed on two different
planes.
13. The method of claim 1, wherein said first multi-element
photodetector is formed with three parallel light-sensitive
elements oriented perpendicular to the line dividing the light
returning from the disk into a first half part and a second half
part, each of which is capable of generating an opto-electrical
signal whose magnitude is proportional to the intensity of the
returning light from the optical disc spotted thereon, the three
opto-electrical signals generated respectively by said three
light-sensitive elements of said first multi-element photodetector
being designated by A, B, and C; and said second multi-element
photodetector is formed with three parallel light-sensitive
elements oriented perpendicular to the line which divides the light
returning from the disk into first and second half parts, each of
which is capable of generating an opto-electrical signal whose
magnitude is proportional to the intensity of the returning light
from the optical disc spotted thereon, the three opto-electrical
signals generated respectively by said three light-sensitive
elements of said second multi-element photodetector being
designated by A*, B*, and C*.
14. The method of claim 13, wherein the focus error signal FES is
obtained from the following equation: FES=A+C-B-(A*+C*-B*)
15. The method of claim 13, wherein in the case of using DPD
tracking method, the tracking error signal is obtained from the
phase difference between (A+A*) and (C+C*).
16. The method of claim 13, wherein in the case of using the
heterodyne tracking method, the tracking error signal is obtained
by mixing (A+A*-C-C*) with (A+B+C+A*+B*+C*).
17. The method of claim 13, wherein in the case of using the
push-pull tracking method, the tracking error signal is obtained
from (A+B+C)-(A*+B*+C*).
18. The method of claim 1, wherein said first multi-element
photodetector is formed with four parallel light-sensitive elements
oriented perpendicular to the line which divides the light
returning from the disk into first and second half parts, each of
which is capable of generating an opto-electrical signal whose
magnitude is proportional to the intensity of the returning light
from the optical disc spotted thereon, the four opto-electrical
signals generated respectively by said four light-sensitive
elements of said first multi-element photodetector being designated
by A, B, C, and D; and said second multi-element photodetector is
formed with four parallel light-sensitive elements oriented
perpendicular to the line which divides the light returning from
the disk into first and second half parts, each of which is capable
of generating an opto-electrical signal whose magnitude is
proportional to the intensity of the returning light from the
optical disc spotted thereon, the four opto-electrical signals
generated respectively by said four light-sensitive elements of
said second multi-element photodetector being designated by A*, B*,
C*, and D*.
19. The method of claim 18, wherein the focus error signal is
obtained from (A+D-B-C)-(A*+D*-B*-C*).
20. The method of claim 18, wherein in the case of using the DPD
tracking method, the tracking error signal is obtained from the
phase difference between (A+B+A*+B*) and (C+D+C*+D*).
21. The method of claim 18, wherein in the case of using the
heterodyne tracking method, the tracking error signal is obtained
by mixing (A+B+A*+B*-C-D-C*-D*) with (A+B+C+D+A*+B*+C*+D*).
22. The method of claim 18, wherein in the case of using the
push-pull tracking method, the tracking error signal is obtained
from (A+B+C+D)-(A*+B*+C*+D*).
23. An apparatus for control of the focus/tracking of a pickup head
of an optical drive to read data from an optical disc, which
comprises: laser means for generating a laser beam of a specific
wavelength; an objective lens, optically coupled to said laser
means, for focusing the laser beam onto the optical disc; beam
splitting means, optically coupled to said objective lens, capable
of splitting the returning light from the optical disc in half into
a first half part and a second half part and directing the first
half part of the returning light along a first optical axis and the
second half part of the returning light along a second optical
axis; and a first multi-element photodetector disposed on the first
optical axis, said first multi-element photodetector being formed
with a plurality of light-sensitive elements capable of generating
a first set of opto-electrical signals in response to the first
half part of the returning light spotted thereon; and a second
multi-element photodetector disposed on the second optical axis,
said second multi-element photodetector being formed with a
plurality of light-sensitive elements capable of generating a
second set of opto-electrical signals in response to the second
half part of the returning light spotted thereon; and wherein a
focus error signal and a tracking error signal are obtained from
said first and second sets of opto-electrical signals from said
first and second multi-element photodetectors, said focus error
signal being used for feedback control the focusing of the laser
beam until the laser beam is focused precisely on the optical disc,
and said tracking error signal being used for feedback control of
the tracking of the laser beam until the laser beam is spotted on
the target data track.
24. The apparatus of claim 23, wherein said laser means is a
multiple-wavelength laser source.
25. The apparatus of claim 23, wherein said laser means includes: a
plurality of laser sources, each being selected for use to
generating a laser beam of a specific wavelength; and a combinator
for guiding each one of the various laser beams generated by the
plurality of laser sources to the same optical axis.
26. The apparatus of claim 24, wherein said laser means includes a
plurality of laser sources, each capable of generating a laser beam
of a unique wavelength, and which are connected via optical fibers
to a coaxial position, allowing each one of the laser beams
generated by the laser sources to be emitted along the same optical
axis in the same direction.
27. The apparatus of claim 26, wherein said optical fibers are
linked to an optical coupler which allows each one of the laser
beams from the plurality of laser sources to be guided to a source
fiber which emits the selected laser beam along the same optical
axis in the same direction.
28. The apparatus of claim 26, wherein said optical fibers are
disposed in parallel and close proximity with each other, allowing
each one of the laser beams to be emitted along the same optical
axis in the same direction.
29. The apparatus of claim 23, wherein said laser means includes a
laser diode mounted on a substrate, said laser diode being capable
of generating various wavelengths of laser beams.
30. The apparatus of claim 23, wherein said beam splitting means is
a holographic beamsplitter having a first holographic region and a
second holographic region which are disposed to receive and split
the returning light from the optical disc in half into said first
and second half parts
31. The apparatus of claim 23, wherein said beam splitting means is
a reflective-type beamsplitter having: a first reflective surface
arranged to receive and reflect a half part of the returning light
from the optical disc along the first optical axis toward a first
multi-element photodetector, and a second reflective surface
arranged to receive and reflect another half part of the returning
light from the optical disc along the second optical axis toward a
second multi-element photodetector.
32. The apparatus of claim 23, wherein said beam splitting means is
a refractive-type beam splitting means including: a standard
beamsplitter coupled to receive the returning light from the
optical disc; and a refractive beamsplitter element coupled to said
standard beamsplitter, said refractive beamsplitter element having
a first refracting surface and a second refracting surface which
are disposed to receive and split the returning light from the
optical disc in half into said first and second half parts.
33. The apparatus of claim 23, wherein said first multi-element
photodetector is disposed behind the converging point of the first
half part of the returning light, and said second multi-element
photodetector is disposed in front of the converging point of the
second half part of the returning light.
34. The apparatus of claim 33, wherein said first multi-element
photodetector is disposed in front of the converging point of the
first half part of the returning light, and said second
multi-element photodetector is disposed behind the converging point
of the second half part of the returning light.
35. The apparatus of claim 33, wherein said first and second
multi-element photodetectors are disposed on two different
planes
36. The apparatus of claim 23, wherein said first and second
multi-element photodetectors are disposed in parallel on the same
plane.
37. The apparatus of claim 23, wherein said first and second
multi-element photodetectors are disposed on a plane which is
parallel to the surface of the optical disc.
38. The apparatus of claim 23, wherein said first multi-element
photodetector is formed with three parallel light-sensitive
elements, each of which is capable of generating an opto-electrical
signal whose magnitude is proportional to the intensity of the
returning light from the optical disc spotted thereon, the three
opto-electrical signals generated respectively by said three
light-sensitive elements of said first multi-element photodetector
being designated by A, B, and C; and said second multi-element
photodetector is formed with three parallel light-sensitive
elements, each of which is capable of generating an opto-electrical
signal whose magnitude is proportional to the intensity of the
returning light from the optical disc spotted thereon, the three
opto-electrical signals generated respectively by said three
light-sensitive elements of said second multi-element photodetector
being designated by A*, B*, and C*
39. The apparatus of claim 38, wherein the focus error signal FES
is obtained from the following equation: FES=A+C-B-(A*+C*-B*)
40. The apparatus of claim 38, wherein in the case of using DPD
tracking method, the tracking error signal is obtained from the
phase difference between (A+A*) and (C+C*).
41. The apparatus of claim 38, wherein in the case of using the
heterodyne tracking method, the tracking error signal is obtained
by mixing (A+A*-C-C*) with (A+B+C+A*+B*+C*).
42. The apparatus of claim 38, wherein in the case of using the
push-pull tracking method, the tracking error signal is obtained
from (A+B+C)-(A*+B*+C*).
43. The apparatus of claim 23, wherein said first multi-element
photodetector is formed with four parallel light-sensitive
elements, each of which is capable of generating an opto-electrical
signal whose magnitude is proportional to the intensity of the
returning light from the optical disc spotted thereon, the four
opto-electrical signals generated respectively by said four
light-sensitive elements of said first multi-element photodetector
being designated by A, B, C, and D; and said second multi-element
photodetector is formed with four parallel light-sensitive
elements, each of which is capable of generating an opto-electrical
signal whose magnitude is proportional to the intensity of the
returning light from the optical disc spotted thereon, the four
opto-electrical signals generated respectively by said four
light-sensitive elements of said second multi-element photodetector
being designated by A*, B*, C*, and D*.
44. The apparatus of claim 43, wherein the focus error signal is
obtained from (A+D-B-C)-(A*+D*-B*-C*).
45. The apparatus of claim 43, wherein in the case of using the DPD
tracking method, the tracking error signal is obtained from the
phase difference between (A+B+A*+B*) and (C+D+C*+D*).
46. The apparatus of claim 43, wherein in the case of using the
heterodyne tracking method, the tracking error signal is obtained
by mixing (A+B+A*+B*-C-D-C*-D*) with (A+B+C+D+A*+B*+C*+D*).
47. The apparatus of claim 43, wherein in the case of using the
push-pull tracking method, the tracking error signal is obtained
from (A+B+C+D)-(A*+B*+C*+D*).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the priority benefit of Taiwan
application Ser. No. 86115108, filed Oct. 15, 1997, the full
disclosure of which is incorporated herein by reference
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to optical drives used to read
information from optical discs, and more particularly, to a
focus/tracking method and system for use on an optical drive, which
is capable of detecting both the focusing error and the tracking
error of the pickup head to thereby control the focus/tracking of
the same during read or write operation on an optical disc. This
invention allows the focus/tracking method and system for the
optical drive to be simplified in structural complexity, thereby
saving manufacturing cost.
[0004] 2. Description of Related Art
[0005] Pickup heads for optical discs must produce signals that
indicate whether the optical stylus is in focus on the disc surface
and the position of the optical stylus with respect to the
information track besides just reading the coded information from
the disc. As information is being recorded on the discs with
ever-increasing density and in multiple layers, and as the number
of styles of optical discs that a single pickup is expected to read
is also increasing, more robust and versatile methods for producing
these signals are called for.
[0006] It is well known that single-beam tracking performs better
than three-beam tracking (which is commonly used for CD drives)
when the tracks are spaced more closely together and there are
multiple layers of information on the discs as, for example, in DVD
discs. Single-beam tracking, which includes the methods of pushpull
tracking, heterodyne tracking, and differential phase detection
(DPD), also has the advantage over three-beam tracking of being
generated directly from the disc information track rather than
requiring critical alignment of tracking spots. Heterodyne and DPD
tracking further have an advantage over pushpull tracking in that
the pit depth which maximizes these tracking signals is the same as
the depth which maximizes the information signal; whereas for
pushpull tracking, its signal is at maximum at the pit depth which
minimizes the information signal.
[0007] Multiple wavelength sources are required in pickups which
are used to handle a wide variety of disc media. For example, red
lasers of around 650 nm wavelength are required for reading DVD
discs, while write-once CD-R media must be read using an infra-red
laser with a wavelength around 780 nm. In order to avoid
multiplying the number of components in the pickup, one set for
each wavelength, a new means for generating the focus/tracking
signals is needed which can be aligned properly for all of the
wavelengths simultaneously. The differential spot-size detection
method for focus-error signal generation is such a system that can
be aligned for multiple wavelengths simultaneously but it is
incompatible with heterodyne and DPD single-beam tracking
methods.
[0008] Prior art for this invention includes a description of
differential spot-size detection disclosed in Japanese Laid-Open
Patent Document Number 63-229640 dated Sep. 26, 1988. The essential
information processing scheme is reproduced in FIG. 1A, which
includes a laser source 10, a holographic beamsplitter element 11,
an objective lens 12, and a pair of 3-element photodetectors 16,
17. The holographic beamsplitter element 11 is used to divide the
beam returning from the disc 13 into two beams 14 and 15, which are
incident respectively on two 3-element photodetectors 16, 17. The
holographic beamsplitter element I 1 further has a focusing effect
which causes the first beam to focus in front of one of the
3-element photodetectors and causes the second beam to focus behind
the other 3-element photodetector. The spots on the photodetectors
are diagrammed in FIGS. 1B-1D for various cases of the focus of the
optical stylus on the information surface of the disc. The focus
error signal (FES) indicating the focus error of the optical stylus
with respect to the information surface in the disc is given by
combining the electrical signals generated by the photodetector
elements as follows:
FES=(S.sub.A"+S.sub.C"-S.sub.B")-(S.sub.A'+S.sub.C'-S.sub.B')
[0009] In the case shown in FIG. 1B, the stylus is focused behind
the information surface which causes the spots from the two beams
to have different sizes on their respective 3-element
photodetectors and FES to be positive. For the case shown in FIG.
1C, the stylus is focused properly on the information surface, the
spots from the two beams have the same size on their respective
3-element photodetectors, and FES=0. For the case shown in FIG. 1D,
the stylus is focused in front of the information surface causing
the spots from the two beams vary in a complementary manner to the
case shown at the top and FES to be negative. For different
wavelengths, the diffraction angles of the two beams from the
holographic element vary, causing the spots to move along the
photodetectors parallel to lines dividing the detector into three
elements. This does not affect the resulting FES. Other prior art
disclosing similar differential spot-size detection is found in
U.S. Pat. No. 5,111,448 (May 1992). The drawback of these methods
is that, since the complete beam area is incident on both of the
3-element photodetectors, there is no way to access the heterodyne
and DPD tracking information which is embedded in an interference
pattern in the beam.
[0010] An example of the interference pattern embedded in the beam
is given in FIG. 2. A beam after experiencing diffraction from the
information surface of the disc is shown centered on a coordinate
system with quadrants labeled I, II, III, and IV. The arcs drawn
within the main circular beam represent the overlap of the main
circular beam and diffraction orders created by diffraction from
the disc information surface. There is interference between these
diffracted orders and the main beam. As the optical stylus moves on
and off the information track, the intensity of these interference
regions changes. The shaded areas indicate the interference regions
that contribute to heterodyne and DPD tracking signals. Signals
from each of the four quadrants must be available separately in
order to generate these tracking signals. As stated above, the
prior art for differential spot-size focus detection does not
provide separate signals from these four quadrants and therefore
cannot be used to generate these tracking signals. The astigmatic
focus detection method is described in any introductory text to
optical disc technology (e.g. A. B Marchant, Optical Recording,
Addison Wesley Publishing, Reading, Mass., *990) and is a method
which does provide separate access to the signals in each of the
four quadrants. Moreover, for instance, U.S. Pat. No. 4,731,772
(March 1988) uses a quadrant detector to provide separate signals
from each of the four quadrants as shown in FIGS. 3A-3D. However,
since the spot must remain centered on the quadrant photodetector,
this approach is not tolerant of position shifts that will occur
with multiple wavelength sources.
SUMMARY OF THE INVENTION
[0011] This invention uses a new method to combine some the best
features of previously incompatible differential
spot-size-detection focus-error and signal-beam tracking-error
signal generation techniques to create a focus/tracking system that
is well suited to multiple layer, high density and multiple
wavelength optical disc systems while requiring a minimum number of
components to implement.
[0012] This invention is compatible with all of the above-mentioned
tracking methods, however the preferred embodiments utilize its
special ability to produce heterodyne and DPD tracking signals in a
multiple-wavelength system.
[0013] In accordance with the foregoing and other objectives of the
present invention, a focus/tracking method and system for the
pickup head of an optical drive is provided. The method of the
invention includes the following steps of: generating a laser beam;
focusing the laser beam on the optical disc; splitting the
reflected light from the optical disc in half into a first half
part and a second half part; guiding the first half part of the
reflected light to a first optical axis while guiding the second
half part of the reflected light to a second optical axis; at a
fixed position on the first optical axis, detecting the first half
part of the reflected light to thereby generating a first set of
opto-electrical signals; at a fixed position on the second optical
axis, detecting the second half part of the reflected light to
thereby generating a second set of opto-electrical signals; and
from the first and second set of opto-electrical signals, obtaining
a focus error signal and a tracking error signal, the focus error
signal being used for feedback control of the focusing of the laser
beam until the laser beam is focused precisely on the optical disc,
and the tracking error signal being used for feedback control of
the tracking of the laser beam until the laser beam is spotted on
the target data track
[0014] The system of the invention includes the following
constituent components: laser means for generating a laser beam of
a specific wavelength; an objective lens, optically coupled to the
laser means, for focusing the laser beam onto the optical disc;
beam splitting means, optically coupled to the objective lens,
capable of splitting the reflected light from the optical disc in
half into a first half part and a second half part and directing
the first half part of the reflected light to a first optical axis
and the second half part of the reflected light to a second optical
axis; a first photodetector disposed on the first optical axis, the
first photodetector being formed with a plurality of
light-sensitive elements capable of generating a first set of
opto-electrical signals in response to the first half part of the
reflected light spotted thereon; and a second photodetector
disposed on the second optical axis, the second photodetector being
formed with a plurality of light-sensitive elements capable of
generating a second set of opto-electrical signals in response to
the second half part of the reflected light spotted thereon. With
the foregoing focus/tracking system, a focus error signal and a
tracking error signal can be obtained from the first and second
sets of opto-electrical signals from the first and second
photodetectors. The focus error signal is used for feedback control
of the focusing of the laser beam until the laser beam is focused
precisely on the optical disc, while the tracking error signal is
used for feedback control of the tracking of the laser beam until
the laser beam is spotted on the target data track.
[0015] The foregoing focus/tracking method and system of the
invention allows both of the focus error signal and the tracking
error signal to be obtained from the same set of photodetectors,
while still providing the benefits of maintaining alignment over
multiple wavelengths and compatibility with single-beam tracking
methods. The photodetectors used in the invention are each formed
with a plurality of parallel light-sensitive elements. The
light-sensitive elements of one photodetector are also in parallel
with those on the other photodetector and perpendicular to the line
which splits the light spot into two halves. This design scheme
allows the photodetectors used in the invention to provide separate
access to the signals in the four quadrants of the light spot as
used in single-beam tracking methods. Furthermore, the elongated
dimension of the light-sensitive elements is parallel to the
direction light is deflected by the beamsplitter element allowing
proper alignment between the beam and the light sensitive elements
to be maintained even when the laser beam is changed in wavelength
that causes a shift in the spotted location on these
photodetectors. The invention is therefore suitable for use on an
optical drive with a multiple-wavelength laser source.
BRIEF DESCRIPTION OF DRAWINGS
[0016] The invention can be more fully understood by reading the
following detailed description of the preferred embodiments, with
reference made to the accompanying drawings, wherein:
[0017] FIGS. 1A-1D are prior art representing a typical
implementation of differential spot-size detection for generating a
focus error signal;
[0018] FIG. 2 is a schematic diagram showing the interference
pattern embedded in the optical beam by diffraction from the
information surface of the disc;
[0019] FIGS. 3A-3D is prior art representing a typical
implementation of the astigmatic method for generating a focus
error signal;
[0020] FIG. 4 is a schematic diagram depicting a first preferred
embodiment of the focus/tracking method according to the
invention;
[0021] FIG. 5 is a schematic diagram depicting a second preferred
embodiment of the focus/tracking method according to the
invention;
[0022] FIG. 6 is a schematic diagram depicting a third preferred
embodiment of the focus/tracking method according to the
invention,
[0023] FIGS. 7A through 7C are schematic diagrams used to depict
the structure and use of one example of the photodetectors used in
the focus/tracking method and system of the invention; and
[0024] FIGS. 8A through 8C are schematic diagrams used to depict
the structure and use of another example of the photodetectors used
in the focus/tracking method and system of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] The three preferred embodiments of this invention are
disclosed in the following with reference to FIGS. 4,5, and 6,
respectively. Each of them includes a laser light source with one
or more wavelengths. These light sources include one or more lasers
emitting at one or more wavelengths. Many possible methods might be
used to accomplish the multiple wavelength sources, including; (1)
a single laser which can be controlled to oscillate at different
wavelengths; (2) a number of different lasers may be combined so
that their light is coaxial using beam combiner elements such as
standard reflective-type beamsplitters; (3) using optical fibers to
carry light from separate laser sources and combining their light
into a single fiber source using an optical fiber coupler or simply
aligning the fibers side-by-side so as to create a nearly coaxial
grouping of separate laser sources; (4) mounting one or more laser
diode chips of various wavelengths onto a single substrate in such
a way that the light emitted from each of the lasers is nearly
coaxial with light emitted from the other lasers. Typically though
not necessarily, only one of the laser wavelengths is turned on at
any given instant
[0026] First Preferred Embodiment
[0027] FIG. 4 is a schematic diagram depicting a first preferred
embodiment of the focus/tracking method and system according to the
invention for focus/tracking control of the laser beam being used
to read data from an optical disc, as designated by the reference
numeral 43. As shown, the system includes a laser source 40, a
holographic beamsplitter 41 having a first holographic region 44
and a second holographic region 45, a first multi-element
photodetector 48, and a second multi-element photodetector 49.
[0028] The laser source 40 is used to generate a laser beam which
then propagates in the forward direction through the holographic
beamsplitter 41 toward the disc 43. At the holographic beamsplitter
41, the beam is diffracted, but only the 0-order (or undiffracted
part) is used (this action is considered separate from any
auxiliary grating device that may be combined with the holographic
beamsplitter 41 to create multiple beams for 3-beam tracking). The
0-order component of the diffracted light is then focused by the
objective lens 42 onto the information surface of the disc 43 where
the data to be read out are located.
[0029] The returning light from the disc 43 then passes through the
objective lens 42 back to the holographic beamsplitter 41. The
returning light is again diffracted by the holographic beamsplitter
41 and one or more diffraction orders other than the 0-order are
used to implement the beamsplitter action. The two holographic
regions 44, 45 are used to diffract the returning light into
different directions, which are designed in such a manner that they
share a common boundary which divides the returning light into two
essentially equal halves: a first half part and a second half
part.
[0030] The first half part of the returning light is received and
diffracted by the first holographic region 44 of the holographic
beamsplitter 41 to converge at a first focus point 46 in front of
the first multi-element photodetector 48; and then be incident on
the first multi-element photodetector 48, while a second half part
of the returning light is received and diffracted by the second
holographic region 45 which steers and focuses the light towards a
second focus point behind the second multi-element photodetector 49
and which is incident on said second multi-element photodetector
49.
[0031] The first and second multi-element photodetectors 48, 49 are
identical in structure and form and mounted in the same plane. Each
of the first and second multi-element photodetectors 48, 49 is
formed with a plurality of elongated parallel light-sensitive
elements. The parallel borderlines between the light-sensitive
elements on the multi-element photodetectors 48, 49 are oriented in
perpendicular to the boundary dividing the returning light into the
two half-beams The first and second multi-element photodetectors
48, 49 are disposed on the same plane.
[0032] FIGS. 7A-7C and FIGS. 8A-8C show two examples of the first
and second multi-element photodetectors used in the three preferred
embodiments (i.e., 48, 49 in the first preferred embodiment, 57, 58
in the second preferred embodiment, and 68, 69 in the third
preferred embodiment). The two multi-element photodetectors shown
in FIGS. 7A-7C are here designated instead by the reference
numerals 70 and 71, while the two multi-element photodetectors
shown in FIGS. 8A-8C are here designated instead by the reference
numerals 80 and 81.
[0033] Referring to FIGS. 7A-7C, in the first example, the first
multi-element photodetector 70 includes three parallel
light-sensitive elements A, B, C; and similarly, the second
multi-element photodetector 71 includes three parallel light
sensitive areas A*, B*, C*. The design of the holographic
beamsplitter 11 and the positioning of the detectors 70 and 71 are
arranged so that the following is true.
[0034] In the case of the laser beam from the laser source being
focused at a point in front of the information surface of the disc,
the spotted area of first half part of the returning light on the
first multi-element photodetector 70 will be larger than that of
the second half part of the returning light on the second
multi-element photodetector 71, as indicated by the half-circled
shaded areas in FIG. 7A.
[0035] In the case of the laser beam from the laser source being
focused precisely on optical disc, the spotted area of first half
part of the returning light on the first multi-element
photodetector 70 will be equal to that of the second half part of
the returning light on the second multi-element photodetector 71,
as indicated by the half-circled shaded areas in FIG. 7B.
[0036] In the case of the laser beam from the laser source being
focused behind the information surface of the disc, the spotted
area of first half part of the returning light on the first
multi-element photodetector 70 will be smaller than that of the
second half part of the returning light on the second multi-element
photodetector 71, as indicated by the half-circled shaded areas in
FIG. 7C.
[0037] It is desired that the first half part and the second half
part of the returning light respectively on the first and second
multi-element photodetectors 70, 71 have equal spotted areas as
illustrated in FIG. 7B. The focus error signal is therefore
obtained from the opto-electrical signals generated from the
light-sensitive elements of the first and second multi-element
photodetectors 70, 71 in accordance with the following
FES=A+C-B-(A*+C*-B*)
[0038] where FES is the focus error signal; A, B, C represent the
magnitudes of the opto-electrical signals generated respectively by
the three light-sensitive elements of the first multi-element
photodetector 70; and A*, B*, C* represent the magnitudes of the
opto-electrical signals generated respectively by the three
light-sensitive elements of the second multi-element photodetector
71.
[0039] Depending on the particular tracking method used by the
optical drive, the tracking error signal can be obtained in
different manners. For example, in the case of the DPD method, the
tracking error signal TES is as follows:
TES.sub.(DPD)=Phase(A+A*)-Phase(C+C*)
[0040] In the case of the hetero dyne tracking method,
TES.sub.(heterodyne)=Mixer Combination of (A+A*-C-C*) and
(A+B+C+A*+B*+C*)
[0041] In the case of the push-pull tracking method,
TES.sub.(push-pull)=(A+B+C)-(A*+B*+C*)
[0042] Referring to FIGS. 8A-8C, in the second example, the first
multi-element photodetector (here designated by the reference
numeral 80) includes four parallel light-sensitive elements A, B,
C, D; and similarly, the second multi-element photodetector (here
designated by the reference numeral 81) includes four parallel
light sensitive areas A*, B*, C*, D* This embodiment is devised in
particular to provide more precise separate access to the
interference regions used in tracking. The dividing line between
elements B and C in 80 and elements B* and C* in 81 is centered in
the detectors and divides the incident beams into left and right
quadrants. The design of the beamsplitter element 11 and the
positions of the detectors 80 and 81 is arranged so that the
following is true.
[0043] In the case of the laser beam from the laser source being
focused at a point in front of the information surface of the disc,
the spotted area of first half part of the returning light on the
first multi-element photodetector 80 will be larger than that of
the second half part of the returning light on the second
multi-element photodetector 81, as indicated by the half-circled
shaded areas in FIG. 8A.
[0044] In the case of the laser beam from the laser source being
focused precisely on the information surface of the disc, the
spotted area of first half part of the returning light on the first
multi-element photodetector 80 will be equal to that of the second
half part of the returning light on the second multi-element
photodetector 81, as indicated by the half-circled shaded areas in
FIG. 8B.
[0045] In the case of the laser beam from the laser source being
focused behind the information surface of the disc, the spotted
area of first half part of the returning light on the first
multi-element photodetector 80 will be smaller than that of the
second half part of the returning light on the second multi-element
photodetector 81, as indicated by the half-circled shaded areas in
FIG. 8C.
[0046] It is desired that the first half part and the second half
part of the returning light respectively on the first and second
multi-element photodetectors 80, 81 have equal spotted areas as
illustrated in FIG. 8B. The focus error signal is therefore
obtained from the opto-electrical signals generated from the
light-sensitive elements of the first and second multi-element
photodetectors 80, 81 in accordance with the following:
FES=(A+D-B-C)-(A*+D*-B*-C*)
[0047] where FES is the focus error signal; A, B, C, D represent
the magnitudes of the opto-electrical signals generated
respectively by the four light-sensitive elements of the first
multi-element photodetector 80; and A*, B*, C*, D* represent the
magnitudes of the opto-electrical signals generated respectively by
the four light-sensitive elements of the second multi-element
photodetector 81.
[0048] Depending on the particular tracking method used by the
optical drive, the tracking error signal can be obtained in
different manners. For example, in the case of the DPD method, the
tracking error signal TES is as follows:
TES.sub.(DPD)=Phase(A+B+A*+B*)-Phase(C+D+C*+D*)
[0049] In the case of the heterodyne tracking method,
TES.sub.(heterodyne)=Mixer Combination of (A+B+A*+B*-C-D-C*-D*) and
(A+B+C+D+A*+B*+C*+D*)
[0050] In the case of the push-pull tracking method,
TES.sub.(push-pull)=(A+B+C+D)-(A*+B*+C*+D*)
[0051] During the focusing operation, if the laser beam to be
focused on the disc 43 is defocused in such a manner that the focus
point is in front of the information surface of the disc 43, the
spotted area of the first half part of the returning light on the
first multi-element photodetector 48 will be larger than the
spotted area of the second half part of the returning light on the
second multi-element photodetector 49; and if the focus point is
back of the disc 43, the spotted area of the first half part of the
returning light on the first multi-element photodetector 48 will be
smaller than that of the second half part of the returning light on
the second multi-element photodetector 49; and if the focus point
is right on the disc 43, the spotted area of the first half part of
the returning light on the first multi-element photodetector 48
will be equal to that of the second half part of the returning
light on the second multi-element photodetector 49.
[0052] Accordingly, whether the laser beam is focused precisely on
the disc 43 can be determined by comparing the opto-electrical
signals generated from the light-sensitive elements of the first
and second multi-element photodetectors 48, 49 in response to the
returning light from the disc 43. The magnitude of the
opto-electrical signal generated from each light-sensitive element
is proportional to the intensity of the light spotted thereon. The
difference in the opto-electrical signals generated by the first
and second multi-element photodetectors 48, 49 is then taken as a
focus error signal, which is then used as a feedback signal to
control the objective lens 42 to be shifted to the right position
that allows the laser beam from the laser source 40 to be focused
precisely on the disc 43. Moreover, a tracking error signal can be
obtained from these opto-electrical signals generated from the
light-sensitive elements of the first and second multi-element
photodetectors 48, 49.
[0053] Second Preferred Embodiment
[0054] FIG. 5 is a schematic diagram depicting a second preferred
embodiment of the focus/tracking method according to the invention
for focus/tracking control of the laser beam being used to read
data from an optical disc, as designated by the reference numeral
54. In particular, this embodiment differs from the previous one in
that a reflective-type beamsplitter (as designated by the reference
numeral 51) is here used in place of the holographic beamsplitter
41 in the previous embodiment. Further, an optional quarter-wave
retarder 52 can be inserted in the optical path to improve light
utilization. This reflective-type beamsplitter 51 may be of the
polarizing beam splitter type where one polarization of light is
transmitted and the other reflected, or of the non-polarizing type
where portions of both light polarizations are reflected and
transmitted. By design in this preferred embodiment, the
reflective-type beamsplitter 51 includes a first reflective surface
55 and a second reflective surface 56. Similar elements in this
system (which are labeled here by different reference numerals)
include a laser source 50, an objective lens 53, a first
multi-element photodetector 57, and a second multi-element
photodetector 58.
[0055] The laser source 50 is used to generate a laser beam which
then propagates to the reflective-type beamsplitter 51. The
reflective-type beamsplitter 51 can be of the type that is known as
a polarizing beamsplitter which transmits one polarization
component of the incident beam while reflecting the orthogonal
polarization component from the slanted reflecting surfaces 55 and
56, or it can be of the type known as a non-polarizing beamsplitter
where the slanted surfaces 55 and 56 are partially reflecting,
transmitting part of the beam and reflecting the rest of the beam
with no dependence on the polarization of the beam. The light
passing through the reflective-type beamsplitter 51 then passes
through the optional quarter-wave retarder 52 and subsequently
focused by the objective lens 53 onto the information surface of
the disc 54 where the data to be read out are located.
[0056] The returning light from the disc 54 then passes through the
objective lens 53 and subsequently propagates through the optional
quarter-wave retarder 52 back to the reflective-type beamsplitter
51. The first reflective surface 55 and the second reflective
surface 56 are arranged in such a manner that the first reflective
surface 55 receives and reflects a first half part of the returning
light from the disc 54 toward the first multi-element photodetector
57 while the second reflective surface 56 receives and reflects a
second half part of the returning light toward the second
multi-element photodetector 58. The first half part of the
returning light then converges at a focus point in front of the
first multi-element photodetector 57, while the second half part of
the returning light converges at a focus point behind the second
multi-element photodetector 58.
[0057] The first and second multi-element photodetectors 57, 58 are
identical in structure and form as those depicted FIGS. 7A-7C and
FIGS. 8A-8C, so detailed description thereof will not be repeated.
The first and second multi-element photodetectors 57, 58 are
disposed on the same plane. Each of the first and second
multi-element photodetectors 57, 58 is formed with a plurality of
parallel light-sensitive elements which are oriented perpendicular
to the line along which the beam reflected from disk 54 is divided
into first and second half parts. In a similar manner as the
previous embodiment, a focus error signal and a tracking error
signal can be obtained from the opto-electrical signals generated
by the first and second multi-element photodetectors 57, 58.
[0058] Third Preferred Embodiment
[0059] FIG. 6 is a schematic diagram depicting a third preferred
embodiment of the focus/tracking method according to the invention
for focus/tracking control of the laser beam being used to read
data from an optical disc, as designated here by the reference
numeral 64 In particular, this embodiment differs from the previous
one in that a refractive-type beamsplitter which is an assembly of
a standard beamsplitter (as designated by the reference numeral 61)
and a specially-designed refractive beamsplitter element (as
designated by the reference numeral 65) is used here in place of
the reflective-type beamsplitter 51 in the previous embodiment of
FIG. 5. Further, an optional quarter-wave retarder 62 can be
inserted in the optical path to improve light utilization. The
refractive beamsplitter element 65 is formed with a first
refracting surface 66 and a second refracting surface 67. Similar
elements in this system (which are designated here by different
reference numerals) include a laser source 60, an objective lens
63, a first multi-element photodetector 68, and a second
multi-element photodetector 69.
[0060] The laser source 60 is used to generate a laser beam which
then passes through the beamsplitter 61 and the quarter-wave
retarder 62 and subsequently focused by the objective lens 63 onto
the disc 64 where the data to be read out are located. The
returning light from the disc 64 then passes through the objective
lens 63 and subsequently through the quarter-wave retarder 62 back
to the beamsplitter 61 where it is subsequently reflected towards
the refractive beamsplitter 65. The refractive beamsplitter element
65 has one side formed into a flat surface attached to the
beamsplitter 61 and the opposite side formed into two inclined
surfaces serving as a first refracting surface 66 and a second
refracting surface 67. The first and second refracting surfaces 66,
67 are designed and shaped in such a manner that the first
refracting surface 66 receives and diffracts a first half part of
the returning light from the beamsplitter 61 toward the first
multi-element photodetector 68, while the second refracting surface
67 receives and diffracts a second half part of the same returning
light toward the second multi-element photodetector 69. The first
half part of the returning light converges at a first focus point,
while the second half part converges at a second focus point, with
the first focus point and the second focus point being at two
equidistant points from the refractive beamsplitter element 65. The
first and second multi-element photodetectors 68, 69 are identical
in structure and form as those depicted FIGS. 7A-7C and FIGS.
8A-8C, so detailed description thereof will not be repeated. The
first multi-element photodetector 68 is positioned such that, when
the optical stylus is properly focused on the information surface
of the disk, the first focus point lies between the photodetector
68 and the beamsplitter surface 66. Under this same condition, the
second photodetector 69 is positioned such that it is located
between the second focus point and beamsplitter surface 67. The
first and second multi-element photodetectors 68, 69 are identical
in structure and form. The first and second multi-element
photodetectors 68, 69 are each formed with a plurality of parallel
light-sensitive elements which are oriented perpendicular to the
common boundary between refractive beamsplitter surfaces 66 and 67.
In a similar manner as the previous embodiment, a focus error
signal and a tracking error signal can be obtained from the
opto-electrical signals generated from the light-sensitive elements
of the first and second multi-element photodetectors 68, 69.
[0061] One feature of the invention is the combination of a method
for focusing control of the pickup head which accpts multiple
wavelengths without requiring realignment and the single-beam
tracking method for tracking control of the pickup head.
[0062] Another feature of the invention is the splitting of the
returning light from the disc into two half parts, each being then
directed to a specially designed multi-element photo-detector which
is formed with a plurality of parallel light-sensitive elements.
The light-sensitive elements of one multi-element photodetector are
also in parallel with those on the other multi-element
photodetector and perpendicular to the line which divides the
optical beam into two half beams. This design scheme allows the
multi-element photodetectors used in the invention to provide the
function of the conventional quadrant multi-element photodetector
used in single-beam tracking method. The elongated dimension of the
light-sensitive elements allows the optical beams to remain
properly aligned with the photodetector elements even when a change
in wavelength causes the light spot to shift along the
photodetectors. The invention is therefore suitable for use on an
optical drive with a multiple-wavelength laser source that allows
the optical drive to read high-density or multi-layer discs.
[0063] Still another feature of the invention is the provision of a
specially designed beamsplitter, which can be a holographic
beamsplitter, a reflective-type beamsplitter, or a refractive-type
beamsplitter, capable of splitting the returning light from the
disc into two half parts which can be detected to obtain the focus
error signal and the tracking error signal.
[0064] Still another feature of the invention is the capability of
obtaining the focus error signal and the tracking error signal from
the same set of multi-element photodetectors. The structure of the
pickup head thus can be simplified to include less number of
constituent components, allowing a reduction in manufacturing
cost.
[0065] The invention introduces the combination of a
special-function beamsplitter element and appropriately designed
multi-element photodetectors to provide a focus/tracking system
which can produce single-beam tracking error signals in a
multi-wavelength optical pickup head with minimal number of
components. It provides a way for a multi-wavelength-tolerant focus
error detection to produce signals for the DPD tracking method,
which is well-suited for use with high information density,
multi-layer optical disc storage systems. Prior art versions of
differential spot size focus detection could be used in a
multi-wavelength system, but are not compatible with the
single-beam tracking methods of heterodyne tracking and DPD. On the
other hand, the astigmatic focus error detection method given in
the prior art is compatible with these single-beam tracking
methods, but, since it requires strict alignment in two orthogonal
directions, it is not suitable for direct use in multi-wavelength
systems.
[0066] The invention has been described using exemplary preferred
embodiments. However, it is to be understood that the scope of the
invention is not limited to the disclosed embodiments. On the
contrary, it is intended to cover various modifications and similar
arrangements. The scope of the claims, therefore, should be
accorded the broadest interpretation so as to encompass all such
modifications and similar arrangements.
* * * * *