WO2008110972A1

WO2008110972A1 - Optical scanning device

Info

Publication number: WO2008110972A1
Application number: PCT/IB2008/050854
Authority: WO
Inventors: Alexander M. Van Der Lee; Erwin Altewischer
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-03-13
Filing date: 2008-03-10
Publication date: 2008-09-18
Also published as: TW200903473A

Abstract

The invention relates to optical data storage systems and, more particularly, to an apparatus and optical scanning device for scanning data stored on an optical record carrier having multiple information layers. When scanning multi-information-layer optical record carriers, the reflection from an out-of-focus information layer interferes with the reflection from the in-focus information layer. This interference pattern changes when the distance between the layers varies, resulting in coherent crosstalk. The tracking error signal can be distorted by this coherent cross-talk. In the situation that a three-beam tracking method is applied mainly caused by distortions of the tracking signals from the satellite spots. The invention provides a solution to reduce this coherent cross-talk.

Description

Optical scanning device

FIELD OF THE INVENTION

The invention relates to optical data storage systems and, more particularly, to an apparatus and optical scanning device for scanning data stored on an optical record carrier having multiple information layers.

BACKGROUND OF THE INVENTION

Optical data storage systems, i.e. optical recording systems or optical data drives, provide means for storage of large quantities of data on an optical record carrier, e.g. a disk. An optical scanning device in the optical data drive is used for scanning the information layer or layers of the carriers. Various optical data storage media formats and systems are well known and already commonly used, such as media according to the CD and DVD media standard, either for only reading data from prerecorded data such as ROM or Video, or for recording data on recordable or rewritable media such as CD-R, DVD+R, DVD-R or CD- RW, DVD+RW, DVD-RW, DVD-RAM. CD media having a capacity of about 650MB to 700MB are recordable and readable using a semiconductor laser emitting a radiation beam having a wavelength of about 780nm and an objective lens with a numerical aperture (NA) of 0.45 to about 0.55. The data is being read and/or written through a standard transparent layer of 1.2mm thickness.

DVD media having a capacity of about 4.7GB are recordable and readable using a semiconductor laser emitting a radiation beam having a wavelength of about 650nm (a DVD radiation beam) and an objective lens with a NA of 0.60 to about 0.65. The standard transparent layer thickness of a DVD disk is 0.6mm. In order to increase the total capacity of such media also dual information layer disks have been introduced for DVD read-only and recordable media having a capacity of about twice the capacity of a single information (data) layer disk. The separation between both information layers of a dual-layer DVD disk is about 55 μm.

A recently introduced higher capacity standard optical record carrier for a new media type of optical record carrier according to the Blue-ray Disc (BD) standard has a capacity of about 25 GB per layer. The standard wavelength of the applied radiation beam is about 405nm and the standard NA of the objective lens focusing the radiation beam onto the information layer is about 0.85. The radiation beam is focused through a standard transparent cover layer of 0.1mm thickness. In view of even higher data storage capacity requirements BD also includes a dual-layer disk having a capacity of 50GB. The spacing between both information layers of this dual-layer BD disk is about 25μm. For even higher capacity requirements also more than two information layers is being worked on.

It is to be understood that an information layer of an optical record carrier can be a prerecorded information layer such as e.g. for data distribution, video distribution, etc., or a recordable information layer for e.g. data and/or video recording. Scanning an information layer may be considered to mean reading and/or recording e.g. data on such an information layer.

With increasing the capacity requirements, the dimensions of the data structures (bits) on the disk are decreasing from CD to DVD to BD. This is e.g. achieved by applying a reduction in the wavelength of the radiation beam and an increase in NA of the objective lens from the CD to the DVD to the BD system. The scanning spot dimension is proportional to λ/NA, hence a reduction in the scanning spot dimensions from about 1.5μm in the CD system to about 1.Oμm in the DVD system to about 0.48μm in the BD system. In order to generate a radiation spot of sufficient optical quality the optical scanning device in the optical data drive requires at least focusing and tracking controls in order to keep the scanning spot on track in axial (perpendicular to the disc surface) as well as in radial

(perpendicular to the track and in the plane of the disc) direction. Deviations from track and optimal focus position may, for example, lead to reduction in the quality of the reproduced data or in off-track data during recording.

An example of a well-known focusing method is the astigmatic focusing method. However, also other focusing methods may be applied such as the knife-edge

(Foucault) focusing method or spot size detection focusing method. For the tracking methods there is also a number of well-known possibilities such as, for example, the push-pull tracking method, the three beam (or three spots) tracking method, or the differential push-pull tracking method. A commonly applied combination of focusing and tracking method for recordable optical disk systems is the astigmatic focusing method with the three spots differential push-pull tracking method. For example, a cylindrical lens and/or plan-parallel plate may be used to generate the astigmatism for the astigmatic focusing method into the radiation beam towards the radiation detector. A diffraction grating may be applied to generate a main and two satellite radiation beams out of the radiation beam emitted by the radiation source, e.g. a semiconductor laser. A commonly applied intensity ratio for the intensity of the main radiation beam with respect to intensity in each satellite beam is about 10 to 15 over 1 for recordable systems, but may have a different ratio. A high radiation power level in the main beam is advantageous for the recording speed in the application.

A radiation detector geometry suitable for cooperation with the astigmatic focusing three spots differential push-pull tracking method comprises a main detector and two satellite detectors (opposite to each other with respect to the main detector).

The main radiation beam reflected by the information layer in the disk is projected via the objective lens onto the main detector, which is used for generating the data readout signal (data signal). The main detector is usually also split up into four quadrant segments (corresponding to a radial and a tangential direction with respect to the tracks on the disk) to be able to generate a focus error signal based on the astigmatic method. The satellite beams reflected by the information layer are each projected via the objective lens onto one of the satellite detectors. Each satellite detector is split up into two segments

(corresponding to the radial direction with respect to the tracks on the disk) in order to be able to generate a push-pull signal per satellite beam. By combining the push-pull signals of the main and two satellite detectors a three spots differential push-pull signal can be generated as radial tracking error signal. The focus error signal and radial tracking signals are used in servo control electronics to accurately align the scanning spot onto the track to be scanned.

Multilayer disks comprise stacks of information layers; the information layers being separated by a spacer layer. An example of such a multilayer disk is the dual- layer BD, comprising a stack of two information layers Ll and LO separated by a spacer layer of about 25 μm and the total covered by a transparent cover layer of 0.075 mm thickness (a single layer BD disc has a transparent cover layer of 0.1 mm thickness). Ll may be assumed to be the closest to the radiation incident surface of the disk, while the LO is then assumed to be farther away from the radiation incident surface of the disk. Ll is not fully reflective as it is preferable to scan the LO layer in order to make use of the capacity of this second information layer. Hence, while scanning the Ll information layer, some radiation is transmitted towards the LO information layer and reflected back into the objective lens to be projected towards the radiation detector. When scanning the LO information layer the Ll information layer also reflects some radiation that is projected towards the radiation detector. In both situations these additional reflected radiation beams may cause unwanted radiation to occur on the main and satellite detectors which may cause optical interferences with the radiation spots of the reflected main and satellite beams on the detector related to the scanned information layer.

When the spacer layer between LO and Ll is varying in thickness, for example, along the track and/or perpendicular to the track direction, the resulting interference patterns are also varying causing crosstalk, the so-called coherent crosstalk. As a result the focus and/or tracking error signals or the data signal may be disturbed by this crosstalk, which may result in incorrect tracking, focusing and/or data recording or data reproduction.

As the intensity of the satellite beams projected onto the satellite detectors in recordable systems is much lower than the intensity in the main beam, the effect of the crosstalk on the tracking error signal, such as the push-pull signals, can be that large such that scanning of dual-layer media is becoming unstable.

It is an object of the invention to provide an optical scanning device in which the interlayer crosstalk on the servo-signals is reduced.

SUMMARY OF THE INVENTION

The object of the invention is obtained by providing an optical scanning device for scanning an optical record carrier having multiple information layers, the device having a radiation source for generation of a radiation beam, a diffraction means for generation of a zero-order radiation beam and at least two higher-order diffracted radiation beams from the radiation beam by means of a diffractive pattern, wherein the optical scanning device comprises a phase-modulating means arranged to generate a phase- modulation between the at least two higher-order diffracted radiation beams and the zero- order radiation beam.

By introducing a phase-modulating means in the optical scanning device arranged to generate a phase-modulation between the at least two higher-order diffracted radiation beams (e.g. the plus and minus first order diffracted radiation beams) also referred to as satellite beams) and the zero-order radiation beam, the interference between the two reflected higher-order diffracted radiation beams on the radiation detector and the zero-order radiation beam being reflected by such other information layer on the radiation detector is modulated. This results in an averaging in time of the interference pattern, reducing the distortion in the servo-signals due to the interlayer crosstalk.

According to an aspect of the invention the phase-modulation means is arranged to modulate the position of the diffractive pattern of the diffraction means. By modulating the position of the diffractive pattern the phase of the two reflected higher-order diffracted radiation beams is modulated, but the phase of the zero-order radiation beam remains unaffected.

The diffraction means can be, for example, a diffraction grating as used for the generation of satellite beams for a three beam tracking method. This diffraction grating can be modulated in position, preferably in a direction substantially perpendicular to the diffraction pattern, which may consist of grating grooves. The larger the deviation from the perpendicular direction the less efficient the modulation of the position is and therefore the less efficient the reduction of the interlayer crosstalk.

The phase-modulation means may comprise a displacement means for displacing the diffraction grating, such as, for example, a piezo-actuator or an electromagnetic actuator for modulating the position of the diffractive pattern of the diffraction grating. Preferably, the displacement means is arranged to displace the diffraction grating substantially perpendicular to the grating grooves of the diffraction grating.

Another aspect of the invention is provided by an optical scanning device, wherein the diffraction means comprises the phase-modulation means and are preferably formed as one. With such an integrated device cost can be reduced and the device as a whole can be reduced in size. Examples of such an integrated diffraction and phase-modulation means are an acousto-optical modulator or an electro-optical modulator.

An acousto-optical modulator can be adapted to generate a diffraction pattern as well as a phase-modulation between the at least two higher order diffracted radiation beams and the zero-order radiation beam is can generate. This will also result in a phase modulation between the higher- order diffracted radiation beams reflected from the information layer being scanned and the zero-order radiation beam being reflected by another information layer than the layer being scanned. The sound waves propagating in the crystal produce a pressure wave in the crystal inducing refractive index modulations, so a diffractive pattern or grating pattern, which pattern will move in the crystal (due to the propagating sound wave) thereby modulating the phase of the higher-order diffracted radiation beams.

A phase-modulating means can also be an electro-optical modulator. The electro-optical modulator may comprise a liquid crystal material and a patterned electrode configuration for generating a diffraction grating as well as a phase-modulation between the at least two higher order diffracted radiation beams and the zero-order radiation beam. A modulation supplied to a patterned electrode of the electro -optical modulator, for example as a modulated voltage or current, can generate a phase modulation in the diffracted radiation beams without affecting the zero order radiation beam. Preferably, the phase modulation is such that amplitude is substantially π, resulting in a seemingly averaging out of the interference pattern on the detector between the higher- order diffracted radiation beams reflected from the information layer being scanned and the zero-order radiation beam being reflected by another information layer than the layer being scanned. An optical data drive for reading and/or writing data (such as audio, video and/or computer data) on an optical record carrier, comprising such an optical scanning device according to the invention will have a better reading and/or writing performance on record carriers having more than one information layer, due to the reduced crosstalk in the servo signals. The resulting more stable servo signals will improve the tracking (as well as focusing) characteristics of the optical data drive. Preferably, the frequency of the phase- modulation of the higher-order diffracted radiation beams is larger than the servo-loop bandwidth of the push-pull servo electronics in such an optical data drive.

Furthermore, the invention provides a method of reducing crosstalk in servo signals obtainable from an optical scanning device for scanning an optical record carrier having multiple information layers is provided, by applying an optical scanning device comprising a radiation source for generation of a radiation beam, the radiation beam having a phase and a diffraction means for generation of a zero-order radiation beam and at least two higher order diffracted radiation beams from the radiation beam, wherein the method comprises the step of generating a phase-modulation between the at least two higher-order diffracted radiation beams reflected from the information layer being scanned and the zero- order radiation beam being reflected by another information layer than the information layer being scanned.

Preferably, the method also comprises the step of setting the frequency of the phase-modulation of the higher-order diffracted radiation beams at a higher frequency than the servo-loop bandwidth of the push-pull servo electronics. For optical data drives with various scanning speed settings the servo-loop bandwidth may be adapted to the scanning speed which allows a subsequent adaptation of the phase-modulation frequency for optimization of the crosstalk reduction.

These and other aspects of the invention will be apparent from and elucidated with reference to the drawings and embodiments described hereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 : Schematic view of the layout of an optical scanning device. Fig. 2 comprising figures 2a and 2b with schematic views of the interference pattern on the detector of the optical scanning device when scanning a multi- information layer optical record carrier.

Fig. 3: Schematic view of two embodiments according to the invention with in Fig. 3a a piezo-actuator and in Fig 3b an electromagnetic actuator.

Fig. 4: Schematic view of another embodiments according to the invention using an acousto-optical modulator.

Fig. 5: Schematic view of yet another embodiment according to the invention using an electro-optical modulator.

DETAILED DESCRIPTION

Fig. 1 shows a schematic set up of an example of an optical scanning device 1 for scanning a multi information- layer record carrier 15, such as for example BD, according to a prior art without any filtering means according to the invention. A radiation source 10 (e.g. a semiconductor laser) emits a radiation beam 17. A beamsplitter 12 reflects the radiation beam towards a collimator lens 13 that collimates the radiation beam to a parallel radiation beam that is focused by the objective lens 14 onto an information layer, in this figure on layer Ll, of an optical record carrier 15. The objective lens can be a single-lens or multiple-lens objective lens. The optical scanning device may comprise other optical components, such as a quarter- wave plate or a sensor lens, but these are not shown in Fig. 1. The radiation reflected by information layer Ll is reflected back into the optics and imaged (or projected) onto the detector. Layer Ll can be scanned by the focused radiation beam using well known focusing and tracking error methods, such as for example the astigmatic focusing method and push-pull tracking method, and the related servo controls and actuators (not shown) for actuating e.g. the objective lens and/or the optical scanning device with respect to the track(s) on the information layer. The beamsplitter 12 in this example generates astigmatism in the radiation beam 19 towards the radiation detector 16. When applying another type of beamsplitter such as e.g. a prism type or diffractive type the astigmatism has to be introduced by other means, such as an additional cylindrical lens or an astigmatism introducing diffractive optical structure.

As Ll information layer is transmitting part of the focuses radiation beam towards another layer LO, which is not being scanned, some radiation 18 is reflected by LO back into the optics. The optical system images, or projects, also this reflected radiation towards the radiation detector 16. As this radiation is out of focus of the objective lens, the radiation is imaged as a large radiation spot over the radiation detector surface.

Fig. 2a schematically shows the radiation distribution on detector 16 in the situation for the scanning of Ll as described in relation to Fig. 1. Radiation spot 20 is the radiation scanning spot on Ll spot projected by the optics onto a quadrant detector 22 for focus error (FE) signal generation and RF-signal generation according to known methods. For a single spot push-pull astigmatic tracking method the following formula to derive the FE, RF and RE signals from the set of main detector elements A, B, C and D of detector 22 may be applied: FE = (A+C) - (B+D) RF = A + B + C + D

The tracking error (RE) signal may be generated by the push-pull method as RE = (A+B) - (C+D)

The radiation spot 21 is the image of the radiation reflected by another layer which is not scanned (in this case LO). Although drawn for convenience with a circular shape in Fig.2, the actual shape of the radiation spot 21 can have a different shape, however, this is not considered to be relevant for the invention.

The overlapping portions of radiation spot 20 and radiation spot 21 will show optical interference that may cause fluctuations in the FE, RE and RF signals when the interference pattern is fluctuating due to, for example, variations in the spacer layer thickness between layer Ll and LO.

It can be understood that the above problem is not limited to scanning optical record carriers having only two information layers. As also shown in Fig. 1 it is possible to apply the three-beam central aperture tracking method or three-beam push-pull tracking method in the optical scanning device 1. For that the radiation beam 17 is split into a main and two satellite radiation beams by a diffraction means as zero-order radiation beam and two higher order diffracted radiation beams, for example, a diffraction grating 11. The higher order diffraction radiation beams may be the +1 and -1 diffraction order beams, however, it is also possible that it are other diffraction order radiation beams. It may also be possible that more than two higher-order diffracted radiation beams are used, such as for example the +1/-1 and +3/-3 diffraction order radiation beams (e.g. for more complex tracking purposes). The main beam has a larger radiation intensity than the two satellite radiation beams. Commonly used ratios for the intensities in optical scanning devices suitable for recording data are 1 :10:1 or 1 :15:1. However, also other ratios may be applied. The three radiation beams are focused on the information layer being scanned as main spot and first and second satellite spot, according to known methods, orientations and positions with respect to the track(s) on the information layer. The three reflected radiation beams are imaged (or projected) by the optical system towards the radiation detector and imaged (see Fig. 2a) as a main 20, first 23 and second 24 satellite spot onto the respective set of detector elements 22, 25 and 26. The satellite detectors 25 and 26 may each be split up in order to make a three-beam push-pull tracking possible. The tracking error signal based on the three beam central aperture method can be described using the first and second set of detector elements E, F, G and H of the first and second satellite detectors by: RE_3spc_A=(E+F)-(G+H)

When using the three-beam push-pull tracking method the tracking error signal can be described by

REssppp = [(A+B)-(C+D)] - Kp_P. [(E-F) +(G-H)] in which K_pp is a gain factor in the electronics for compensating the radiation intensity differences between the main and satellite spots on the detector.

The overlapping portions of radiation spot 21 and the satellite spots 23 and 24 will also show optical interference that will cause fluctuations in the RE-signals when the interference pattern is fluctuating due to, for example, variations in the spacer layer thickness between layer Ll and LO.

The satellite beams focused on the information layer that is scanned will also partially be reflected by the layer not being scanned and will thus also result in a large spot similar to radiation spot 21 onto the various sets of detector elements. However, as the intensity in these satellite beams usually are much less than the intensity in the main radiation beam the disturbances due to optical interference is much less and not causing the main problem to be solved. In Fig. 2b the interference pattern is shown as calculated in a computer simulation. For convenience the overlapping area between the satellite spot and radiation distribution of the zero-order beam reflected by an information layer that is not being scanned is chosen to be a square. The calculated interference patterns in the satellite spot are for a spacer thickness of 25.000 μm (on the left) and 25.065 μm (on the right). The grey area is indicative for the total surface of the detector. It can be seen that all the fringes that are white for a 25.000 μm spacer are black for a 25.065 μm spacer. So, a spacer thickness variation of only 65 nm can already cause large variations in the interference pattern. This will also have influence on the servo-signals to be obtained from the detection system, such as for example the three-beam push-pull signal. Measurements have shown that on typical dual-layer BD- disks variations in the spacer thickness of 200 to 300 nm over the circumference are not uncommon. This implies that large variations in the push-pull signals can be expected when scanning such dual-layer BD disks.

It will be clear to the skilled person that the situation will be analogous for an optical scanning device that is to scan optical record carriers having more than two information layers.

In the conventional optical scanning devices location of the diffractive pattern of the diffraction grating is fixed in place. In the optical scanning device according to an embodiment of the invention the diffractive pattern is modulated in its position with respect to the optical path of the radiation beams. By moving the diffractive pattern the phase of the diffracted radiation beams will change, but the phase of the zero-order radiation beam is not affected. After reflection by the optical record carrier, the interference between the satellite spot and the reflection of the zero-order radiation beam from the out-of focus information layer (the layer that is not being scanned) will also be modulated due to this changing phase. The position of the grating x, with period a, determines the phase Δφ of the diffraction orders m (|m|>0), so when the grating moves Δx the phase of the orders m is given by:

Δφ = 2 m π Δx / a

The movement of the grating, i.e. diffraction pattern, can be done mechanically, for example using an electromagnetic actuator, a piezo-actuator (as shown in Fig 3a resp. 3b) or other suitable mechanical displacement means.

In Fig. 3a a schematic view of an embodiment is shown a phase-modulation means 30 having diffraction grating 31 attached to a piezo-actuator 33. In Fig. 3b a schematic view of an embodiment is shown of a phase-modulation means 30' having an electromagnetic actuator with one or more coils 34 cooperating with one or more magnets 35 for the displacement of the diffractive pattern of the grating 31. The grating is displaced in the Δx- direction.

Preferably the displacement is a periodic displacement as this limits the overall displacement and size of the device. When the displacement is perpendicular to the optical axis of the optical scanning device, the positions of the satellite spots on the detector are not affected, so, this is a preferred displacement direction of the diffractive pattern. Usually a three-beam grating has straight line grooves and when the displacement is substantially perpendicular to these grooves the effect of the modulation is optimum, which is a preference. When, for example, the angle is 45 degrees the effect is reduced to about 70% due the reduction of the effective Δx with the sinus of the angle, however, still usable.

Depending on the mechanical requirements of the actuator in the optical scanning device the most suitable displacement angle can be chosen. The frequency of the phase modulation is depending on the speed of the displacement of the diffraction pattern.

Preferably, the frequency of the phase modulation is larger than the servo-loop bandwidth in order to have a sufficiently large modulation frequency of the phase of the satellite spots so as to average out the interference pattern. In practice, the speed of displacement of the diffraction pattern can be limited due to, for example, power restrictions in the actuator. For high accelerations of the diffraction pattern the forces needed, and hence the power, increase rapidly. As the servo-loop bandwidths of the servo electronics may depend on the scanning speed of the system, the modulation frequency may need to be adapted to the different servo-loop bandwidth settings in the system during scanning, such that, preferably, the frequency of the phase modulation is larger than the applied setting of the servo-loop bandwidth of the servo electronics. The tracking error signals of the satellite spots do not have to contain the full bandwidth of the servo, as variations in, for example, beamlanding or spacer layer thickness are usually relatively low- frequency distortions. The main spot tracking signal can be used for the high-frequency variations.

Preferably, the variation of the phase of the satellites spots is faster than the expected variations of the beamlanding and the coherent cross-talk during scanning. The push-pull signal of the satellite spots will be sufficient to compensate for the beamlanding, while the coherent cross-talk contribution will be averaged. Preferably, a low-pass filter or band-pass filter can be used in the servo-loop electronics of the optical scanning device to eliminate the modulation in the push-pull of the satellite spots due to the moving grating.

In another embodiment, as schematically shown in Fig. 4, according to the invention the diffraction pattern is formed by a creating pressure wave in a suitable material 42 having opto-acoustical characteristics, such as an opto-acoustical modulator 40. The pressure wave will induce refractive index modulations that generate the diffractive pattern 41. As the sound wave propagates the pattern will move (indicated by the direction of the arrow in Fig. 4) and this will lead to the modulation of the phase of the satellite spots. The sound velocity and the frequency of the sound wave determine the period of the pattern. The frequencies than can be achieved in an acousto -optical modulator are usually in the order of 10-100 MHz (depending on the type of acousto-optical modulator and material used). The frequency f needed to obtain a diffraction pattern with period p is given by f = v/(p-n) , with sound velocity v and (optical) refractive index of the material n. Typical values for material parameters are v about 3 km/s and n about 2, which for a p= 50 μm results in a f of about 30 MHz. As can be understood, these values depend on the materials used in the acousto-optical modulator and the required grating period to obtain the proper diffraction angles of the diffracted radiation beams.

In this way the diffraction pattern moves fast enough in comparison with the servo bandwidth (which is usually in the order of some 10 kHz's) and the interference pattern will average out.

In yet another embodiment of the invention the diffractive pattern is generated using an electro-acoustic modulator 50 having electro-optical modulated material, for example, a liquid crystal material 52 as shown schematically in Fig. 5. The diffractive pattern is generated using patterned electrode 51, for example, made of transparent ITO-materials. By supplying a voltage on the pattered electrode the phase depth of the pattern can be changed. By, for example, switching the voltage form +V to -V the diffractive pattern can be inverted. In one state (e.g. with +V applied) the phase of the radiation passing through the electrodes is advanced by Δφ compared to the radiation traveling through the other parts. In the other state (e.g. -V applied) the radiation passing through the electrodes has a phase lag of Δφ compared to the radiation traveling through the other parts. In this way the phase of the higher-order diffracted order radiation beams can have a phase switch of π. If this switching is done fast enough compared to the servo bandwidth the interference pattern in the satellite spots on the radiation detector will average out. Alternatively, a low-pass filter or band-pass filter can be used in the servo-loop electronics of the optical scanning device or optical data drive to eliminate the modulation in the push-pull of the satellite spots due to the moving grating.

In all above embodiments the satellite radiation beams can be the first order diffracted radiation beams, but also higher order diffracted radiation beams. It is also possible that the application of the optical scanning device is requiring first and higher order diffracted radiation beams for e.g. specific tracking servo-signal purposes or disk format compatibility requirements.

Depending on the type of device used for the phase-modulation a sinusoidal, saw-tooth, triangular, binary or any other suitable modulation shape may be applied.

Although the invention is described in detail in relation to an optical scanning device for scanning an optical record carrier having two information layers such as a BD or a DVD, the invention also can be applied in combination with an optical scanning device capable of scanning more types of optical record carriers, such as for all three types e.g. BD, DVD and CD, as well as scanning optical record carriers having more than two information layers.

Although the invention is explained in relation to an astigmatic focusing method the invention can also be applied in combination with other focusing methods such as spot-size detection or knife-edge method. Also a differential astigmatic focusing method may be applied.

Claims

CLAIMS:

1. An optical scanning device (1) for scanning an optical record carrier (15) having multiple information layers (LO, Ll), the device comprising: a radiation source (10) for generation of a radiation beam (17) and a diffraction means (11, 31, 41) for generation of a zero-order radiation beam and at least two higher-order diffracted radiation beams from the radiation beam by means of a diffractive pattern, characterized in that the optical scanning device comprises a phase-modulating means (30, 30', 40, 50) arranged to generate a phase-modulation between the at least two higher-order diffracted radiation beams and the zero-order radiation beam.

2. The optical scanning device of claim 1, wherein the phase-modulation means is arranged to modulate the position of the diffractive pattern of the diffraction means.

3. The optical scanning device according to claim 1 or 2, wherein the diffraction means comprises a diffraction grating (31).

4. The optical scanning device according to claim 3, wherein the phase- modulating means comprises a displacement means (33; 34, 35) for displacing the diffraction grating in the radiation beam.

5. The optical scanning device according to claim 4, the diffraction grating having grating grooves, and wherein the displacement means is arranged to displace the diffraction grating substantially perpendicular to the grating grooves.

6. The optical scanning device according to claim 4 or 5, the displacement means being a piezo-actuator (33).

7. The optical scanning device according to claim 4 or 5, the displacement means being an electro -magnetic actuator (34, 35).

8. The optical scanning device according to claim 1 or 2, wherein the diffraction means comprises the phase-modulating means.

9. The optical scanning device according to claim 8, wherein the phase- modulating means is an acousto-optical modulator (40).

10. The optical scanning device according to claim 9, wherein the acousto-optical modulator is adapted to generate a diffraction pattern (41) as well as a phase-modulation between the at least two higher order diffracted radiation beams and the zero-order radiation beam.

11. The optical scanning device according to claim 8, wherein the phase- modulating means is an electro-optical modulator (50).

12. The optical scanning device according to claim 11, wherein the electro-optical modulator comprises a liquid crystal material (52) and a patterned electrode configuration (51) for generating a diffraction grating as well as a phase-modulation between the at least two higher order diffracted radiation beams and the zero-order radiation beam.

13. The optical scanning device of claim 12, the electro-optical modulator being adapted to cooperate with a modulation to be supplied to the patterned electrodes in order to generate a phase modulation in the at least two diffracted radiation beams such that the phase modulation amplitude is substantially π.

14. An optical data drive comprising an optical scanning device according to any one of claims 1 to 13.

15. The optical data drive according to claim 14, adapted to generate push-pull servo signals for scanning an optical record carrier, the frequency of the phase-modulation of the higher-order diffracted radiation beams being larger than the servo-loop bandwidth of the push-pull servo electronics.

16. A method of reducing coherent crosstalk in an optical scanning device for scanning an optical record carrier having multiple information layers due to interference between radiation beams reflected by the information layer being scanned and radiation beams being reflected by information layers not being scanned, the optical scanning device comprising a radiation source for generation of a radiation beam, the radiation beam having a phase and a diffraction means for generation of a zero-order radiation beam and at least two higher-order diffracted radiation beams from the radiation beam, characterized in that the method comprises the step of generating a phase-modulation between the at least two higher- order diffracted radiation beams and the zero-order radiation beam.

17. The method of claim 16, further comprising the step of setting the frequency of the phase-modulation of the higher-order diffracted radiation beams at a higher frequency than the servo-loop bandwidth of the push-pull servo electronics.