EP2005428A2

EP2005428A2 - A method and apparatus for controlling laser power for a plurality of laser beams

Info

Publication number: EP2005428A2
Application number: EP07735201A
Authority: EP
Inventors: Dayu Chen; Henk Goossens; Qiang Zhan
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2006-03-30
Filing date: 2007-03-21
Publication date: 2008-12-24
Also published as: TW200739566A; JP2009531800A; WO2007113715A2; WO2007113715A3; KR20090005086A; RU2008142953A

Abstract

A method of controlling laser power for a plurality of laser beams, the method comprising the steps of: - successively switching on (710) the writing of each laser beam at different starts of segment for writing data, - successively switching off (720) the writing of each laser beam that arrives at the start of a segment previously written by another laser beam, - detecting (730) only one laser beam that has kept on writing, - calibrating (732) laser power of the only one laser beam, - switching off (750) the writing of said only one laser beam, and - shifting (760) all laser beams.

Description

A METHOD AND APPARATUS FOR CONTROLLING LASER POWER FOR A

PLURALITY OF LASER BEAMS

FIELD OF THE INVENTION

The invention relates to a method and apparatus for controlling laser power for a plurality of laser beams.

BACKGROUND OF THE INVENTION

In recent years, video/audio data have been rapidly digitized. In particular, video signals produce a very large amount of data when digitized. Therefore, an optical disc player for video signals, such as a VCD (video compact disc) player, a DVD (digital versatile disc) player, a BD (Blu-ray disc) player, is especially required to be capable of having a large capacity and have a high data transfer rate. Various attempts have been made to realize a large data capacity and high data transfer rate for an optical disc player. According to such method, an input data sequence is divided into a plurality of portions which are respectively supplied to a plurality of heads, thereby resulting in parallel writing/reproducing of signals in a plurality of tracks on a disc, in order to realize a high transfer rate. This method makes it possible to increase the overall transfer rate performed by the plurality of tracks, without increasing the transfer rate of each head.

As the number of heads increases, the above method causes problems to arise of complicated control and increase in cost. Therefore, it is contemplated to employ a multi-beam semiconductor in which a plurality of laser diodes is provided instead of a plurality of heads, so as to write signals on a plurality of tracks simultaneously. A multi- beam semiconductor has a plurality of laser diodes and a photo-detector. By employing such a multi-beam semiconductor, it becomes possible to write signals at a high transfer rate in spite of a small number of heads.

Laser power is related to current flow through a laser diode. Higher current introduces higher laser power. But the relationship between laser power and current is influenced by temperature. That is to say, the laser power will change with a variation of temperature even if the current through the diode is fixed. So, the photo-detector provided in the multi-beam semiconductor is used for monitoring the laser power emitted by the laser emitting elements. Specifically, when one of the laser diodes radiates a laser beam on a track for writing, a portion of the laser beam also illuminates the photo-detector, which is disposed near the laser diode. The photo-detector receives this portion of the laser beam and outputs a monitored current in proportion with the power of the laser beam emitted by the laser diode. Accordingly, by converting the monitored current into a monitored voltage and feeding back the monitored voltage to driving circuitry of the laser diode, it becomes possible to control the power of the laser beam output by the laser diode at a constant level.

Many multi-beam semiconductors only have one photo-detector for a plurality of laser diodes (multiple photo-detectors are expensive and need very precise mechanical adjustment). As a result, the photo-detector can output only one monitored current in response to the laser beams output by the plurality of laser diodes. In a multi- beam semiconductor of this configuration, the photo-detector is not capable of distinguishing the laser power of one laser beam from the laser power of another laser beam. This makes it impossible to perform simultaneous power control for more than one of the plurality of laser beams. In other words, when performing power control, all the laser diodes, except for the laser diodes that are subjected to power control, must be in an off-state, i.e., not emit a laser beam.

However, implementing the method mentioned above to control laser power may waste writing space of an optical disc, because when all laser beams are in the "off state except for one laser beam which is in the "on" state without writing simultaneously, the optical disc is still rotating, which causes the fact that some spaces cannot be written.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method of controlling laser power for a plurality of laser beams.

The method according to the invention is intended to control laser power during the writing of data on an optical disc by a plurality of laser beams, the optical disc comprising segments which are each identified by a start of segment. The method comprises the steps of:

- successively switching on the writing of each laser beam at different starts of segments for writing data,

- successively switching off the writing of each laser beam that arrives at the start of a segment previously written by another laser beam, - detecting that only one laser beam has kept on writing,

- calibrating laser power of the only one laser beam,

- switching off the writing of said only one laser beam, and

- shifting all laser beams.

The invention also proposes an apparatus comprising processing means for implementing the different steps of said method according to the invention.

This method and apparatus are advantageous; the laser beams, after shifting, start writing from the starts of segments and by a whole segment at one time, which requires a lower precision compared to that of the prior art, since a precision in the wobble wavelength order is sufficient. As a result, consecutive data sequences can be written by a plurality of laser beams without data overlapping data and data lost. Furthermore, the writing space of the optical disc cannot be wasted due to calibration of the laser power of the only one laser beam, because the laser beams that are switched off when the power of the only one laser beam is calibrated, have to be switched off because they have to wait for starts of segments to arrive. Since it is arbitrary about which laser beam will be the only one laser beam left, so after every time shift, any one of the laser beams may have a chance to act as the only one laser beam.

The invention provides another improved method of controlling laser power during the writing of data on an optical disc by a plurality of laser beams, said optical disc comprising segments which are each identified by a start of segment. The method comprises the steps of:

- successively, during the switching on of the writing of each laser beam, detecting a first laser beam switched on and calibrating laser power of the first laser beam,

- successively switching off the writing of each laser beam that arrives at the start of a segment previously written by another laser beam,

- detecting only one laser beam that has kept on writing, - switching off the writing of said only one laser beam, and

- shifting all laser beams.

The optical writing space cannot be wasted due to the calibration laser power of the first laser beam, because the laser beams switched off when the laser power of the first laser beam is calibrated, have to be switched off because they have to wait for a new start of segment. Since, it is arbitrary which laser beam will be the first one, so after every time shift, any one of the laser beams could have the chance to act as the first laser beam to write.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the method and apparatus according to the invention for controlling laser power for a plurality of laser beams will become apparent from and will be elucidated with respect to the implementations and embodiments described hereinafter and with reference to the accompanying drawings, wherein:

Figs. 1 to Fig. 6 illustrate by way of example, different and consecutive states resulting from the steps of the method according to the invention,

Fig. 7 depicts a flow chart of the method of controlling laser power for a plurality of laser beams according to the invention,

Fig. 8 depicts an apparatus for implementing the method according to the invention previously described.

Same reference numerals are used to denote similar parts throughout the figures.

DETAILED DESCRIPTION OF THE INVENTION

For the sake of understanding, the invention will first be explained by way of example using two laser beams, then secondly generalized to a plurality of laser beams.

During the writing process, the optical disc and the laser beams have a relative rotating movement. The optical disc comprises segments intended to store data and defined along the spiral tracks on the optical disc. Each segment is identified by a start of segment detected by a structure on the optical disc.

In the following, the innermost laser beam means the laser spot landing position that has the smallest radius with respect to other laser spots landing positions on the disc, while the outermost laser beam means the laser spot landing position that has the largest radius with respect to other laser spots landing positions on the disc.

In the following, the laser beams have the same relative angular variation as the optical disc.

In the following, a laser beam "switched off will refer to a state where the writing process is "off, i.e. not activated, e.g. a laser beam is not applied to the optical disc or a laser beam is applied to the optical disc in the read mode. A laser beam "switched on" will refer to a state where the writing process is "on", i.e. activated, namely, the laser beam is in the write mode.

Figs. 1 to Fig. 6 illustrate by way of example, different and consecutive states resulting from the steps of the method according to the invention, via the use of two laser beams A and B applied to the optical disc so as to write data in parallel.

In this example, the innermost laser beam is laser beam A, and the outermost laser beam is laser beam B. The writing is supposed to be done from inner tracks to outer tracks, but it will be apparent to a skilled person that similar explanations apply if the writing is done from outer tracks to inner tracks.

In Fig.l, at the start of the writing process, the initial position of the laser beam A is located at the start of segment 1, and the initial position of laser beam B is located somewhere inside segment 4. The laser beam A being positioned at the start of segment 1, is switched on from the start of segment 1. The laser beam B being located inside segment 4, and not located at a start of a segment, the laser beam B is switched off until the laser beam B arrives at a subsequent start, in this example start of segment 5, as depicted in Fig. 2. In Fig. 2, the laser beam B arrives at the start of segment 5. The laser beam B now being positioned at the start of a segment, is switched on from the start of segment 5. The laser beam A is still switched on.

In Fig. 3, the laser beam A arrives at the start of segment 5, i.e. the start of a segment previously written by another laser beam, namely by laser beam B. To avoid laser beam B overwriting previously written data, the laser beam A is switched off. The laser beam B becomes the only remaining laser beam that is switched on, until laser beam B arrives at a subsequent start of segment, such as the start of segment 13 in this example (or start of segment 14 ...).

In Fig. 4, the laser beam B arrives at a subsequent start of segment, segment 13 in the example, and is thus switched off. Laser beam A and Laser B being both switched off, they are shifted simultaneously, so that laser beam A (i.e. the innermost laser beam) gets positioned at said subsequent start of segment 13, i.e. gets positioned at a start of a segment where laser beam B has been switched off.

Fig.5 shows an intermediate state before laser beam A arrives at the start of segment 13, just after the shift of the laser beams A and B. Indeed, since the optical disc is rotating and the shift is not instantaneous, laser beam A actually arrives in a segment situated before the start of segment 13. In this example, laser beam A arrives in segment 12 and laser B arrives in segment 23.

Concerning laser beam A, it is switched on when arriving at the stored start of segment 13.

Concerning laser beam B, it is also switched on when arriving at the start of a segment :

- either switched on before laser A has been switched on, as shown in Fig.6 A, where laser beam B is switched on at the beginning of segment 24,

- or switched on after laser A has been switched on, as shown in Fig.βB, where laser beam B is switched on at the beginning of a next segment 25. The following of the process is then similar to that described in accordance with Fig. 2.

Fig. 7 depicts a flow chart of the method of controlling laser power for a plurality of laser beams according to the invention. The method of controlling laser power during the writing of data on an optical disc by a plurality of laser beams positioned along inner tracks to outer tracks of said optical disc, the optical disc comprises segments which are each identified by a start of segment, said method comprising the steps of:

- successively switching on (710) the writing of each laser beam at different starts of segment for writing data: this switching can be done on the basis of starts of segments detected from pre-groove information of the optical disc or data information previously written on the optical disc. The segment start detection being known as such, no further explanation will be given.

- successively, during the switching on of the writing of each laser beam, detecting (712) a first laser beam switched on and calibrating (714) laser power of the first laser beam.

- successively switching off (720) the writing of each laser beam that arrives at the start of a segment previously written by another laser beam: this step implies to store the previous segment starts of laser beams when they are switched on.

- detecting (730) only one laser beam that has kept on writing: as long as more than one laser beam is still switched on, the process goes back to step 720.

- calibrating (732) laser power of the only one laser beam.

- obtaining (740) a subsequent start of segment accessed by an outermost laser beam if the writing is done from inner tracks to outer tracks, or by an innermost laser beam if the writing is done from outer tracks to inner tracks.

- switching off (750) the writing of said only one laser beam.

- shifting (760) all laser beams so that the innermost laser beam gets positioned at said subsequent start if the writing is done from inner tracks to outer tracks, or shifting all laser beams so that the outermost laser beam gets positioned at said subsequent start if the writing is done from outer tracks to inner tracks: this shift implies a simultaneous displacement of all laser beams, for example via a translation movement in a radial direction of the optical disc.

According to an additional decision step 770, the process either goes back to step 710, or stops if the criteria of said additional decision step are fulfilled (e.g. no more data to be written).

The calibrating (714, 732) step can be implemented by a conventional single laser beam power control system, and the calibrating (714, 732) may comprise the steps of:

- monitoring the current of the only one laser beam and the first laser beam,

- converting the monitored current of the only one laser beam and the first laser beam into voltage, and

- generating adaptive power for the only one laser beam and the first laser beam.

Fig. 8 depicts an apparatus for implementing the method according to the invention previously described. This apparatus for controlling laser power during the writing of data on an optical disc by a plurality of laser beams positioned along inner tracks to outer tracks of said optical disc, which optical disc comprises segments each identified by a start of segment, said apparatus comprising:

- control means (810) for successively switching on the writing of each laser beam at different starts of segment for writing data: the start of a segment may, for example, be detected from pre-groove information of the optical disc or data information written on the optical disc via a detector. The detection of starts of segment via pre-groove information or data information written on the optical disc being known as such, no further explanation will be given. When a given laser beam is switched on, its corresponding start of segment is stored in a memory.

- detection means (820) for detecting a first laser beam switched on for writing and calibration means (830) for calibrating the laser power of the first laser beam during successively switching on of the writing of each laser beam.

- control means (810) for successively switching off the writing of each laser beam that arrives at the start of a segment previously written by another laser beam: the start of a segment is detected via a detector. This change of state may be done on the basis of a comparison via a comparator between a previously stored start of segment and a current start of segment.

- detection means (820) for detecting only one laser beam that has kept on writing.

- calibration means (830) for calibrating laser power of the only one laser beam.

- control means (810) for obtaining a subsequent start of a segment accessed by an outermost laser beam if the writing is done from inner tracks to outer tracks, or by an innermost laser beam if the writing is done from outer tracks to inner tracks.

- control means (810) for switching off the writing of said only one laser beam.

- control means (810) for shifting all laser beams so that the innermost laser beam gets positioned at said subsequent start of segment if the writing is done from inner tracks to outer tracks, or shifting all laser beams so that the outermost laser beam gets positioned at said subsequent start of segment if the writing is done from outer tracks to inner tracks: this shift implies a simultaneous displacement of all laser beams, for example via a translation movement in a radial direction of the optical disc.

The calibration means (830) may comprise a photo-detector and a driving circuit of a laser diode, wherein:

- the photo-detector is used for monitoring current of the only one laser beam and the first laser beam,

- the photo-detector is used for converting the monitored current into voltage, and

- the driving circuit of the laser diode is used for generating adaptive power for the only one laser beam and the first laser beam. This apparatus also comprises means for shifting simultaneously all laser beams so that the innermost laser beam gets positioned at said subsequent start of segment if the writing is done from inner tracks to outer tracks, or for shifting all laser beams so that the outermost laser beam gets positioned at said subsequent start of segment if the writing is done from outer tracks to inner tracks.

This shift implies a simultaneous displacement of all laser beams, for example via a translation movement in a radial direction of the optical disc. For example, the shift could be done via similar actuation means (not shown) usually in charge of displacing a laser beam in the radial direction, under the control of the control means (810).

This apparatus may advantageously be integrated with a recorder for optical discs (e.g. computer disc drive, standalone apparatus...).

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention and that those skilled in the art will be able to design alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of elements or steps not listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements and by means of a suitably programmed computer. In the unit claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The usage of the words first, second and third, etcetera does not indicate any ordering. These words are to be interpreted as names.

Claims

1. A method of controlling laser power during the writing of data on an optical disc by a plurality of laser beams, the optical disc comprising segments each being identified by a start of segment, said method comprising the steps of:

- successively switching on (710) the writing of each laser beam at different starts of segment for writing data,

- successively switching off (720) the writing of each laser beam that arrives at the start of a segment previously written by another laser beam,

- detecting (730) that only one laser beam has kept on writing,

- calibrating (732) laser power of the only one laser beam,

- switching off (750) the writing of said one laser beam, and

- shifting (760) all laser beams.

2. A method as claimed in claim 1, further comprising a step of:

3. A method as claimed in claim 2, wherein the calibrating step (732, 714) comprises the steps of:

- monitoring current of the only one laser beam and the first laser beam,

- converting the monitored current of the only one laser beam and the first laser beams into voltage, and

4. A method as claimed in claim 1, wherein the starts of segments are detected from pre- groove information of said optical disc or data information written on said optical disc.

5. A method of controlling laser power during the writing of data on an optical disc by a plurality of laser beams, the optical disc comprising segments each identified by a start of segment, said method comprising the steps of: - successively switching on (710) the writing of each laser beam at different starts of segment for writing data,

- successively, during the switching on of the writing of each laser beam, detecting (712) a first laser beam switched on and calibrating (714) laser power of the first laser beam,

- detecting (730) only one laser beam that has kept on writing,

- switching off (750) the writing of said one laser beam, and

- shifting (760) all laser beams.

6. A method as claimed in claim 5, further comprising a step of:

- calibrating (732) laser power of the only one laser beam.

7. A method as claimed in claim 6, wherein the calibrating step (732, 714) step comprises the steps of:

- monitoring current of the only one laser beam and the first laser beam,

8. A method as claimed in claim 5, wherein the starts of segments are detected from pre- groove information of said optical disc or data information written on said optical disc.

9. An apparatus for controlling laser power during the writing of data on an optical disc by a plurality of laser beams, the optical disc comprising segments each being identified by a start of segment, said apparatus comprising:

- control means (810) for successively switching on the writing of each laser beam that arrives at different starts of segment for writing data,

- control means (810) for successively switching off the writing of each laser beam that arrives at the start of a segment previously written by another laser beam, - detection means (820) for detecting only one laser beam that has kept on writing,

- calibration means (830) for calibrating laser power of the only one laser beam,

- control means (810) for switching off the writing of said one laser beam, and

- control means (810) for shifting all laser beams.

10. An apparatus as claimed in claim 9, wherein the detection means (820) are used to detect a first laser beam switched on for writing and said calibration means (830) are used to calibrate the laser power of the first laser beam during the successive switching on of the writing of each laser beam.

11. An apparatus as claimed in claim 10, wherein the calibration means (830) comprise means for:

- monitoring current of the only one laser beam and the first laser beam,

12. An apparatus for controlling laser power during the writing of data on an optical disc by a plurality of laser beams, the optical disc comprising segments each being identified by a start of segment, said apparatus comprising:

- detection means (820) for detecting a first laser beam switched on for writing and calibration means (830) for calibrating the laser power of the first laser beam during successive switching on of the writing of each laser beam ,

- control means (810) for successively switching off of each laser beam that arrives at the start of a segment previously written by another laser beam, - detection means (820) for detecting only one laser beam that has kept on writing,

- control means (810) for switching off the writing of said one laser beam, and

- control means (810) for shifting all laser beams.

13. An apparatus as claimed in claim 12, wherein the calibration means (830) are used to calibrate the laser power of the only one laser beam.

14. An apparatus as claimed in claim 13, wherein the calibration means (830) comprise means for:

- monitoring current of the only one laser beam and the first laser beam,

15. A recorder for recording an optical disc, said recorder comprising an apparatus as claimed in claim 9 or 12.