GB2307548A - Optical disk pickup using a six segment photodetector for tracking control - Google Patents
Optical disk pickup using a six segment photodetector for tracking control Download PDFInfo
- Publication number
- GB2307548A GB2307548A GB9523938A GB9523938A GB2307548A GB 2307548 A GB2307548 A GB 2307548A GB 9523938 A GB9523938 A GB 9523938A GB 9523938 A GB9523938 A GB 9523938A GB 2307548 A GB2307548 A GB 2307548A
- Authority
- GB
- United Kingdom
- Prior art keywords
- optical
- detector
- error signal
- track error
- photo
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/13—Optical detectors therefor
- G11B7/131—Arrangement of detectors in a multiple array
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0943—Methods and circuits for performing mathematical operations on individual detector segment outputs
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
- G11B7/13—Optical detectors therefor
- G11B7/133—Shape of individual detector elements
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
- G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B7/0901—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following only
Landscapes
- Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Mathematical Physics (AREA)
- Optical Recording Or Reproduction (AREA)
Abstract
An optical disc pick-up device has one laser beam reflected from the disc onto a six segment photodetector (9). Signals from the photodetector determine data, focussing error signals and track error signals. Division into 6 subfields is made by a horizontal (x), a vertical (y) and an oblique (45{) axis (z). With those six subfields a track error signal can be defined as: ```TE = (Xs - G1 . Ys - G2. Zs) where ```Xs = (A + D1 + D2) - (B1 + B2 + C) ```Ys = (D1 + D2 + C) - (A + B1 + B2) ```Zs = (B2 + D2) - (B1 + D1), and G2 = 2.828 Thus the influence of an offset of the light spot incident on the photo-detector can be minimized or virtually eliminated.
Description
Track Offset Compensation for an Optical Pick-up System
The invention relates to an optical pick-up device for optical disk systems using one laser beam, wherein a track error signal is generated by a photo-detector, and a method for compensating the offset of the track error signal.
More particular, this invention relates to the transversal push-pull compensation, which is for objective lens movement and to disc tilt compensation.
Such optical pick-up devices are known and used for example in CD players, video disk players or other magneto-optical recording and reading devices.
The US patent No. 4,661,944 shows an apparatus for recording and/or reading information to/from a track of a radiation reflecting record carrier by means of a light beam which is incident on the record carrier, wherein the apparatus comprises a light source for producing a light beam, an objective for focusing the light beam onto the surface of the record carrier and for directing the reflected light onto a focusing error detection system comprising an astigmatic element and a light sensitive detector which comprises four subdetectors forming each a quadrant of the light detector. With these four subdetectors a data signal and a focusing error signal are derived, wherein an index m (m being an integer from 1 to 4) is assigned to the four subdetectors SM in a clockwise sense.
The reflected light from the disk surface form in the focused state a circular light spot located symmetrically onto the four detectors. If the optical disk is for example tilted this spot can be located in a nonsymmetrical position which in turn influences the focusing error signal. As a result the light beam is not focused accurately on the record carrier by the objective. For this reason the control signal according to this document is given by formula:
SF = a [(S1 - S4)/(S1 + S4) + (S3 - S2)/(S3 + a being a constant.
With this standardization the focusing error signal is less sensitive to a non-symmetrical positioning of the light spot on the four detectors. A compensation of the track error signal is not described.
EP patent No. 0 454 740 B1 discloses an optical scanning device including an objective lens for a recording and/or reproducing device in which a light beam is focused on a record carrier in accordance with a single beam method and is reflected from the record carrier onto a fourth quad rant photo-detector comprising four photo diodes of which one axis runs parallel to the track direction, in which a tracking error signal TE is obtained from the output signals of the four quadrants by forming sums and differences.Because the objective lens is movable relative to the four quadrant photo-detector in a direction forming a predetermined angle alpha relative to the normal of the track direction, the tracking error signal
TE is calculated by the following formula: TE = k {(A + B) - (C + D) - (f kA+D)-(B+C)ll, where k and f are proportionality factors.
This method for compensation the tracking error signal has the disadvantage, that it delivers reasonable results in the known moving direction under a predetermined angle alpha. Therefore no compensation is performed for an offset of the light spot on the four quadrants, if the direction of the offset is different from the direction alpha, which can happen because of, for example, thermal expansion of the objective or eccentricity of the disk.
It is therefore an object of the present invention to reduce or avoid said offset, which is an undesired DC offset, within said track error signal.
This object is solved by the subject matter of claim 1.
According to the invention the photodetecting sensor is separated into six subfields, wherein the sensor is divided by a horizontal, a vertical and an oblique (45 ) axis.
The present invention comprises an optical pick-up system for an optical disk system using a single laser beam, which is focused onto the surface of the optical disk and is reflected from the disk onto an optical photo-detector, wherein the optical photo-detector is divided into six subfields (A, B1, B2,
C, D1, D2). The signals from the subfields are used to determine data, a focusing error signal and a track error signals.
As this invention works according to the one beam push-pull method, it is extremely suitable for any track-pitch fabricated on a disk as compared with the Differential push-pull method, which is known by
SONY devices.
Due to this invention a diffraction grating is not required.
Accordingly, a power loss can be decreased compared to known systems.
Preferably the optical photo-detector of the optical pick-up device has a square shape and is subdivided into the six subfields by the horizontal, vertical and diagonal axis.
Usually the horizontal axis of the optical photo-detector is arranged in the direction of the track movement of the beam on the disk. To achieve a suitable form of the spot on the optical photo-detector the optical pickup device comprises an astigmatical element.
Preferably the track error signal of the optical pick-up device is determined according to the formula
TE = Xs - G1 . Y5 - G2 ZS (1) wherein XS = (A + D1 + D2) - (B1 + B2 + C) Y5 = (D1 + D2 + C) - (A + B1 + B2) Z5 = (B2 + D2) - (B1 + D1), and
G1, G2 being predetermined gain factors.
Preferably said gain factor G2 is set to a value of approximately 2.88 for a tracking angle TE of 45 degrees. By the above given formula and the value of the gain factor G2 an undesired offset within the track error signal can be compensated.
It may be mentioned that the transversal push-pull method, which is realised by this invention, works best with a 45 degree tracking angle.
But this method is not restricted to this angle due to a variable gain factor
G1, which depends on the tracking angle.
Further the compensation of the track error signal is performed using four differential amplifiers, wherein the first differential amplifier computes Xs, the second computes Y5 and the third computes ZS, and wherein the output of the first three differential amplifiers are input to the fourth differential amplifier which computes the track error signal, wherein the output of the third differential amplifier is amplified by a value G2.
A preferred embodiment of the invention will now be described in more detail by way of example, with reference to the drawings, wherein:
Fig. 1 shows a schematic of an optical pick-up device according to the invention,
Fig. 2 shows the photo-detector comprising six fields according to the invention together with a schematic of the tracks of a disk and the tracking direction,
Fig. 3 shows the picture of a spot on the photo-detector with an offset in the X direction together with the diagramms of the three measurement variables,
Fig. 4 shows a case where the spot has an offset in the 45" direction together with the diagramms of the three variables,
Fig. 5 shows the geometrical situation of an offset spot, and
Fig. 6 shows a schemastic circuit for determining the track error signal.
Fig. 1 shows an optical pick-up device comprising a light source 1, which emanates light, a first convex lens 2, which generates a parallel light beam 3, which passes through a beam splitter 4, so that the following objective lens 5 focuses the light beam 3 onto an optical disk 6.
The reflected light passes again through the objective lens 5 falls onto the beam splitter 4, where it is deflected under right angles and directed to a second convex lens 7. The reflected light then passes through a cylindrical lens 8, which generates the astigmatism, and is incident onto a photodetector 9.
Fig. 2 shows in its upper part a plane view of the photo-detector used in the optical pick-up system according to Fig. 1. The photo-detector 9 has a square shape and consists of six independent areas or fields A,
B1, B2, C, D2, D1 (in clockwise direction), wherein these subdetectors or subfields are generated by the division of the photo-detector 9 with the "horizontal" axis X, the "vertical" axis Y and an "oblique" axis Z, which is arranged at a tracking angle TE of 45D to the vertical and the horizontal axis. The photodetector 9 is arranged in such a way, that the horizontal axis X coincides with the tracking direction 10 of the optical disk 6, consisting of neighboring tracks 11, 12 and 13. The subfields of the photodetector 9 are electrically independent of each other.If the light beam 3 is focused onto the disk 6, a circular spot is incident onto the photo-detector 9. This circular spot (not shown in the upper part of Fig. 2) corresponds to the focus spot 14, shown in the lower part of Fig. 2. The circular spot on the photo-detector 9 is under normal circumstances symmetrical to the axis system consisting of the X and Y axis. Due to thermal effects or eccentricity of the disk 6 the position of the spot on the photodetector 9 can be misaligned, in other words, an offset can exist.
Fig. 3 shows the situation of a non-symmetrical spot 15 on the detector 9 consisting of the six subfields A, B1, B2, C, D2 and D1. It has to be kept in mind that each element of the detector 9 is positioned in order that the focus spot 14 on the disk 6 moves in dependence on the laser spot 15 on the detector 9. Fig. 3 shows in its upper part the laser spot 15, which is displaced into the horizontal, positive X axis. The track error signal TE is composed of the signals Xs, Y5 and ZS, wherein as already mentioned XS = (A + D1 + D2) - (B1 + B2 + C), so-called push-pull signal, Y5 = (D1 + D2 + C) - (A + B1 + B2), so-called first track offset compensating signal, and ZS = (B2 + D2) - (B1 + D1), so-called second track offset compensating signal.
In this situations the diagramms of the lower part of Fig. 3 shows from left to right the variables Xs, Ys and Z5. It can be seen that Xs, which is the push-pull signal, oscillates and comprises a DC offset, that Y5 is the signal in the vertical direction and shows no offset and its value is 0, and that the oblique signal ZS oscillates with a lower amplitude and shows a second offset.In other words Xs, Y5 and Ze can be described as X5 = A sin(WT) + Vx, Ys = 0, and ZS = A' sin(WT) + Vz, where W is the angular velocity of the push pull signal frequency, which depends on the disk rotation.
Therefore, the relationship between A/Vx and A'/Vz is
A/Vx > A'/Vz.
The reason is that the push-pull signal in ZS is suppressed by the summation of (B2 - B1) and (D2 - D1). Therefore, by multiplying ZS by a certain gain factor, the track error signal TE, which is defined as
TE = XS - G1 . Y5 - G2 Z5, can be expressed as
TE = (A-A'. G2) sin(WT), wherein A = A' G2. Therefore TE with the gain factor G chosen as
G2 = Vx/Vz is approximately constant when the laser spot on the detector 9 moves around the center of the detector 9. Thereby the range of the movement has to be less than 15 % of the laser spot diameter on the detector.
Fig. 4 shows the situation, where the laser spot 15 on the photodetector 9 is displaced in the positive oblique direction Z. In this situation Xs shows a DC offset, Y5 shows also an DC offset and Z5 has no DC offset as can be seen from the right lower part of Fig. 4. Therefore Xs can be written as X5 = A sin(WT) + Vx, YS can be written as Y5 = Vx, and Ze can be written as us = A' sin(WT).
For this reason the track error signal TE can be written as
TE = (A - A' G2) sin(WT) with
A = A' G2.
Fig. 5 shows the geometrical evaluation of a practical value for the gain factor G2. As can be seen from the drawing, the center of the axis system ist the point 0, i.e. the center of the detector 9, C is the center of the laser spot, P the normal point on the X axis of C, R the normal point on the oblique axis Z of C, Q the cutting point of the oblique axis Z with a
parallel of the X axis through C, and S the cutting point of the parallel
through C of the Y axis. Because the oblique axis Z is at 45" relative to the
X or Y axis the following formulas can be derived:
CS = 1.414 CR OP = PS = CS + PC = 1.414 CR + PC.
Because PC is equivalent to Ys, CR is equivalent to 2 us, and OP is equivalent to Xs, the value of the track error signal TE is approximately 0.
For this reason the track offset of the spot incident onto the photodetector 9 has no influence on the evaluation on the track offset signal defined as above.
Fig. 6 finally shows a circuit for determining the track error signal.
The signals of the photodetector 9 with the six subfields A, B1, B2, C, D1,
D2 are input to three differential amplifiers 16, 17, 18 according to formula (1), so that signal Xs is output from amplifier 16, signal Y5 is output from amplifier 17 and signal ZS is output from amplifier 18. The signal Ze is multiplied with a predetermined gain Value G by an amplifier 19. The three values Xs, Y5 and G-ZS are input to a fourth differential amplifier 20 resulting in the track error signal TE.
Claims (10)
1. Optical pick-up system for an optical disk system using a single laser beam (3), which is focused onto the surface of the optical disk (6) and is reflected from the disk (6) onto an optical photo-detector (9) characterized in that the optical photo-detector (9) is divided into six subfields (A, B1, B2, C,
D1, D2).
2. Optical pick-up device according to claim 1, characterized in that the optical photo-detector (9) determines data, focusing error signals and track error signals.
3. Optical pick-up device according to claim 1 or 2, wherein the optical photo-detector (9) has a square shape and is subdivided into the six subsections (A, B1, B2, C, D1, D2) by the horizontal (X), vertical (Y) and diagonal axis (Z).
4. Optical pick-up device according to one of the preceding claims, characterized in that the optical photo-detector (9) is arranged in the direction of the track movement of the beam (3) on the disk (6).
5. Optical pick-up device according to one of the preceding claims, characterized in that the optical pick-up device comprises an astigmatical element (8).
6. Optical pick-up device according to one of the preceding claims, characterized in that the track error signal is determined according to the formula
TE = X5 - G1 Y5 - G2 Z5 wherein XS = (A + D1 + D2) - (B1 + B2 + C) (D1 + D2 + C) - (A + B1 + B2) Z5 = (B2 + D2) - (B1 + D1), and
G1, G2 are predetermined gain factors.
7. Optical pick-up device according to claim 6, wherein the gain factor (G2) is set to a value of approximately 2.88.
8. Optical pick-up device according to claim 6 or 7, wherein the compensation of the track error signal (TE) is done using four differential amplifiers (16, 17, 18, 20), wherein the first differential amplifier (16) computes X5, the second (17) computes Y5 and the third (18) computes
ZS, and wherein the output of the first three differential amplifiers (16, 17, 18) are input to the fourth differential amplifier (20) which computes the track error signal (TE), wherein the output of the third differential amplifier (18) is multiplied with a gain value G2 by an amplifier (19).
9. Method for determining a track error signal in an optical pick-up device for optical disks (6), wherein one laser beam (3) is focused onto said optical disk (6) and the reflected light from the disk (6) is directed onto an optical photo detecting means (9), characterized in that the optical detecting means (3) is devided into six subsections (A, B1, B2, C, D1, D2) measuring six different measurement values, so that the track error signal is determined by
TE = (Xs - G1 . Y5 - G2 Z5), wherein XS = (A + D1 + D2) - (B1 + B2 + C) Y5 = (D1 + D2 + C) - (A + B1 + B2) Z5 = (B2 + D2) - (B1 + D1), and
G1, G2 are predetermined gain factors.
10. Method for determining the track error signal according to claim 9, wherein the gain value (G2) is set to approximately 2.88.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9523938A GB2307548A (en) | 1995-11-23 | 1995-11-23 | Optical disk pickup using a six segment photodetector for tracking control |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9523938A GB2307548A (en) | 1995-11-23 | 1995-11-23 | Optical disk pickup using a six segment photodetector for tracking control |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9523938D0 GB9523938D0 (en) | 1996-01-24 |
| GB2307548A true GB2307548A (en) | 1997-05-28 |
Family
ID=10784317
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9523938A Withdrawn GB2307548A (en) | 1995-11-23 | 1995-11-23 | Optical disk pickup using a six segment photodetector for tracking control |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2307548A (en) |
Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4631395A (en) * | 1981-09-17 | 1986-12-23 | Tokyo Shibaura Denki Kabushiki Denki | Optical apparatus for detecting the focusing state and positional accuracy of a light beam directed onto an optical disk tracking guide in an optical read/write system |
| US4816665A (en) * | 1987-08-06 | 1989-03-28 | Maxtor Corporation | Sensor array for focus detection |
| EP0321039A1 (en) * | 1987-12-16 | 1989-06-21 | Koninklijke Philips Electronics N.V. | Optical scanning unit and an optical read and/or write apparatus comprising such a unit |
| EP0492515A2 (en) * | 1990-12-20 | 1992-07-01 | Omron Corporation | Optical pick-up apparatus |
| US5233587A (en) * | 1990-06-29 | 1993-08-03 | Kabushiki Kaisha Toshiba | Rotary-type optical head |
| US5396478A (en) * | 1991-10-29 | 1995-03-07 | International Business Machines Corporation | Optical data storage system with aberration compensation for reduced servo crosstalk |
-
1995
- 1995-11-23 GB GB9523938A patent/GB2307548A/en not_active Withdrawn
Patent Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4631395A (en) * | 1981-09-17 | 1986-12-23 | Tokyo Shibaura Denki Kabushiki Denki | Optical apparatus for detecting the focusing state and positional accuracy of a light beam directed onto an optical disk tracking guide in an optical read/write system |
| US4816665A (en) * | 1987-08-06 | 1989-03-28 | Maxtor Corporation | Sensor array for focus detection |
| EP0321039A1 (en) * | 1987-12-16 | 1989-06-21 | Koninklijke Philips Electronics N.V. | Optical scanning unit and an optical read and/or write apparatus comprising such a unit |
| US5233587A (en) * | 1990-06-29 | 1993-08-03 | Kabushiki Kaisha Toshiba | Rotary-type optical head |
| EP0492515A2 (en) * | 1990-12-20 | 1992-07-01 | Omron Corporation | Optical pick-up apparatus |
| US5396478A (en) * | 1991-10-29 | 1995-03-07 | International Business Machines Corporation | Optical data storage system with aberration compensation for reduced servo crosstalk |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9523938D0 (en) | 1996-01-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |