EP1958192A1

EP1958192A1 - System and method of reading/writing data on an optical disc by a plurality of spots

Info

Publication number: EP1958192A1
Application number: EP06831905A
Authority: EP
Inventors: Henk Goossens; Dayu Chen; Rong Liu; Alexander Marc Van Der Lee
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2005-11-29
Filing date: 2006-11-23
Publication date: 2008-08-20
Also published as: WO2007063458A1; KR20080078020A; CN1979647A; CN101317223A; JP2009517793A; BRPI0619048A2; TW200741689A

Abstract

The present invention relates to a method and system of reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of the optical disc. The method comprises the steps of detecting successively that each spot has reached a data area previously read/written; and moving the spot to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks.

Description

SYSTEM AND METHOD OF READING/WRITING DATA ON AN OPTICAL DISC

BY A PLURALITY OF SPOTS

FIELD OF THE INVENTION

The present invention relates to a method and system of reading/writing data on an optical disc, and more particularly, to a method and system of reading/writing data on an optical disc by a plurality of spots.

BACKGROUND OF THE INVENTION

Nowadays, optical discs have become a data carrier widely used for storage of multi-media audio/video information considering its relatively low cost and large data storage capacity. As a result, increasing the data rate has become an important factor of improvement, since data stored on the optical disc can be read faster, or data be stored on the optical disc can be written faster.

A way to increase the data rate of an optical drive is using multiple spots that read or write multiple tracks of an optical disc in parallel.

Due to the spiral structure of most optical discs, spots will encounter a part of the track that already has been read or written by another spot, as illustrated by Fig. IA and Fig. IB.

Fig. IA is a schematic diagram showing the situation where data are recorded/read on the optical disc by spot A from start of block 1, and by spot B from start of block 5. This parallel process of recording/reading by spot A and B goes on until spot A arrives at start of block 5, as illustrated by Fig. IB. In other words, spot A arrives at a block previously recorded/read by spot B. To avoid spot A record new data on the previously recorded/read area by spot B, a displacement of the spots is needed at this point.

Typically, the displacement comprises moving all spots over an equal distance. The advantage of this spot displacement strategy is that it is very easy to implement because only a radial displacement of the objective lens is required. European Patent Application No. EP 0 840 294 A2 (published on 6 May 1998 and submitted by Samsung Electronics Co., Ltd.) discloses a multiple track scanning method for optical pickup using multiple spots, in which this spot displacement strategy is employed.

The disadvantage, however, is that the displacement must be (Nb-I)Ng-I tracks (for a system with Nb spots, with a distance of Ng tracks between the spots) in order to read or write all data. As a result, the data rate of this method is limited to a factor Nb-l/Ng times the data rate for one spot. In practice, Ng is often 1, so the data rate gain will be limited to Nb-I.

Thus, a new displacement strategy for reading and writing an optical disc exceeding the above data rate limit is expected to be implemented in the existing optical drives to meet the great requirement of improving data rate of reading and writing.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and system of reading/writing data on an optical disc by a plurality of spots, for improving the reading and writing data rate.

According to one embodiment of the present invention, a method of reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of the optical disc is disclosed. The method comprises the steps of detecting successively that each spot has reached a data area previously read/written; and moving the spot to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks.

According to one embodiment of the present invention, a system of reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of the optical disc is disclosed. The system comprises a detection device for detecting successively that each spot has reached a data area previously read/written, and generating a detection signal; and an actuation device, triggered by the detection signal, for moving the spot to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks.

Contrary to the prior art where the data rate increase is limited to a factor of Nb-l/Ng, the displacement strategy according to the present invention allows a data rate increase of a factor Nb. In other words, the displacement strategy of moving the spots individually allows to increase the data rate of reading and writing on an optical disc.

Moreover, the strategy of moving the spots can be implemented in an easy way since not all spots have to be moved over an equal distance simultaneously, resulting in a cost- effective solution.

The other objects and effectiveness of the present invention will be apparent from the description of the present invention with reference to the following drawings and claims, and will allow one to have a thorough understanding of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. IA and FIG. IB illustrate schematic diagrams showing the situation of a spot encountering a region that has been read or written by another spot.

FIGs. 2A-2D illustrate different stages of multi-spot readout of an optical disc using the spot displacement strategy according to one embodiment of the present invention.

FIG.3 illustrates a schematic diagram showing that a mirror array is used to displace individual spots from a laser array.

FIG.4 illustrates a schematic diagram showing the spot arrangement on an optical disc.

FIG.5 shows angular displacements of the mirrors which follow a saw tooth pattern over time. FIG.6 illustrates a schematic block diagram showing the structure of a closed loop control system for controlling the spots on an optical disc.

FIG.7 illustrates a schematic flowchart showing a method of reading data on an optical disc in an open-loop control according to one embodiment of the present invention.

In the above drawings, the same reference numeral indicates the same, similar or corresponding element or function.

DETAILED DESCRIPTION OF THE INVENTION

The technical measures of the present invention will be described in detail hereinafter by way of embodiments with reference to the drawings.

It should be noted that the present invention can be used for both reading and writing an optical disc. In this embodiment, "reading" is used for reasons of simplicity. Persons skilled in the art should understand that at any points of the specification "reading" could be exchanged with "writing".

Fig.2A to Fig.2D illustrates different stages of multi-spot readout of an optical disc using the spot displacement strategy according to one embodiment of the present invention. This strategy is illustrated by the use of three spots A, B and C being distant by one track from each other. Three spots, Spot A, Spot B and Spot C, are positioned along inner tracks to outer tracks of the optical disc, such as at Tracks Tl, T2 and T3 respectively, and read tracks in parallel with the orientation of the arrow in Fig.2A (the regions which have been read are marked with shadow in Figs. 2B-2D), wherein the distance between all spots are kept constant by at least one track and along the radial direction. A detection device detects successively the position of each spot and generates a detection signal. As soon as Spot A reaches a part of the track that has been read before by another spot, as illustrated by Fig.2A, Spot A becomes useless and a detection signal is generated indicating that Spot A has to be moved to continue reading data. In this embodiment, Spot A is moved by an actuation device in response to the detection signal to a position adjacent to (i.e. next to) the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to (i.e. next to) the current innermost spot if the reading/writing is done from outer to inner tracks. In this example, as shown in Fig.2B, Spot A is moved to Track T4, next to Spot C.

It is noted that the term "adjacent" means that the spot which is moved will be positioned next to the current outermost/innermost spot, and will be distant by one track (as in the example given by Fig.2) or by a plurality of tracks from said current outermost/innermost spot (for example in the case where the spots are distant by more than one track from each other).

The same strategy as that applied to Spot A is applied to other Spots. For example, as shown in Fig.2C, Spot B also encounters a part of the track that has been read before. Similarly as Spot A, Spot B is also moved to Track T5, as shown in Fig.2D.

With respect to the implementation of the above strategy, there are different ways that can be used to implement this jump strategy. However, it is important that each of the spots should be controlled individually.

Fig.3 is a schematic diagram illustrating realization of the jump strategy by means of a mirror array to displace individual spots in a dual laser system for improving the data rate of the drive. The system comprises a laser array with two laser sources 4 and 5. For laser beams (see dashed lines in Fig.3) emitted from the laser array, there is a mirror array used to deflect the laser beams. The mirror array comprises two mirrors 1 and 2. Mirror 1 deflects laser beam A from the laser source 5 and mirror 2 deflects laser beam B from the laser source 4. Each of mirrors 1 and 2 can be rotated over a small angle αl, α2 and the rotation axis 3 approximately lies in the plane going through the optical axis of the laser beams. By rotating one of mirrors 1 and 2, the laser beam after deflection by the mirror is changed so that the corresponding spot on optical disc is controlled. For example, if the mirror rotates counter clockwise around the axis 3, the spot will be moved toward the edge of the optical disc. On the contrary, the spot will be moved toward the centre. By this way, a mirror array is employed to realize the displacement of individual spots from a laser array.

A preferred embodiment of the present invention is to position the spots in an almost tangential orientation. This is advantageous because it minimizes the mechanical movement of components during displacement of the spots and therefore simplifies the actuation.

Although this displacement is not the shortest possible to jump from the previous track to the next, the difference is very small and causes no problems in practice. This means, the rotation axes do not have to be very precisely aligned, thereby simplifying the construction of the mirror array.

In order to deflect only the laser beam from one laser source per mirror, the mirrors have to be placed closely to the laser array at a distance where the beams do not overlap yet. As a result, small mirrors, for example with width and height around 50-100 micron, are preferred due to the advantage that the mirrors can be displaced very quickly and can be integrated with the laser array in the same housing to form one component. Thus, a simpler application in an optical light path is provided to carry out the present invention.

The mirror array may also be placed elsewhere in the light path. For example, in order to create beams which do not overlap, a telescope that creates focal points is needed. The mirror array is then placed near these focal points.

In the above embodiment, a mirror array is used to displace each spot. Alternatively, the displacement of each spot may be implemented by other means, such as a group of lens which is used to change the orientations of the laser beams. Another possible solution is that displacement of individual spot is managed by controlling the deflection angle of each of different parallel non-overlapping beams with a liquid crystal module in a plane perpendicular to the optical axis. Moreover, the liquid crystal module has a constant index of refraction gradient. Thus, varying the index of refraction gradient by variation of the voltages over the module will also vary the beam angle.

The light source corresponds to a laser array. Other light sources, for example a single- beam laser source split by a grating or collimated by a lens, can also be used in the embodiments of the present invention. In particular, a mirror array used to deflect the collimated laser beams is suitable for a read only system. Alternatively, the light source can be formed by using an acousto-optic modulator that can both split an incoming beam into modulated beam at different (and variable) angles, or discrete lasers of which the beams are combined into the same light path.

According to an embodiment of the present invention, it is preferable to let each mirror slowly go back to its starting position after each jump because if the spot encounters another region which has been read, another jump may be needed. Repetitive displacement of a spot would otherwise lead to an over-increasing displacement of the mirror. The normal radial tracking system including an objective lens actuator will make that all spots stay on their respective tracks while the mirrors return to their initial position.

Fig.4 shows the typical arrangement of the spots on the disc, in view of explaining the result of choosing the rotation axis approximately in the plane of laser beams. For example, the spots are distant by lOμm, and the track pitch is 320 nm (e.g. according to Blu-ray Discs standard). The spots are therefore on a line with only a small angle with respect to the tracks. The displacement of a spot after rotation of one mirror with its rotation axis in the plane of the laser array's optical axes is perpendicular to this line (as indicated with the arrow).

Fig.5 shows angular displacements of the mirrors which follow a saw tooth pattern over time, wherein, αl, α2 represent angles of rotation of the mirrors, as indicated by the solid line and the dash line. The mirrors are controlled individually as shown in Fig.5. Moreover, the decreasing lines represent the duration of the mirror going back to its starting point and the steep increasing lines represent the duration of displacement. In an alternative implementation, the mirrors return to their initial position more quickly, and then stay constant for a short time. Depending on the variations in the disc revolution time (e.g. depending on the mode of operation of the drive being constant angular or constant linear velocity), either of the two methods might be easier to implement.

The objective lens actuator which is generally one part of a typical radial feedback loop control system is used to focus the spot on its track.

Fig.6 is a schematic block diagram showing the structure of a closed loop control system for controlling the spots on an optical disc. The system comprises an optics 601 which includes an light path, a disc, and a returning light path with a photo-detector and some other electronics. The optics 601 transmits the measured results to a radial tracking error generator 602 for generating an electrical signal, called radial error signal, which is more or less proportional to the distance of the spot on the disc to the center of the data track. This distance can be adjusted by the radial actuator 603, which moves the objective lens in radial direction (i.e. the objective lens actuator). The force (amplitude and sign) with which the radial actuator 603 has to be moved to keep the spot on the center of the track and thus reduce the radial error to zero defined by a controller module, radial controllers 604. The radial controller 604 takes the radial error signal as input and calculates an output signal such that it will reduce its input to zero.

The radial controller 604 is an electronic filter that has different characteristics in different frequency regions. A typical, and often applied example of such a controller is a

Proportional-Integrator-Differentiator (PID) controller, which has an integrator function dominating the controller output at low frequencies, a proportional gain dominating the output at middle frequencies and a differentiator dominating the output at high frequencies.

Alternatively, the average tracking error of all spots can be fed back. The feedback loop contains a Low Pass Filter (LPF) 605 of the top part, such that the movement of the spots is only controlled up to a certain bandwidth. The portions of the tracking errors above that bandwidth are removed by the other feedback loops (bottom part) controlling the rotatable mirrors. Because the mirrors are small and light, a very high bandwidth can be achieved.

Note that the wide arrows indicate a collection of signals, one for each spot. The signal starts from the optics and then the radial tracking errors are generated for all spots. These radial tracking errors are filtered by High Pass Filters (HPF) 606, such that only higher frequency components are addressed. Obviously, the cut off frequencies of the LPF 605 and HPF 606 have similar values. To this high pass filtered error signals, the jump signals from jump signal generator 607 as shown in Fig.6 are added to signal mixer 608. Via a set of N controllers 609, for example of the PID-type, each controller being in charge of controlling a spot or mirror, the signals are fed to the actuated mirrors 610, which in turn deflect the individual beams to the correct positions on the tracks of disc.

In the above embodiment, not only the present invention can be implemented, but also the additional function of compensating high frequency tracking errors by displacing the spot on the disc is achieved. By adding LPF and HPF, high frequency signals are separated and used to modulate actuated mirrors. This is beneficial for recording and read out of the optical disc at high spinning speeds.

The displacement of the spots on the disc also causes a displacement of the spots in the photo-detector plane in charge of generating electrical signals from which the data signal (High Frequency - HF - signal), the focus error and the radial tracking error signals can be derived from. This leads to disturbances in the optical signals. For example, it can lead to offsets in the push-pull radial signal, in the same way classical beam landing does. This can be avoided by moving the photo-detectors in accordance with the spot displacement.

In any of the implementations above, an open-loop or closed-loop control can be chosen to implement the displacement of the spots. One difference between open-loop and closed- loop control is that in an open loop scheme the rotation is done by feeding a predefined electric signal into an actuator, while for a closed loop reversing jump, the electric signal going into the actuator depends on the measured radial error signal from one of the non- static spots. Fig.7 is a schematic flowchart showing a method of reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of the optical disc in an open-loop control according to one embodiment of the present invention. At step S710, whether each spot has reached a data area previously read/written is successively detected.

Then, at step S720, the spot which has been detected is moved to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks. Therefore, the spot displacement is finished. A method in open-loop control is in general simpler to be carried out. In a closed-loop control, the position of the moving spots is measured during displacement and the movement of each spot is controlled on the basis of the measured signal.

Results have shown that for a dual beam Blu-Ray disc system using the present invention

(Ng=I, assuming infinitely short jump times), the data rate improvement is a factor 1.9, whereas with the typical method the data rate factor would drop below 1. Moreover, the present invention can be applied in almost all optical storage systems with multiple spots, such as CD, DVD and Blu-Ray drives.

The above embodiments described are only illustrative, and not intended to limit the technique approaches of the present invention. Although the present invention is described in details referring to the preferable embodiments, those skilled in the art will understand that the technique approaches of the present invention can be modified or equally displaced without departing from the spirit and scope of the technique approaches of the present invention, which will also fall into the protective scope of the claims of the present invention.

Claims

1. A method of reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of said optical disc, said method comprising the steps of: detecting successively that each spot has reached a data area previously read/written, moving said spot to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks.

2. A method as claimed in Claim 1, further comprising the steps of: moving photo-detectors to follow the movements of said spots, measuring the positions of said spots.

3. A method as claimed in Claim 1, wherein the distance between all said spots are kept constant by at least one track and along the radial direction.

4. A method as claimed in Claim 1, wherein said step of moving is rotating a mirror for deflecting a single laser beam.

5. A system for reading/writing data on an optical disc by a plurality of spots positioned along inner tracks to outer tracks of said optical disc, said system comprising: a detection device for detecting successively that each spot has reached a data area previously read/written, and generating a detection signal, - an actuation device, triggered by said detection signal, for moving said spot to a position adjacent to the current outermost spot if the reading/writing is done from inner to outer tracks, or to a position adjacent to the current innermost spot if the reading/writing is done from outer to inner tracks.

6. A system as claimed in Claim 5, further comprising a rotatable mirror array, connected to said actuation device, for deflecting laser beams used to generate said plurality of spots.

7. A system as claimed in Claim 5, further comprising a liquid crystal module, connected to said actuation device, for deflecting laser beams.

8. A system as claimed in Claim 5, wherein the distance between all said spots are kept constant by at least one track and along the radial direction.

9. A system as claimed in Claim 6, wherein the rotation axis of said mirror array substantially lies in the plane going through an optical axis of said laser beams.

10. A system as claimed in Claim 6, wherein said mirror array is integrated with laser sources emitting said laser beams.

11. A system as claimed in Claim 6, wherein said laser beams are formed by splitting a single laser source through a grating or an acousto-optic modulator.

12. A system as claimed in Claim 6, wherein the time for returning each mirror of said rotatable mirror array, after rotation, to its initial position depends on the disc revolution time.