WO2003046897A2

WO2003046897A2 - Optical scanning device

Info

Publication number: WO2003046897A2
Application number: PCT/IB2002/005082
Authority: WO
Inventors: Martin D. Liess
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2001-11-29
Filing date: 2002-11-29
Publication date: 2003-06-05
Also published as: WO2003046897A3; AU2002351079A1; AU2002351079A8

Abstract

An optical scanning device for an optical record carrier, said device including a radiation source (9) for generating a radiation beam to be scanned across a record carrier and optical elements for directing the beam to form a spot in the optical record carrier, said optical elements including an electro-optical element (16) which includes material having a refractive index which is variable in accordance with an applied signal, said electro-optical element being arranged to steer the beam, in response to the applied signal, in a beam steering direction so as to move the beam spot in a direction across a scanning surface of the optical record carrier and/or along the beam axis.

Description

Optical scanning device

This invention relates to an optical scanning device for scanning optical record carriers.

In known optical scanning devices, for example compact disc (CD) and digital versatile disc (DVD) recorders and players, an actuated optical pick up unit (OPU) is used. The OPU includes a lens which is moved radially, perpendicular to the tracks of the optical disc, and vertically, towards and away from the disc, in order to perform fine track following and fine focus adjustments. The movement is performed using mechanical actuators of a high bandwidth in order to provide fast servo-based control in response to the detection of tracking error or focus error in the signal detection part of the optical scanning device.

A paper by Stankovic et al., "Integrated Optical Pickup System for Axial Dual Focus", Applied Optics, Vol. 40, No. 5, 10 February 2001 describes an adaptive liquid crystal element that uses liquid crystal material to create two axial foci separated by a distance corresponding to the separation of the two layers of a dual-layer DVD. It would be desirable to reduce the size of optical scanning devices, to allow optical data storage media to be used as data storage devices for smaller and more lightweight consumer electronics devices.

In accordance with one aspect of the present invention, there is provided an optical scanning device for an optical record carrier, said device including a radiation source for generating a radiation beam to be scanned across a record carrier and optical elements for directing the beam to form a spot in the optical record carrier, said optical elements including an electro-optical element which includes material having a refractive index which is variable in accordance with an applied signal, said electro-optical element being arranged to steer the beam, in response to the applied signal, in a beam steering direction so as to move the beam spot in a direction across a scanning surface of the optical record carrier.

Use of such an electro-optical element allows for the omission, or reduction in size, of the mechanical tracking actuators in the optical head of a scanning device, thereby allowing miniaturization of the device. According to a further aspect of the invention, there is provided an electro- optical element comprising a liquid crystal layer and an electrode structure arranged to provide a varying refractive index in the liquid crystal layer in dependence upon voltages applied thereto, wherein said electrode structure includes a plurality of electrode zones arranged on a surface of the element, each zone comprising a first contact means and a second contact means, said device being arranged such that each said first contact means receives a first common voltage and each said second contact means receives a second common voltage.

This arrangement allows a Fresnel-type configuration providing an increased beam steering or lensing effect with available voltages and available variations in the liquid crystal refractive index under the application of a common, variable voltage.

Further objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings, in which:

Figure 1 shows a scanning device according to the invention;

Figure 2 shows a cross-section of a first embodiment of electro-optical element used in the arrangement of Figure 1;

Figure 3 shows a cross-section of a second embodiment of electro-optical element;

Figure 4 shows a cross-section of a third embodiment of electro-optical element; and

Figure 5 shows a cross-section of a fourth embodiment of electro-optical element.

Figure 1 shows elements of an optical scanning device, arranged in accordance with an embodiment of the invention, including an optical head for scanning an optical record carrier 2. The record carrier is in the form of an optical disk comprising a transparent layer 3, on one side of which an information layer 4 is arranged. The side of the information layer facing away from the transparent layer is protected from environmental influences by a protection layer 5. The side of the transparent layer facing the device is called the entrance face 6. The transparent layer 3 acts as a substrate for the record carrier by providing mechanical support for the information layer. Alternatively, the transparent layer may have the sole function of protecting the information layer, while the mechanical support is provided by a layer on the other side of the information layer, for instance by the protection layer 5 or by a further information layer and a transparent layer connected to the information layer 4. Information may be stored in the information layer 4 of the record carrier in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in Figure 1. The marks may be in any optically readable form, e.g. in the form of pits, or areas with a reflection coefficient or a direction of magnetization different from their surroundings, or a combination of these forms.

The scanning device 1 comprises a radiation source in the form of a semiconductor laser 9 emitting a radiation beam 7. The radiation beam is used for scanning the information layer 4 of the optical record carrier 2. A beam splitter 13 reflects the diverging radiation beam 12 on the optical path towards a collimator lens 14, which converts the diverging beam 12 into a collimated beam 15. The collimated beam 15 is incident on a transparent electro-optical element 16, which modifies the wavefront of the collimated beam. The beam 17 from the electro-optical element 16 is incident on an objective system 18. The objective system may comprise one or more lenses and/or a grating. The objective system 18 in Figure 1 consists in this example of two elements, a first lens 18a and a second lens 18b. The objective system 18 has an optical axis 19. The objective system 18 changes the beam 17 to a converging beam 20 incident on the entrance face 6 of the record carrier 2. The objective system has a spherical aberration correction characteristic adapted for passage of the radiation beam through the thickness of the transparent layer 3. The converging beam 20 forms a spot 21 on the information layer 4.

Radiation reflected by the information layer 4 forms a diverging beam 22, transformed into a substantially collimated beam 23 by the objective system 18 and subsequently into a converging beam 24 by the collimator lens 14. The beam splitter 13 separates the forward and reflected beams by transmitting at least part of the converging beam 24 towards a detection system 25. The detection system captures the radiation and converts it into electrical output signals 26. A signal processor 27 converts these output signals to various other signals, which are processed by signal processing circuits 29 and 31. The processing circuits 27, 29 and 31 are located in the scanning device separately from the optical head 1.

One of the signals is an information signal 28, the value of which represents information read from the information layer 4. The information signal is processed by an information processing unit for error correction 29. Other signals from the signal processor 27 are the focus error signal and radial error signal 30. The focus error signal represents the axial different in height between the spot 21 and the information layer 4. The radial error signal represents the distance in the plane of the information layer 4 between the spot 21 and the center of a track in the information layer to be followed by the spot.

The focus error signal and the radial error signal are fed into a servo circuit 31, which converts these signals to a focus error signal for controlling a mechanical focus actuator (not shown) in the optical head and a tracking error signal 32 for controlling the electro-optical element 16, respectively. The mechanical focus actuator controls the position of the objective system 18 in the focus direction 33, thereby controlling the actual position of the spot 21 such that it coincides substantially with the plane of the information layer 4. A further mechanical actuator, such as a radially movable arm, alters the position of the optical head 1 in a radial direction 34 of the disk 2, thereby coarsely controlling the radial position of the spot 21 to lie above a track to be followed in the information layer 4. The tracks in the record carrier 2 run in a direction perpendicular to the plane of Figure 1.

Figure 2 illustrates a first embodiment of electro-optical element 16 used to perform fine tracking error correction. The electro-optical element includes two parallel transparent plates 40, 41 separated by spacers 42, 43, to form an enclosed rectangular space containing a planar layer of liquid crystal material 44. The liquid crystal material may be any suitable type of liquid crystal material providing a refractive index variation dependent upon the orientation of the liquid crystal molecules inside the electro-optical element 16. In one example, a ferroelectric liquid crystal material is used, in order to provide fast response times whereby the electro-optical element can provide tracking control at fast response speeds, i.e. to provide a high bandwidth modulation of the beam.

The inside surfaces 45, 46 are covered with an orientation layer, to provide an initial alignment of the liquid crystal molecules without the application of a voltage signal, and a transparent electrode, covering the whole of each of inner surfaces 45, 46, formed for example of indium tin oxide (ITO) material. The electrode on one of the inner surfaces, for example surface 46, is adapted for the application of a single control voltage over the entire electrode, via electrical contact 47. The electrode on the other inner surface 45 is adapted for the application of a linearly- varying voltage signal across its width, between electrical contact lines 48, 49, which are arranged to be parallel, normal to the surface of the page as shown in Figure 2. During operation, voltage V₀ is applied to contact 47, voltage Vi is applied to contact 48, and voltage V₂ is applied to contact 49. Voltage V] and/or voltage V₂ are varied in accordance with the radial tracking error signal 32. The transparent conductive layer on the inner surface 45 is uniformly resistive, such that a linear voltage gradient is applied across the width of the beam 15.

As shown in Figure 2, the beam position may be altered in a beam steering direction S by control of the applied voltages in accordance with a tracking error signal. The beam may be steered from an axial position parallel to optical axis 19, which is achieved when the voltage across inner surface 45 is constant so that the exit beam 17 is parallel to the incoming beam 15, to an off-axis position, which is achieved when the voltage across the inner surface 45 varies linearly across the full width of the beam so that the exit beam 17' is angled with respect to the incoming beam 15. The beam is steered off- axis as shown, if a generally linear gradient in refractive index is achieved in the liquid crystal layer 4. By applying similar voltages to Vi and V₂ in reverse, a similar off-axis steering can be achieved in the opposite direction. The objective system 18 converts varying beam positions into varying positions of the beam spot 21 in the information layer 4. By varying the voltages Vi and V₂ applied to the contacts 48 and 49, the position of the beam spot on the disk may be continuously varied between two extreme off-axis positions in order to effect servo-based tracking using the electro-optical element 16.

Figure 3 illustrates a second embodiment of electro-optical element 116. In Figure 3, components are labeled with the same numbers, incremented by 100, as similar components in the first embodiment, and descriptions thereof, and of alternatives thereto, will not be repeated but apply here also. In the second embodiment, instead of operating in a transmissive mode, the electro-optical element 116 operates in reflective mode, with inner surface 145 including a radiation mirroring material. The element 116 is angled at approximately 45° with respect to the incoming beam 115, and the outgoing beam 117 is angled at approximately 90° with respect to the incoming beam. In Figure 3, the incoming beam 115 is in a divergent state, since collimator 14 has been omitted, although it is to be appreciated that the collimator may also be used in this embodiment.

By varying the voltage Vi applied to contact 148 and/or the voltage V₂ applied to contact 149, the exit beam may be steered in direction S from an axial position, achieved when the voltage across inner surface 145 is constant so that the exit beam 117 is at 90° to the incoming beam 115, to an off-axis position, which is achieved when the voltage across inner surface 145 varies linearly across the full width of the beam. In the off-axis position the exit beam 117', is angled at less than 90° to the incoming beam 115. Again, the steering can be achieved in the opposite direction by applying the voltages in a reverse order. In this embodiment, since the radiation beam passes through the liquid crystal layer 114 of the electro-optical element 116 twice, an increased radial corrective effect may be achieved by use of a liquid crystal layer of a similar thickness, thereby maintaining a response speed whilst increasing the beam steering effect. The electro-optical element 116 of the second embodiment may be placed in the arrangement shown in Figure 1 in place of element 16, with the optical path forming an additional 90° fold. Alternatively, the element 116 may be placed between the radiation source 9, which in this case is angled at 90° with respect to the arrangement shown in Figure 1, and the beam splitter 13. If, as is well-known in the art, beam splitter 13 is a polarization- dependent beam splitter, the element 116 may be used in place of such a beam splitter 13, by including a polarization-dependent mirroring element on the inner surface 145, or adjacent the plate 141. The polarization-dependent mirror element may for example be in the form of a wire grid polarizer.

Figure 4 shows a third embodiment of the invention. Elements similar to those of the first embodiment are provided with the same reference numerals, incremented by 200, and descriptions thereof, and of alternatives thereto, will not be repeated but apply here also. In this embodiment, the electro-optical element 216 is capable not only of tracking error conection but also focus error correction. In order to provide the tracking error correction, a variable linear voltage gradient is applied across the inner surface 245, similar to the arrangements in the first and second embodiments.

In order to provide the focus error correction, a voltage is applied to the inner surface 246 which is radially varying but circularly symmetric with respect to the center of the beam. In order to provide the desired voltage pattern, the electrode may for example be formed in a circular ring concentric with the beam axis. A varying voltage V is applied to a contact 247 supplying the voltage to the electrode in order to perform focus conection.

Thus, the liquid crystal layer 244 may be controlled to provide a refractive index pattern within the liquid crystal layer 244 which has both a component which varies linearly across the width of the beam and a component which varies radially from the center of the beam outwards. By zeroing voltages V_{1 ;} V₂ and V₃, a non-modulating effect is obtained, whereby the output beam 217 remains parallel with the input 215 and collimated, in the design of Fig. 1. By maintaining a zero effective voltage V₃, to provide no voltage variation across the inner surface 246, and a different voltage at each of Vi and V₂ to obtain a linearly varying voltage across inner surface 245, a tracking enor correcting effect may be obtained to steer the beam in direction S. For example, as shown, exit beam 217' is steered off-axis as in the first embodiment. By applying a similar voltage at Vi and V₂, but a different voltage at V₃, a focus enor conecting effect may be obtained, so that exit beam 217" is made convergent. A divergent beam may be obtained by applying the two different voltages in reverse. Furthermore, by controlling the various voltages applied at V], V and V₃, continually varying variations of the beam modulation effect may be obtained to provide both continual servo-based tracking error conection and focus error correction.

Figure 5 illustrates a fourth embodiment of the invention in which an electro- optical element 316 is provided for tracking enor conection and focus enor conection functions in reflective mode. The element functions as a folding minor and is ananged at 45° to the optical axis of the incident beam. Elements similar to that of the second embodiment, are indicated with the same reference numerals, incremented by 200, and descriptions thereof, and of alternatives thereto, will not be repeated but apply here. In this case, a variable linear voltage gradient is applied across transparent inner surface 353, between parallel contact lines 350 and 351. A variable radial voltage variation is generated across reflective inner surface 354 by use of an electrode driver 352 providing a plurality, for example five, of variable voltages V₃, V , V₅, V₆ and V₇ to a corresponding plurality of concentric elliptical electrodes formed on inner surface 354. The electrodes are elliptical to provide a circular phase variation in the beam; thus the shapes of the elliptical electrodes, when projected onto a plane arranged at 45° to the element, form circular outlines. The electrode driver 352 varies the voltages applied to the electrode formation in accordance with a focus enor signal F_s, whilst the tracking enor signal is used to vary the voltage gradient across inner surface 353. The position of the beam may be varied in direction S between an axial beam position 317 and an off-axis beam position 317' for tracking conection. Similarly, the vergence of the beam may be varied between a beam 317 of an initial vergence and a less divergent beam 317" by varying the voltages applied radially by electrode driver 352. Opposite effects may be obtained by reversing the applied voltages, and continually varying variations of the beam modulation effect may be obtained as required.

Figure 6 illustrates the electrode arrangement, taking a plan view of the electro-optical element 316. The surface containing the electrodes is covered in a resistive layer 360, onto which electrodes 362 are applied. As shown, the electrodes are in the form of a plurality of concentric elliptical electrodes, each receiving a different voltage V₃-V .

In a fifth embodiment, not shown, an electro-optical element as in the second embodiment is altered in a similar manner to that of the third embodiment, whereby an electro-optical element with both tracking enor conection and focus enor conecting functions may be provided for use in reflective mode without use of a multi-electrode driver.

Figure 7 illustrates in plan view an alternative anangement of electrodes for use in providing beam-steering using an electro-optical element 416. The anangement may be used on the respective surface of any of the above-described elements. In this case, the elements are ananged in a Fresnel configuration, using an inter-digitating anay of contacts 404, 406 arranged upon separate rectangular transparent resistive electrode zones 402. The zones 402 are separated by insulating material and each zone consists of a uniform sheet of resistive material. A linear voltage gradient is created in the liquid crystal material by applying different voltages V_{l s} V₂, across each of the different zones 402, thus creating a generally sawtooth-like profile in refractive index in the liquid crystal layer, with each sawtooth having a gradient which is equal and generally linear. In this way, the effect of a single linear refractive index gradient across the full beam width is achieved, in terms of steering a beam, however an increased amount of beam steering can be achieved with the available voltage differences, since the refractive index gradients are greater.

Figure 8 illustrates in plan view a further anangement of annular electrodes suitable for use in relation to the anangement of the third and fourth embodiments of the invention. This provides a Fresnel-type configuration, in which the effect of a Fresnel lens is achieved by including a plurality of annular resistive electrode zones 502 separated by insulating areas on the reflective surface of the electro-optical element 516. Each zone 502 consists of a uniform sheet of resistive material. One of each of a set of circular contacts 504 is located at the outermost circumference of each zone, each receiving a common first voltage Vi. One of each of a set of circular contacts 506 is located at the innermost circumference of each annular film, and receives a common second voltage V₂. Thus, in profile along the radius of the element 516, a sawtooth-like voltage profile is produced, to provide a suitable refractive index profile, or component thereof, in the electro-optical element 516. The voltage profile generated is shaped similarly to the physical profile of a Fresnel lens, namely the innermost of the zones 502 occupy a greater radial distance than the outermost zones, with a gradual decrease in the zone width towards the outside of the element 516. The voltage gradient, and hence the refractive index gradient produce, in each zone generally increases from the center of the lens outwards, thereby providing a lensing effect.

By ananging the lens in this way, the lens can be driven, to provide a variable focusing effect, by use of only two different voltages Vj, V₂ common to the electrodes, thereby simplifying the drive for the electro-optical element 516, and providing an increased lensing effect with the available voltage differences.

Note that Figure 8 illustrates an arrangement of electrodes which is circular, and suitable for use in the third embodiment. For use in the fourth embodiment a similar anangement of elliptical electrodes should be used.

In further alternative embodiments, the third or fourth embodiments may be altered to include both electrode configurations as illustrated in Figures 7 and 8. The electric field and refractive index pattern that is generated in the liquid crystal layer, resulting from the superposition of the linear and radial sawtooth from both respective sides, can be very complicated, however it can be generated from only two voltages of which one controls the radial tracking and the other the focal depth.

Note in relation to the above that, where the electro-optical element provides focus enor conection, the focus enor conecting functions of the electro-optical element may replace the mechanical focus control actuator of the optical head. Alternatively, the electro- optical element may be used for fine focus enor conection whilst an additional mechanical actuator may be used for coarse focus enor conection.

Note that, herein, the term "circular" and "elliptical" are not intended to be limited to perfectly circular or elliptical anangements; for example some degree of linear asymmetry, such as up to one tenth radial variation between different halves of the pattern, may be included in order for example to compensate for comatic or other abenations created in the beam.

The above embodiments are to be understood as illustrative examples of the invention. It is to be noted that the location of the electro-optical element in the optical path may be different to that described above. In the above it is stated that the voltage variation across one side of the liquid crystal layer varies linearly. A different voltage variation may also be applied in order to achieve a gradually monotonically varying refractive index, which is preferably linear, across the full width of the beam, or parts thereof, in order to provide the tracking enor signal conecting functions. It is to be understood that any feature described in relation to one embodiment may also be used in other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS:

1. An optical scanning device for an optical record carrier, said device including a radiation source for generating a radiation beam to be scanned across a record carrier and optical elements for directing the beam to form a spot in the optical record carrier, said optical elements including an electro-optical element which includes material having a refractive index which is variable in accordance with an applied signal, said electro-optical element being arranged to steer the beam, in response to the applied signal, in a beam steering direction so as to move the beam spot in a direction across a scanning surface of the optical record canier.

2. An optical scanning device according to claim 1, wherein said material is ananged in a planar layer in said electro-optical element.

3. An optical scanning device according to claim 1 or 2, wherein said material comprises a liquid crystal material.

4. An optical scanning device according to any preceding claim, wherein said electro-optical element is ananged to be capable of generating, in response to an applied signal, a variation in refractive index across the beam.

5. An optical scanning device according to claim 4, wherein said electro-optical element is ananged to be capable of generating, in response to an applied signal, a monotonically varying refractive index across the full width of the beam in the beam steering direction.

6. An optical scanning device according to claim 4 or 5, wherein said electro- optical element is ananged to be capable of generating, in response to an applied signal, a refractive index varying with a sawtooth-like profile across the beam in the beam steering direction.

7. An optical scanning device according to any preceding claim, wherein said electro-optical element comprises a resistive electrode or electrodes ananged to exhibit, in response to an applied signal, a gradual variation in voltage in the beam steering direction.

8. An optical scanning device according to any preceding claim, wherein the optical record carrier is adapted to store data in the form of tracks, wherein the device is ananged to generate a tracking error signal during scanning of the optical record carrier, and wherein said electro-optical element is ananged to move the beam spot in the beam steering direction in response to the tracking enor signal.

9. An optical scanning device according to any preceding claim, wherein said electro-optical element is ananged in a path of the radiation beam such that the beam passes through the material at least twice before reaching the optical record carrier.

10. An optical scanning device according to claim 9, wherein the element is ananged at an angle of approximately 45° in relation to an axis of the optical beam from the radiation source, and wherein the element operates in a reflective mode.

11. An optical scanning device according to any preceding claim, wherein said electro-optical element is ananged to be capable of generating, in response to an applied signal, a refractive index having at least a component varying generally with a circular symmetry in relation to the beam to provide beam focus enor conection.

12. An optical scanning device according to claim 11, wherein said electro-optical element comprises an annular electrode.

13. An electro-optical element comprising a liquid crystal layer and an electrode structure ananged to provide a varying refractive index in the liquid crystal layer in dependence upon voltages applied thereto, wherein said electrode structure includes a plurality of electrode zones ananged on a surface of the element, each zone comprising a first contact means and second contact means, said device being ananged such that each said first contact means receives a first common voltage and each said second contact means receives a second common voltage.

14. An electro-optical element according to claim 13, wherein said zones are separated by insulating material and wherein said first and second contact means are ananged at opposite peripheries of each respective zone.

15. An electro-optical element according to claim 13 or 14, wherein said zones are generally rectangular and ananged side-by-side.

16. An electro-optical element according to claim 13 or 14, wherein said zones are generally annular and ananged concentrically.