WO2008117228A1

WO2008117228A1 - Method for tracking the information track of an optical disc

Info

Publication number: WO2008117228A1
Application number: PCT/IB2008/051087
Authority: WO
Inventors: Alexander Marc Lee; Erwin Altewischer; Emile Johannes Karel Verstegen
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2007-03-23
Filing date: 2008-03-24
Publication date: 2008-10-02
Anticipated expiration: 2009-09-23
Also published as: TW200849232A

Abstract

A method of radially tracking the information track of a multi-layer optical record carrier is disclosed. The method comprises generating a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of-focus layers of the multi-layer optical record carrier. The method is useful for optical pick-up units used for multi-layer discs such as Blu-ray discs, HD-DVD and DVD.

Description

Method for tracking the information track of an optical disc

FIELD OF THE INVENTION:

The subject matter relates to a method for tracking the information track of an optical disc, and more specifically, to tracking the information tracks in multi-layer optical discs.

BACKGROUND OF THE INVENTION: US Patent 5923632 discloses an optical pick-up device for a multi-layer optical disc wherein a light beam generated by a light source is separated into a main beam, a first side beam and a second side beam. The main beam is focused onto the main spot; the first side beam and the second side beam are focused on to the first and second side spots respectively on the multi-layer optical discs. Reflected light beams from the main spot, the first and the second side spots are photo-electrically converted by the main beam receiving means, the first side beam receiving means and the second side beam receiving means to obtain push-pull tracking signals. The first side beam receiving means and the second side beam receiving means are spaced a distance apart from the main beam receiving means so that no interference with the first and the second side beam receiving means results from the main beam reflected from an unfocused information layer of the multi-layer optical disc. This solution may be technically challenging because positioning the first side beam receiving means and the second side beam receiving means with a high degree of precision may be difficult to achieve. There may be some amount of interference from the main beam reflected from an unfocussed information layer of the multi-layer optical disc. This may result in inaccurate push-pull tracking signal affecting the recording/reproducing of the data.

It would be advantageous to have a method for obtaining accurate push-pull tracking signals. It would also be advantageous to have an optical pick-up unit for obtaining accurate push-pull tracking signals.

SUMMARY OF THE INVENTION:

A method of radially tracking the information track of a multi- layer optical record carrier is disclosed. The method comprises generating a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi-layer optical record carrier.

An optical pick-up unit for a multi-layer optical record carrier is disclosed. The optical pick-up unit comprises a control unit arranged to generate a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi-layer optical record carrier.

Furthermore, the method of obtaining accurate push-pull tracking signal for a multi-layer optical record carrier could be implemented with a computer program.

BRIEF DESCRIPTION OF THE DRAWINGS:

These and other aspects, features and advantages will be further explained by the following description, by way of example only, with reference to the accompanying drawings in which same reference numerals indicate same or similar parts, and in which: Fig. 1 schematically illustrates an exemplary optical pick-up unit;

Fig. 2 schematically illustrates an exemplary optical detector used for obtaining radial push-pull tracking signals;

Fig. 3 schematically illustrates the positional relationship between the main spot, the first side spot and the second side spot in an exemplary differential push-pull method;

Fig. 4 schematically illustrates the distribution of the light intensity on the optical detector plane showing a background spot along with the main spot, the first side spot and the second side spot, the background spot resulting from reflections from out-of-focus layer of an exemplary multi-layer Blu-ray disc; Fig. 5A and Fig. 5B graphically illustrates the magnitude of distortion of the push-pull tracking signal with the side spots (i.e. with the first side spot and the second side spot) and without the side spots (i.e. without the first side spot and the second side spot); Fig. 6 shows the method of generating the push-pull tracking signal according to an embodiment of the present subject matter;

Fig. 7 shows schematically an exemplary liquid crystal grating; and

Fig. 8 shows schematically an exemplary acousto-optic modulator;

Optical record carrier 2 can be a multi-layer Recordable (R) or Rewritable (RW) type, where information may be stored or recorded on information tracks, such as DVD+RW, DVD-RW, DVD+R, HD-DVD and BD+RW.

Referring to Fig. 1, the optical pick-up unit 30 has a radiation generator 31 (e.g. laser diode), arranged to generate a radiation beam 32a. The radiation beam 32a passes a beam splitter 33 and an objective lens 34. The objective lens 34 focuses the radiation beam 32b in a focal spot F on the optical record carrier 2. The radiation beam 32b reflects from the optical record carrier 2 (reflected radiation beam 32c) and passes the collimator lens 34a and the objective lens 34b and the beam splitter 33 (beam 32d) and passes through an astigmatic lens 36 (for focusing error signal generation) to reach an optical detector 35.

For achieving and maintaining correct focusing of the radiation beam 32b on a desired location of the optical record carrier 2, the objective lens 34 is mounted axially displaceable.

Referring to Fig. 2, the optical detector 35 comprises a plurality of detector segments, in this case four detector segments, 35a, 35b, 35c and 35d, capable of providing individual detector signals A, B, C, and D, respectively, indicating the amount of radiation incident on each of the four detector quadrants. A centerline 37 separates the first and fourth segments 35a and 35d from the second and third segments 35b and 35c. Furthermore, the optical detector 35 comprises two detector segments 35e and 35f capable of providing detector signals E and F, indicating the amount of light incident on each of the detector segments (i.e. 35e and 35f). Furthermore, the optical detector 35 comprises two detector segments 35g and 35h capable of providing individual detector signals G and H, indicating the amount of light incident on each of the detector segments (i.e. 35g and 35h).

Push-pull methods and three beam methods have generally been employed as tracking servo methods for recording/reproducing the data on/from an optical record carrier. Among the methods, typical one is a differential push-pull method.

The principle of differential push-pull method is schematically illustrated in Fig. 3. The radiation beam generated by the radiation generator 31 is passed through the optical means 39 (Cf. Fig. 1) and is separated into a main radiation beam, a first side radiation beam and a second side radiation beam. The main radiation beam results in the main spot M, the first side radiation beam results in the first side spot Si and the second side radiation beam results in the second side spot S₂ on the optical record carrier 2. The three radiation beam spots (a main spot M and the side spots Si and S₂) formed by the optical pick up 31 are positioned so that the side spots Si and S₂ are shifted with respect to the main spot M by half of a track pitch P. Reflected radiation beams from the main spot M and both side spots Si and S₂ are photo-electrically converted by the optical-detector 35 so that push-pull tracking signals can be obtained for the spots M, Si and S₂. The photo-electrically converted signals are used to obtain data and servo signals such as radial, focus and tilt control signals that are required by the disc drive. For instance, a data signal, a tracking signal and a focus error signal can generally be obtained using the following equations: Data = (A+ B+ C+ D)

Tracking = [(A + D) - (B + C)] - k [(E + G) - (F+H)], where k is a multiplication factor, Focus = (A + C) - (B + D) It is to be noted that main spot here refers to central spot or primary spot and the side spot refers to satellite spots or sub-spot or secondary spot. Further, the optical system 30 can be suitably modified to form the main spot M and the side spots Si and S₂.

DETAILED DESCRIPTION OF THE EMBODIMENTS:

The inventors have found that for dual layer or multi-layer optical record carriers, distortions appear in the three spots push-pull tracking signal used for radial tracking. The inventors have also found that the cause of these distortions is not only the coherent cross talk (i.e. interference of the first side spot and the second side spot with the out-of-focus layer spot), but for a large part variation present in the out-of- focus spots itself. It is clear from Fig. 4 that the reflected radiation beam from the main spot M and the first side spot Si and the second side spot S₂ fall on the optical detector 35. It is to be noted that a background spot is also formed resulting from the reflections from out-of-focus layers of the optical record carrier 2. Further, the reflected radiation beams from the background spot is incident on all the detector segments (namely 35a, 35b, 35c, 35d, 35e, 35f, 35 g, and 35 h). The measurements without the side spots (i.e. without Si and S₂), showed variation in the signals detected at the position of the side spots Si and S₂. This is shown in Fig. 5 A and 5B.

In Fig. 5A, the horizontal axis represents the push-pull tracking signal values and the vertical axis represents the magnitude of the push-pull tracking signal with the first side spot and the second side spot. In Fig. 5B, the horizontal axis represents the push-pull tracking signal values and the vertical axis represents the magnitude of the push-pull tracking signal without the first side spot and the second side spot. It can be observed from Fig. 5A and Fig. 5B that the magnitude of distortion of the push-pull tracking signal is very similar to the distortion where no side spots are present. Referring to Fig. 5B, the fluctuations in the magnitude of the push-pull tracking signal values is caused by the fluctuation in the spot on the optical detector 35 reflected by the out-of- focus layer.

Accordingly, a method of radially tracking the information track of a multilayer optical record carrier is disclosed. The method comprises generating a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi-layer optical record carrier.

The push-pull tracking signals are generated taking into account the fluctuations in the background spot and hence results in accurate push-pull tracking signal values. In essence, the method disclosed here separates the cause of the distortion in the three spots push-pull tracking signal, namely the variation in the background spot, from the signal with the side spots on from which the beam landing can be deduced.

In an embodiment, the method 600 of generating the push-pull tracking signal for the main spot, the first side spot and the second side spot comprises the steps illustrated in Fig. 6. Now referring to Fig. 6, in step 602, the first side radiation beam and the second side radiation beam resulting in the first side spot Si (Cf. Fig. 3) and the second side spot S₂ (Cf. Fig. 3) is switched off. In step 604, a first signal associated with the background spot is measured by photo-electrically converting reflected radiation beams coming from the background spot. In step 606, the first side radiation beam and the second side radiation beam resulting in the first side spot Si (Cf. Fig. 3) and the second side spot S₂ (Cf. Fig. 3) is switched on. In step 608, a second signal associated with the background spot, the first side radiation beam, and the second side radiation beam is measured by photo-electrically converting reflected radiation beams coming from the background spot, the first side radiation beam and the second side radiation beam. In step 610, a corrected signal is computed based on a difference of the first signal and the second signal. In step 612, the push-pull tracking signal for the main spot, the first side spot and the second side spot is generated by using the corrected signal on the photo-electrically converted (i.e. photo-electric signals) reflected radiation beams coming from the main spot, the first side spot and the second side spot. Switching on and switching off the first side spot and the second side spot is advantageous since the signal without the side spots (i.e. without the first side spot and the second side spot) can be measured and used as a measure associated with the background spot. The signal with the side spots (i.e. with the first side spot and the second side spot) can be measured and this can now be compensated for beam landing as (normal) in the three spots push-pull tracking signals. The push-pull tracking signals are obtained considering the fluctuations in the background spot and hence the push-pull tracking signals are accurate. This improves the recording/reproducing of the data from the optical record carrier.

In a further embodiment, switching on and switching off of the first side radiation beam and the second side radiation beam includes transmitting the main radiation beam through a switchable grating having an ON state that generates, besides the main radiation beam, the first side radiation beam and the second side radiation beam and an OFF state that transmits only the main radiation beam. In other words, in the OFF state the first side radiation beam and the second side radiation beam is not transmitted. This is an advantageous implementation as a small and relatively simple optical element can be easily introduced in the optical light path and hence allows a cost effective solution with minimal changes to the rest of the architecture of the optical drive. In a still further embodiment, the switchable grating comprises one of a liquid crystal material and an acousto-optic modulator. The grating is made switchable by liquid crystal material as shown in Fig. 7. The grating is made via a cell in which a transparent structure is present, e.g. created by photo-polymerization or by photolithography. The structure has the shape of the desired grating, given the optical constants of the materials used. The cell substrates consist of a transparent electrode necessary to enable the liquid crystal to be addressed. Fig. 7 shows an extra parallel layer of liquid crystal material on top of the structures. It is to be noted that the extra parallel layer is for manufacturing purposes only and does not influence the function of the optical pick up unit 31. It is also possible to omit this layer. Referring to Fig. 7, preferably one of the refractive indices of the switchable liquid crystal material should be matched to the non-switchable part of the grating. The ON or OFF state then corresponds to diffraction or no diffraction depending on whether a liquid crystal with positive or negative dielectric anisotropy is used. Hence, by applying a voltage to the grating it starts or stops to diffract. By sequentially applying a high and low voltage (ON and OFF), the signal of the background spots without the side spots (i.e. without the first side spot and the second side spot) and the signal of the background spots including the side spots (i.e. with the first side spot and the second side spot) can be measured separately.

A simple reconstruction algorithm is given below: a_m = side spot signal (t=mT) = signal without side spots b_m= side spot signal (t=l/2T+mT) = signal with side spots c_m = side spot signal corrected for background spot = b_m - a_m where m is an integer number and T is the time period it takes to complete a cycle of switching the granting off and on.

Hence, the new three spots signal (i.e. the main spot, the first side spot and the second side spot) is constructed by sampling the side spot push-pull tracking signal in phase with on/off switching of the spots that results in the samples a_m and b_m, from which c_m is calculated. c_m consists of the side spot push-pull value without the background spot. The signal is then subtracted from the push-pull tracking signal of main spot to arrive at the three spots signal.

When the switching of the grating is below the bandwidth of the actuator, the discreteness of the sampling influences the radial tracking performance. When the switching is within the bandwidth of the actuator, the bandwidth of the side spot push-pull tracking signal will be reduced. The side spot signal is used to compensate for beam landing, which is also a slow varying signal (the roundtrip frequency or the second, third harmonic of this). So, as long as the switching time is well above the frequency needed to compensate for beam landing, this solution will work adequately. The idea is that by switching the liquid crystal material the light will only encounter a phase shift and not a polarization rotation; this should be accomplished by the right design of the liquid crystal cells and their orientation with respect to the incoming polarization direction.

As an alternative to the preferred grating in which a physical structure is present a grating could be made in which the entire cell would be filled with liquid crystal, but now with a patterned electrode conformation. The conformation should be chosen such as to enable the proper phase shift to be realized. For both the grating with a solid structure inside as well as for the grating made from liquid crystal, only the pattern to be created is in the form of a binary grating with a duty cycle of 50 %. The intensity of the side spots should typically be in the order of l/10^th to l/15^th of the intensity of the main spot.

For a binary phase grating the intensity of a the zero and first order intensity is given by 0^th order : I₀ = ¹A [I + cos(φ)] I_1n 1^st order: I₁ = [2 - 2 cos(φ)] /(π²) I_1n

Where I_1n is the intensity of the radiation beam and φ(phi) is the phase depth of the binary grating.

For an intensity difference of a factor of 10, the phase depth difference is 0.15 of a wavelength. For an intensity difference of a factor of 15, the phase depth difference is 0.12 of a wavelength.

For liquid crystal materials, the refractive index difference between the two states is typically 0.1. So, in order to achieve above mentioned phase difference of 0.15λ, a depth of the liquid crystal material of 1.5λ, which is for Blu-ray disc (λ=405 nm) a depth of 608 nm, is a convenient cell depth, and allows for fast switching of the cell.

The switchable grating can also be an acousto-optic modulator as shown in Fig. 8. Referring to Fig. 8, switching ON/OFF the acoustic wave generates the grating profile and will induce the switching ON and switching OFF of the grating. Basically, by switching on and off the sound wave, the grating can be switched on and off. The processing of the side spots can be carried out as described in the previous paragraph with regard to liquid crystal grating. The only difference is that the switching time of an acousto-optic modulator can be very short, in the order of 1 - 5 micro seconds.

In a still further embodiment, the liquid crystal material comprises ferro- electric liquid crystalline material. Several gratings can be made utilizing different types of liquid crystals such as the ones using nematic liquid crystal materials and others using ferroelectric liquid crystalline materials. The advantage of using the ferro-electric liquid crystalline materials as compared to nematic liquid crystals is their large difference in switching time approximately 20 μs switching time v/s 5 - 0.1ms, for nematic liquid crystals. The actual switching time depends on cell gap and applied field strength.

Referring to Fig. 1, the optical pick up unit 30 is adapted to perform the method of generating the push-pull tracking signal for the main spot, the first side spot and the second side spot taking into consideration the background spot (Cf. Fig. 4) formed on the multi-layer optical record carrier. To this end, the optical pick-up unit 30 has a control unit 38 arranged to generate a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi- layer optical record carrier.

The optical means 39 (Cf. Fig. 1) comprises a switchable grating arranged to switch off and on the focusing of the first side radiation beam and the second side radiation beam on to the first side spot and the second side spot respectively.

The control unit 38 further comprises: i) a measurement unit 38a arranged to measure a first signal associated with the background spot when the switchable grating is in ON state and to measure a second signal associated with the background spot when the switchable grating is in OFF state, by photo-electrically converting reflected radiation beams coming from the background spot; ii) a computing unit 38b arranged to compute a corrected signal based on a difference of the first signal and the second signal; and iii) a logic unit 38c arranged to generate the push-pull tracking signal for the main spot, the first side spot and the second side spot by applying the corrected signal on photo-electrically converted (i.e. photo-electric signals) reflected radiation beams coming from the main spot, the first side spot and the second side spot.

A drive such as Blu-ray disc drive or a DVD drive or a HD-DVD drive having the optical pick- up unit 30 can obtain accurate push-pull tracking signals and thereby improve recording/reproducing of data. Although the present subject matter has been explained by using embodiments using Blu-ray discs and Blu-ray drives, the technique is applicable to all types of multi-layer record carriers, e.g. write-once media and write-many recordable types (DVD-RW, DVD+RW, HD-DVD). Further, the above technique is not limited to a two-layer one sided record carrier, i.e. a dual layer record carrier, or to a two-layer double sided record carrier, i.e. a dual layer double sided record carrier. A person skilled in the art can implement the described embodiments of the method of generating a push-pull signal in software or in both hardware and software. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art of practicing the claimed subject matter, from a study of the drawings, the disclosure and the appended claims. The use of the verb "comprise" does not exclude the presence of elements other than those stated in a claim or in the description. The use of the indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps. The figures and description are to be regarded as illustrative only and do not limit the subject matter.

Claims

CLAIMS:

1. A method (600) of radially tracking the information track of a multi-layer optical record carrier, the method comprising: generating a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the mult i- layer optical record carrier.

2. The method as claimed in claim 1, further comprising: switching off the first side radiation beam and the second side radiation beam which result in the first side spot and the second side spot; measuring a first signal associated with the background spot by photo- electrically converting reflected radiation beams coming from the background spot; switching on the first side radiation beam and the second side radiation beam which result in the first side spot and the second side spot; measuring a second signal associated with the background spot by photo- electrically converting reflected radiation beams coming from the background spot; computing a corrected signal based on a difference of the first signal and the second signal; and generating the push-pull tracking signal for the main spot, the first side spot and the second side spot by using the corrected signal on the photo-electrically converted reflected radiation beams coming from the main spot, the first side spot and the second side spot.

3. The method as claimed in claim 2, wherein switching on/off of the first side radiation beam and the second side radiation beam comprises: transmitting the main radiation beam through a switchable grating having an ON state that generates, besides the main radiation beam, the first side radiation beam and the second side radiation beam and an OFF state that transmits only the main radiation beam.

4. The method as claimed in claim 3, wherein the switchable grating comprises one of a liquid crystal material and an acousto-optic modulator.

5. The method as claimed in claim 4, wherein in the switchable grating comprising the liquid crystal material the liquid crystal material comprises ferro-electric liquid crystalline material.

6. The method as claimed in any one of the claims 1 - 5, wherein the multi-layer optical record carrier is one of Blu-ray disc and DVD.

7. An optical pick-up unit (30) for a multi-layer optical record carrier, the optical pick-up unit comprising: a control unit (38) arranged to generate a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi-layer optical record carrier.

8. The optical pick-up unit as claimed in claim 7, further comprising: a switchable grating (39) arranged to switch ON/OFF the first side radiation beam and the second side radiation beam resulting in the first side spot and the second side spot respectively; a measurement unit (38a) arranged to measure a first signal associated with the background spot when the switchable grating is in ON state; and to measure a second signal associated with the background spot when the switchable grating is in OFF state, by photo-electrically converting reflected radiation beams coming from the background spot; a computing unit (38b) arranged to compute a corrected signal based on a difference of the first signal and the second signal; and a logic unit (38c) arranged to generate the push-pull tracking signal for the main spot, the first side spot and the second side spot by applying the corrected signal on the photo-electrically converted reflected radiation beams coming from the main spot, the first side spot and the second side spot.

9. The optical pick-up unit as claimed in claim 8, wherein the switchable grating comprises one of liquid crystal switchable grating and an acousto-optic modulator.

10. A drive comprising the optical pick-up unit as claimed in any one of the claims 7-9, wherein the drive is one of Blu-ray drive and DVD drive.

11. A recorder comprising the drive as claimed in claim 10, wherein the recorder is one of Blu-ray disc recorder and DVD recorder.

12. A computer program comprising program code means to perform a method of generating a push-pull signal for a multi-layer optical record carrier, the method comprising: generating a push-pull tracking signal derived from the light of a main spot, a first side spot and a second side spot falling on an optical detector, wherein the main spot, the first side spot and the second side spot are formed by focusing a main radiation beam, a first side radiation beam and a second side radiation beam, respectively, taking into consideration the effect of a background spot resulting from reflections of the main spot from out-of- focus layers of the multi-layer optical record carrier.