WO2004059629A1

WO2004059629A1 - Dual layer birefringent optical component

Info

Publication number: WO2004059629A1
Application number: PCT/IB2003/006064
Authority: WO
Inventors: Emile J. K. Verstegen; Hendrik R. Stapert
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-12-30
Filing date: 2003-12-10
Publication date: 2004-07-15
Also published as: AU2003285653A1

Abstract

An optical component (181) comprises two adjacent birefringent materials (170, 203) with a curved interface (206) between the materials.

Description

Dual layer birefringent optical component

The present invention relates to optical components comprising a birefringent material, devices including such components and methods of manufacturing such components and devices. The component is particularly suitable for but not limited to, use as an optical element in optical scanning devices.

Optical pickup units for use in optical scanning devices are known. The optical pickup units are mounted on a movable support for scanning across the tracks of the optical disk. The size and complexity of the optical pickup unit is preferably reduced as much as practicable, in order to reduce the manufacturing cost and to allow additional space for other components being mounted in the scanning device.

Modern optical pickup units are generally compatible with at least two different formats of optical disk, such as the Compact Disc (CD) and the Digital Versatile Disc (DND) format. Recently proposed has been the Blu-ray Disk (BD) format, offering a data storage capacity of around 25GB (compared with a 650MB capacity of a CD, and a 4.7GB capacity of a DVD).

Larger capacity storage is enabled by using small scanning wavelengths and large numerical apertures (ΝA), to provide small focal spots, (the size of the focal spot is approximately λ/ΝA), so as to allow the readout of smaller sized marks in the information layer of the disk. For instance, a typical CD format utilises a wavelength of 785nm and has an objective lens with a numerical aperture of 0.45, a DND uses a wavelength of 650nm and has a numerical aperture of 0.65, and a BD system uses a wavelength of 405nm and a numerical aperture of 0.85.

Typically, the refractive index of materials vary as a function of wavelength. Consequently, a lens will provide different focal points and different performance for different incident wavelengths. Further, the discs may have different thickness transparent layers, thus requiring a different focal point for different types of discs.

In some instances, storage capacity is further increased by increasing the number of information layers per disc. For example, a dual layer BD-disc has two information layers separated by a 25μm thick spacer layer. Thus, the light from the optical pickup unit has to travel through the spacer layer when focusing on the second information layer. This introduces spherical aberration, the phenomenon that rays close to the axis of the converging cone of light have a different focal point compared to the rays on the outside of the cone. This results in a blurring of the focal spot, and a subsequent loss of fidelity in the read-out of the disc.

To enable dual layer readout and backward compatibility (i.e. the same optical system being used for different disc formats), polarisation sensitive lenses (PS-Lenses) have been proposed to compensate for spherical aberration. Such lenses can be formed of a birefringent material, such as a liquid crystal. Birefringence denotes the presence of different refractive indices for the two polarisation components of a beam of light. Birefringent materials have an extraordinary refractive index (lie) and an ordinary refractive index (n_o), with the difference between the refractive indices being Δn^-n_o PS lenses can be used to provide different focal points for a single or different wavelengths by ensuring that the same or different wavelength(s) are incident upon the lens with different polarisations.

It is an aim of embodiments of the present invention to provide an improved optical component which addresses one or more of the problems of the prior art, whether referred to herein or otherwise.

It is an aim of particular embodiments of the present invention to provide a birefringent lens having a focusing power that varies in a novel manner in dependence upon the polarisation of the incident radiation beam.

It is an aim of particular embodiments of the present invention to provide a birefringent lens that can be switched to a neutral state such that it does not alter the direction of incident light, as well as a method of manufacturing such a lens.

In a first aspect, the present invention provides an optical scanning device for scanning an information layer of an optical record carrier, the device comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam on the information layer, wherein the device includes an optical element comprising at least two adjacent birefringent materials with a curved interface (206) between the materials. In a typical birefringent lens, the focusing power of the lens is dependent upon the polarisation of the radiation beam relative to the ordinary and extraordinary axis of the birefringent material. However, by providing two birefringent materials, the optical function of the curved surface (which may, in some embodiments, function as a lens) is dependent upon the polarisation of the incident radiation beam relative to the orientation of the respective ordinary and extraordinary axis of both materials. This allows the optical element to provide various optical functions. In another aspect, the present invention provides an optical component comprising at least two adjacent birefringent materials with a curved interface between the materials.

In a further aspect, the present invention provides a method of manufacturing an optical scanning device for scanning an information layer of an optical record carrier, the method comprising the steps of: providing a radiation source for generating a radiation beam; providing an optical element , the optical element comprising at least two adjacent birefringent materials with a curved interface between the materials. h a further aspect, the present invention provides a method of manufacturing an optical component, the method comprising: providing at least two adjacent birefringent materials with a curved interface between the materials.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

Fig. 1 illustrates an optical component in accordance with a prefened embodiment of the present invention;

Figs. 2A-2F illustrate method steps in the formation of a first portion of a liquid crystal lens in accordance with a prefened embodiment of the present invention; Figs. 3A-3C illustrate method steps for completing the formation of the liquid crystal lens of Figures 2A-2F in accordance with one embodiment of the present invention;

Figs. 4A-4C illustrate alternative method steps for completing the formation of the liquid crystal lens of Figures 2A-2F in accordance with another embodiment of the present invention; Fig. 5 illustrates a device for scanning an optical record carrier including a liquid crystal lens in accordance with an embodiment of the present invention;

Figs. 6 A and 6B illustrate how the optical system of the scanning device shown in Figure 5 may be used with different polarisations of light to scan different layers within a dual layer optical record carrier; and Fig. 7 illustrates an optical component in accordance with a further embodiment of the present invention.

Optical components (or portions of optical components, optical elements) typically include curved surfaces so as to focus light (e.g. a convex lens) or disperse light (e.g. a concave lens). Birefringent optical components with curved surfaces will provide different focussing or dispersive effects, dependent upon the angle at which the polarised radiation beam is incident on the optical component. The present inventors have realised that, by providing an additional birefringent material adjacent to the curved surface, then the optical function (e.g. focusing or dispersive effect) of the surface will be dependent upon the orientation of the refractive indices of both materials relative to each other, as well as the orientation of the materials relative to the polarisation of the incident radiation. By varying the orientation of the refractive indices of the two birefringent materials, a range of optical functions can be produced, with the optical functions being dependent upon the polarisation angle of the radiation beam. For instance, a lens can be produced that is neutral (i.e. has no focusing or dispersive effect) at a predetermined angle of incidence. Various other functions are described below, and, further functions will be apparent to the skilled person from the teaching herein.

In inorganic birefringent materials (e.g. a crystal such as calcite) the atomic structure is non-symmetric. This leads to an anisotropy in the physical constants of materials in different directions. One of those is the refractive index. Consider a polarised beam of light traversing along different optical axis. There will be one optical axis in which a different refractive index will be observed upon traversion perpendicularly and parallel to the optical axis. In general, but not always, two out of three axes have a refractive index that is higher than the refractive index of the third axis.

In organic crystals, such as a liquid crystal, a similar phenomenon occurs although one can of course not talk about a difference in the atomic structure but only of orientational order within the liquid that resembles a crystal structure. Generally, although not always, two out of three axes have a refractive index that is lower than in the third axis. The direction in which the molecules of a liquid crystal are aligned is called the director. Light propagating with its plane of polarisation parallel to the director experiences the extraordinary refractive index, i-_e. Figure 1 illustrates an optical component 181 in accordance with a prefened embodiment of the present invention. The optical component 181 can be envisaged as being formed of two portions or layers. The first portion is a planoconvex lens formed of a first birefringent material 203. Since the birefringent material is made of a typical liquid crystal, it has two ordinary axes yielding an ordinary refractive index n_oi and one extraordinary axis yielding a refractive index i ei. The second portion of the component comprises a planoconcave lens formed of a second birefringent material 170. The second birefringent material is also a typical liquid crystal, and it has two ordinary axis with a refractive index n^ and one extraordinary axis yielding a refractive index r-_e2. The curved interface 206 between the two portions conesponds to the convex surface of the planoconvex lens mating with the concave surface of the concave lens. An optical axis 19 passes through the first material 203, the second material 170 and the curved interface 206.

If the curved interface 206 of the lens is a spherical interface between the two layers, then the focal length f of the lens is :

R

/ =

(n eff \ ^Ueff 2

where R is the radius of curvature of the spherical interface, n^n is the effective refractive index of the first material 203, and n^ is the effective refractive index of the second birefringent material 170. The effective refractive index is the resultant refractive index experienced by a polarised beam of radiation due to the angle of incidence of the radiation relative to the ordinary and extraordinary axis of the respective material.

In the particular prefened embodiment shown in Figure 1, the same birefringent material is used as the first material 206 and the second material 170. Consequently, noi=n_o2 and n_eι=ns₂. Further, in this embodiment, the material is an anisotropically aligned liquid crystal.

In this embodiment, the first material is oriented with the directors of the liquid crystal in a plane perpendicular to the optical axis 19. Thus, a polarised radiation beam incident on the optical component will experience a refractive index of between i-_oi and n_eι when passing through the first material 203. The precise effective refractive index experienced by the radiation beam will depend upon the angle of polarisation of the radiation beam relative to the director of the liquid crystal. In contrast, in this particular embodiment, the director of the second material 170 is parallel to the optical axis 19. Thus, a polarised radiation beam incident on the optical component along the optical axis will only experience the ordinary refractive index (no ) when traversing the second material 170. It will be appreciated that, when the polarised light is incident upon the optical component 181 along the ordinary axis of the first birefringent material 203 (with its plane of polarisation perpendicular to the director of that material), then as

the light will not experience any lens effect i.e. the component will act as an optically neutral component.

However, when the polarisation of the polarised light incident upon the optical component 181 is no longer perpendicular to the director of the first material, the refractive index of the first material 203 will be greater than the refractive index of the second material 170, such that the optical component acts as a planoconvex lens. The power of the lens will be a function of the angle of tilt of the plane of polarisation of the light relative to the director of the first material. It will however be appreciated that various alternative orientations of the two liquid crystals may exist. For instance, table 1 summarises the lens types achieved using a range of orientations of the first and second materials. In table 1, it is assumed that the same material is utilised as both the first material and the second material (or at least the two materials have identical ordinary and extraordinary refractive indices).

Table 1

In table 1 : " | " indicates the director of the liquid crystal is parallel to the optical axis 19; " — " indicates that the director the liquid crystal is in a plane perpendicular to the optical axis; "•" indicates the director of the liquid crystal is in a plane perpendicular to the optical axis, and the director is also perpendicular to the director shown by " — " ; and V indicates that the director is tilted at an angle with respect to the optical axis. In terms of the lens types achieved, "n" indicates that a neutral lens is achieved; "++" indicates that a positive lens is achieved, "+" indicates that a weakly positive lens is achieved; "-" indicates that a weakly negative lens is achieved, and "- -" indicates that a negative lens is achieved. It will be appreciated that, in all orientations except where the lens is neutral (n), then the exact power of the lens will also be dependent upon the polarisation angle of the incident radiation beam.

Consequently, the same liquid crystal may be used to produce a range of polarisation dependent lenses with different optical powers, the strength (and type) of the lens simply being altered by adjusting the relative orientation of the two materials forming the lens. The two materials forming the lens can of course be the same material e.g. the same liquid crystal or mix of liquid crystals. This allows a uniform manufacturing technique to be applied to produce a range of lenses with a range of properties. Figures 2 A-2F illustrate respective steps in forming a first portion of an optical component in accordance with a prefened embodiment of the present invention.

In the first step, shown in Figure 2 A, mould 100 is provided, the mould having a shaped surface 102 which subsequently serves to define a portion of the shape of the resulting optical component, h this particular instance, the liquid crystal is ultimately photopolymerised, and consequently the mould is formed of a material transparent to the radiation used to polymerise the liquid crystal e.g. glass.

An alignment layer 110 is ananged on the curved surface 102, so as to induce a predetermined orientation (indicated by the anow direction 110) in the liquid crystal subsequently placed upon the alignment layer. In this particular example, the alignment layer is a layer of polyimide (PI). The polyimide may be applied using spincoating from a solution. The polyimide may then be aligned so as to induce a specific orientation (this orientation determimng the resulting orientation of the liquid crystal molecules). For instance, a known process is to rub the polyimide layer with a non-fluff cloth repeatedly in a single direction so as to induce this orientation (110).

A substrate 150, which in this particular embodiment will form part of the optical component, has a bonding layer 120 applied to a first surface 152. The bonding layer is ananged to form a bond with the liquid crystal. In this particular instance, the bonding layer is also an alignment (or orientation) layer comprising polyimide. The bonding layer contains reactive groups ananged to form a chemical bond with the liquid crystal molecules, and in this instance has the same type of reactive group as the liquid crystal molecules, such that when photopolymerising the liquid crystal molecules, chemical bonds with the bonding layer on the substrate are also created. This results in very good adhesion between substrate and the liquid crystal layer. The bonding layer may be deposited on the substrate using the same type of process used to deposit and align the alignment layer on the mould 100. The bonding layer, which in this instance also functions as an alignment layer, is oriented in a predetermined orientation (anow 120) depending upon the desired properties of the resulting liquid crystal components. The bonding layer is aligned so as to be parallel to the direction 110 of the alignment layer on the mould. Preferably, the orientation of the bonding layer is parallel but in the opposite direction to the orientation of the alignment layer.

As illustrated in Figure 2B, a compound 200 incorporating one or more liquid crystals is then placed between the first surface 152 of the substrate 150 and the shaped surface 102 of the mould 100.

In this particular example, as illustrated in Figure 2B, the compound 200 comprises a mixture of two different liquid crystals. These two different liquid crystals have been chosen so as to provide the desired refractive index properties once at least one of the liquid crystals has been polymerised.

A droplet of the liquid crystals 200 is placed on the first surface 152 of the substrate. The compound 200 has been degassed, so as to avoid the inclusion of air bubbles within the resulting optical component. It also avoids the formation of air bubbles from dissolved gases coming out of the solidifying liquid during polymerisation, as the shrinkage during polymerisation leads to a large pressure decrease inside the polymerising liquid. The glass mould is then heated so that the liquid crystal is in the isotropic phase (typically to about 80°C), so as to facilitate the subsequent flow of the liquid crystal into the desired shape.

The substrate and the mould are subsequently brought together, so as to define the shape of the liquid crystal portion 201 of the final resulting optical component (Figure 2C). In order to ensure that the liquid crystal forms a homogenous layer between the mould and the substrate, a pressure may be applied to push the substrate towards the mould (or vice versa).

The substrate/mould/liquid crystal may then be cooled, for instance down to room temperature for 30 minutes, so as to ensure that the liquid crystal enters the nematic phase, coming from the isotropic phase.

When entering the nematic stage, multi domains may appear in the liquid crystal mixture. Consequently, the mixture can be heated to above the clearing point to destroy the multidomain orientation (e.g. the mixture may be heated for 3 minutes to 105°C). Subsequently, the mixture may be cooled to obtain a homogenous orientation 202 (Figure 2D).

The homogenous liquid crystal mixture may then be photopolymerised using light 302 from an ultra violet radiation source 300 (Figure 2E), for instance by applying a UV-light intensity of 10mW/cm² for 60 seconds. At the same time, chemical bonds will be formed between the liquid crystal and the bonding layer.

Subsequently, the first element (or portion) of the optical component (150, 203) can be released from the mould 100 (Figure 2F). This could, for instance, be achieved by slightly bending the mould 100 over a cornered object 400. Alternatively, it could be achieved by pressing a portion of the flat substrate in a flat support, so as to slightly bend the flat substrate. The liquid crystal/substrate element should separate easily from the mould, as a conventional polyimide (without reactive groups) is used on the mould.

The mould can be reused to produce subsequent elements of components, by repeating steps illustrated in Figures 2B-2F. Typically, the alignment layer will remain upon the mould 100, and hence does not need to be reapplied.

If desired, a further processing step can be performed to remove the liquid crystal 202 from the substrate 150. However, in most instances it is assumed that the substrate 150 will form part of the final optical component. Figures 3A-3C illustrate the processing steps in accordance with one embodiment of the invention that can be used to provide the second material to the first portion of the optical component formed by steps 2A-2F, so as to result in the final optical component.

As shown in Figure 3A, the substrate 150 with the polymerised first material 203 is coated with an alignment layer on the existing liquid crystal layer 203 i.e. on surface 206 (Figure 3A). The alignment layer utilised is homeotropic. Consequently, a liquid crystal placed on the alignment layer will attempt to align the directors perpendicular to the alignment layer.

A second substrate 160 is then coated on one surface with a further homeotropic alignment layer. This coated surface of the substrate 160 is then separated from the first substrate 150 by spacers 164 (Figure 3B). The substrates are placed such that the two homeotropic alignment layers are facing each other and, with the addition of the spacers 164, act to define an enclosure, hi order to allow the addition of further liquid crystal between the substrates 150, 160, a fill hole and an air hole is left to provide access to the enclosed space. Subsequently, the enclosed space is filled using capillary cell filling via the fill hole. The fill and air holes are then closed (e.g. by a plug or gluing). The liquid crystal introduced into the enclosed space becomes aligned, due to the alignment layer, so as to be perpendicular to the substrates (and hence also perpendicular to the directors of the first material 203). The orientation of this newly-aligned liquid crystal is then fixed by using UV photo polymerisation, so as to form the second material 170.

Again, if desired, the substrates 150, 160 (and the spacers 164) can be removed from the optical component if desired. In any event, the result is an optical component 181, generally similar to that illustrated in Figure 1.

Figures 4A-4C illustrate an alternative embodiment to that shown in Figures

3A-3C for providing a completed component.

Again, it is assumed that a substrate 150 has been formed with a solidified

(e.g. polymerised) first material 203 with a curved surface 206. However, in this particular embodiment, the substrate 150' has (at least by the time the method steps shown in Figures

4A-4C are performed) a layer of optically transparent conductor on it. For instance, a layer of

ITO (Indium Tin Oxide) could be formed upon the substrate 150'. This layer of conductor could be formed prior to the steps shown in Figures 2A-2F, or subsequent to the processing steps shown in those Figures. A second substrate 160' also includes a layer of transparent conductor e.g.

ITO. In contrast to the steps shown in Figures 3 A-3C, no alignment layers are formed on either the substrate 150' or the substrate 160' for alignment of the second material.

As per Figure 3B, the second substrate 160' is placed parallel to the first substrate 150', and separated from the first substrate 150' by spacers 164. Again, the spacers act to define the gap between the surface of the first material 203 and the flat surface of the second substrate 160' (and hence subsequently the thickness of the second material). The spacers 164 can also act to define the length of the final optical component. In this particular embodiment, the final optical component has a length equal to the width of substrate 150', the width of substrate 160', and the height of the spacers 164. Again, the spacers are placed between the substrates 150', 160', so as to define an enclosure, leaving only fill and air holes free (Figure 4 A). These spacers may, if desired, be glued into position.

Capillary cell filling is then used to introduce the monomer form 168 of the second material into the enclosed volume (Figure 4B). The fill and air holes are then sealed. The monomer 168 is then aligned by applying an electric field across the layer of the monomer. This electric field can then be achieved by providing a voltage N_s from a voltage source 304 to the electrodes formed by substrates 150', 160'. The strong electric field introduces homeotropic liquid crystal alignment. The alignment of the directors of the liquid crystal is subsequently fixed by UN photo polymerisation. Again, the result is an optical component, generally similar to that illustrated in Figure 1.

A suitable polyimide for use in the alignment layer is OPTMER AL-1051 supplied by Japan Synthetic Rubber Co., whilst Merck ZLI2650, spincoated from a solution in γ-butyrolactone can be used as an appropriate reactive polyimide with methacrylate groups as the bonding layer.

As mentioned above, in the prefened embodiment a mixture of two liquid crystals was utilised to obtain the desired n_e and n₀. Two suitable liquid crustals are l,4-di(4- (3-acryloyloxypropyloxy)benzoyloxy)-2-methylbenzene (RM 257) and E7 (a cyanobiphenyl mixture with a small portion of cyanotriphenyl compound) both from Merck, Darmstadt, Germany. The photoinitiator used to ensure the photo polymerisation of the liquid crystals can be Irgacure 651, obtainable from Ciba Geigy, Basel, Switzerland.

Figure 5 shows a device 1 for scanning an optical record carrier 2, including an objective lens 18 according to an embodiment of the present invention. The record carrier comprises a transparent layer 3, on one side of which an information layer 4 is ananged. 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 ananged in substantially parallel, concentric or spiral tracks, not indicated in the Figure. 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 magnetisation different from their sunoundings, or a combination of these forms.

The scanning device 1 comprises a radiation source 11 that can emit a radiation beam 12. The radiation source may be a semiconductor laser. A beam splitter 13 reflects the diverging radiation beam 12 towards a collimator lens 14, which converts the diverging beam 12 into a collimated beam 15. The collimated beam 15 is incident on an objective system 18. The objective system may comprise one or more lenses and/or a grating. 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 abenation conection 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.

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 enor conection 29. Other signals from the signal processor 27 are the focus enor signal and radial enor signal 30. The focus enor signal represents the axial difference in height between the spot 21 and the information layer 4. The radial enor signal represents the distance in the plane of the information layer 4 between the spot 21 and the centre of a track in the information layer to be followed by the spot. The focus enor signal and the radial enor signal are fed into a servo circuit 31, which converts these signals to servo control signals 32 for controlling a focus actuator and a radial actuator respectively. The actuators are not shown in the Figure. The 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. The radial actuator controls the position of the objective lens 18 in a radial direction 34, thereby controlling the radial position of the spot 21 such that it coincides substantially with the central line of track to be followed in the information layer 4. The tracks in the Figure run in a direction perpendicular to the plane of the Figure.

The device of Figure 5 in this particular embodiment is adapted to scan also a second type of record carrier having a thicker transparent layer than the record carrier 2. The device may use the radiation beam 12 or a radiation beam having a different wavelength for scanning the record carrier of the second type. The NA of this radiation beam may be adapted to the type of record carrier. The spherical abenation compensation of the objective system must be adapted accordingly. Figures 6A and 6B illustrate how the lens in accordance with the above embodiment shown in Figure 1 can be utilised to provide two different focal points, suitable for reading a dual-layer optical recording medium 2'. The dual-layer medium 2' has two information layers (4, 4'), a first information layer 4 at a depth d within the transparent layer 3, and a second information layer 4' a further distance Δd beneath the first information layer 4.

In the embodiment shown in Figures 6A and 6B, the objective system 18 comprises a polarisation sensitive lens 181 (comprising liquid crystals 203, 170), a second lens 182, a quarter- wave (λ/4) plate 183, and a twisted nematic (TN) liquid crystal cell 184. The focal point of the obj ective system can be altered by using the variable focal nature of the lens 181. By adjusting the angle of the polarisation of the incident radiation, a range of focal points can be achieved (thus allowing the lens 181 to form part of an objective system for a multi-layer disc, if desired). For ease of explanation, only the two extreme polarisation angles (with conesponding extreme optical functions of the lens 181) will be described.

In the off mode, the TN-cell acts to rotate the polarisation of incident radiation by 90°. For instance, as shown in Figure 6 A, when the TN-cell is off, then incident p- polarised radiation will be rotated by 90° to form s-polarised radiation.

The twisted nematic cell thus acts as a beam rotation means ananged to controllably alter the angle at which the polarised radiation beam is incident on the optical element 181. As an alternative, it will be appreciated that the optical element 181 could instead be rotated, with the polarised radiation beam remaining stationary.

It is assumed that, due to the particular orientation of the first birefringent material within the optical element 181, when the s-polarised radiation is incident on the element 181 , the radiation experiences the ordinary refractive index of the first birefringent material. In both s and p polarisation states, the radiation will experience the ordinary refractive index of the second material (as it is homeotropically oriented). As, in this particular example, the ordinary refractive index of the first material 203 is equal to the ordinary refractive index of the second material 170, the optical element 181 acts as an optically neutral element to s-polarised radiation. In other words, if the s-polarised radiation is a parallel beam incident upon the element 181, then it exits the element as a parallel beam.

After the optical element 181, the s-polarised beam is incident upon the quarter-wave plate, which acts to change the s-polarised beam to right hand circularly polarised light (RHC), which is focused on to the second information layer 4'. Upon reflection from the layer, the RHC light is converted to left hand circularly polarised light (LHC). The LHC light, upon being transmitted through the quarter-wave plate, is converted to p-polarised light. The p-polarised light then passes back through the optical element 181, and is changed to s-polarised light by the TN-cell 184. As shown in Figure 6A, this means that when p-polarised light enters the objective system 18, the light is incident upon the information layer 4', and the reflective light leaves as s-polarised light from the objective system 18. Alternatively when s-polarised light enters the objective system 18, the light is incident upon the information layer 4 and the reflected light leaves the objective system as p-polarised. Consequently, if the beam splitter 13 shown in Figure 5 is a polarising beam splitter, it is easy to ensure that no reflected light is directed back towards the light source 11, but almost all reflected is directed towards the detector 25 since most polarising beam splitters transmit p-polarised light and reflect s- polarised light.

In Figure 6B, the same optical arrangement exists, but in this figure the TN- cell is on, e.g. by applying a sufficiently high voltage over the cell, such that the TN-cell does not change the polarisation of light passing through it. Consequently, p-polarised light is incident upon the optical element 181. The p-polarised light thus experiences a change in refractive index when passing from the second portion of the element 181 to the first portion of the element i.e. it experiences some focusing (convergence) due to the planoconvex birefringent lens that forms the first portion of the element 181.

The p-polarised light, which is now slightly converging, is then incident upon the quarter-wave plate 183. The quarter- wave plate acts to change the p-polarised light to LHC light, which is further focused by the lens 182 so as to be incident upon the first information layer 4. Upon reflection from the first information layer 4, the LHC light turns into RHC light. The RHC light, as it passes through the quarter-wave plate 183, is then changed to s-polarised light, which subsequently passes back through the optical element 181 and the TN-cell 184.

Thus, as shown in Figures 6A and 6B, an optical element may be provided in accordance with an embodiment of the present invention in a scanning device. The element 181 may function as a neutral optical component (as shown in Figure 6 A), or as a focusing element (as shown in Figure 6B). Such an element, as it is optically neutral, allows relatively easy beam shaping within the scanning device.

It will be appreciated that the above embodiments are described by way of example only, and that various alternatives will be apparent to the skilled person. The mould used in the manufacturing process may be formed of any material, including rigid materials such as glass and plastic.

Further, the shaped surface of the mould may be dimensioned so as to allow for any change in shape or volume of the liquid crystal material during the method. For instance, typically liquid crystal monomers shrink slightly upon polymerisation, due to double bonds within the liquid crystal being reformed as single bonds. By appropriately making the optical component shaped defined by the substrate and the mould slightly oversize, an appropriately sized and shaped optical component can be produced. Whilst the substrates have been seen in this particular example as comprising a single sheet of glass, with two flat, substantially parallel sides, it will be appreciated that the substrates can in fact be any desired shape.

An extra adhesion layer may be applied to the mould and/or substrate (prior to deposition of the bonding layer onto the substrate and the orientation layer to the mould), so as to make sure that the applied layers are well attached to the mould and the substrate. For instance, organosilanes may be used to provide this adhesion layer. For the substrate an organosilane comprising a methacrylate group may be used and for the mould an organosilane comprising an amine end group may be used.

It will also be appreciated that the above described optical components are described by way of example only. An optical component (or indeed, an optical element formed according to the present invention i.e. a portion of an optical component) could be formed with different properties to that described above.

For instance, in the above embodiments, it is assumed that the refractive indices ι-_e2 and n_o2 of the second material 170 of the component 181 are respectively equal to n_eι and n_o_. However, it will be appreciated that in fact any values of n_eι, ii_oi, r-_e2 and n_o2 could be used. For instance, an optical component could be formed with

or with

In each instance, the effect (power) of the curved interface within the optical element on the radiation will vary depending upon the differences between the effective refractive indices experienced by the polarised radiation.

Whilst specific examples of materials suitable for forming the optical component have been described, and particular manufacturing steps, these are again provided by way of example only. In the prefened embodiment, it is assumed that the outer surfaces of the optical element (i.e. the surfaces upon which the light enters and exits the element) are two flat, parallel surfaces. However, these surfaces could in fact be any desired shape, including concave or convex. Equally, the optical component 181 can be formed of any birefringent materials, with the ordinary and extraordinary axis of these materials having any predetermined relationship.

For instance, Figure 5 illustrates an optical element 400 in accordance with a further embodiment of the present invention. In this embodiment, the optical element comprises a first portion 402 formed of a first birefringent material, and a second portion 404 formed of a second, different birefringent material. However, in this particular embodiment, the first birefringent material is formed as a convex lens, rather than a planoconvex lens.

In all of the above embodiments, the curved interface between the two birefringent materials of the component can provide a variable optical function for different angles of incident polarised radiation. This allows the optical component to be used in a number of novel and interesting ways. Further, a range of components can be manufactured with different optical properties simply by changing the relative orientations of the two birefringent materials.

Claims

CLAIMS:

1. An optical scanning device for scanning an information layer of an optical record carrier, the device comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam on the information layer, wherein the device includes an optical element comprising at least two adjacent birefringent materials with a curved interface between the materials.

2. A device as claimed in claim 1, wherein the radiation source is ananged to generate a polarised radiation beam, the optical scanning device further comprising beam rotation means ananged to controllably alter the angle at which the polarised radiation beam is incident on the optical element.

3. A device as claimed in claim 2, wherein said beam rotation means is ananged to rotate the element.

4. A device as claimed in claim 2, wherein said beam rotation means is ananged to alter the polarisation angle of the polarised radiation beam.

5. A device as claimed in any one of the above claims, wherein at least one of the two materials is shaped as a lens.

6. A device as claimed in any one of the above claims, wherein at least one of the first material and the second material is shaped as at least one of a planoconcave lens and a planoconvex lens.

7. A device as claimed in any one of the above claims, wherein one of the two materials is shaped as a planoconvex lens and the other of the two materials is shaped as a mating planoconcave lens.

8. A device as claimed in any one of the above claims, wherein both the first birefringent material and the second birefringent material comprise the same birefringent substance.

9. A device as claimed in any one of the above claims, wherein an optical axis passes through the first birefringent material, the curved interface and the second birefringent material, the extraordinary axis of at least one of the birefringent materials being perpendicular to the optical axis.

10. A device as claimed in any one of the above claims, wherein an optical axis passes through the first birefringent material, the curved interface and the second birefringent material, the extraordinary axis of at least one of the birefringent materials being parallel to the optical axis.

11. A device as claimed in any one of the above claims, wherein an optical axis passes through the first birefringent material, the curved interface and the second birefringent material, the extraordinary axis of at least one of the birefringent materials being tilted at an angle with respect to the optical axis.

12. A device as claimed in any one of the above claims, wherein a first of the two birefringent materials has an extraordinary index of refraction that is non-parallel to the extraordinary index of refraction of the other birefringent material.

13. A device as claimed in any one of the above claims, wherein a first of the two birefringent materials has an extraordinary index of refraction that is perpendicular to the extraordinary index of refraction of the other birefringent material.

14. An optical component comprising at least two adjacent birefringent materials with a curved interface between the materials.

15. An optical component as claimed in claim 14, wherein at least one of said materials comprises a polymerised anisotropically oriented liquid crystal.

16. An optical component as claimed in claim 14 or claim 15, wherein at least one of the outer surfaces of the optical component is planar.

17. A method of manufacturing an optical scanning device for scanning an information layer of an optical record carrier, the method comprising the steps of: providing a radiation source for generating a radiation beam; providing an optical element, the optical element comprising at least two adjacent birefiingent materials with a curved interface between the materials.

18. A method of manufacturing an optical component, the method comprising: providing at least two adjacent birefringent materials with a curved interface between the materials.

19. A method as claimed in claim 18, the method comprising: placing a first material between a substrate and a mould, the mould having a shaped surface, at least a portion of the shaped surface having an alignment layer formed thereon, and the substrate having a first surface on which is formed a bonding layer; bringing the mould and the substrate together so as to sandwich the first material between the first surface of the substrate and the shaped surface of the mould; polymerising the first material so as to form the first birefringent material; adhering the material to the bonding layer; removing the substrate with the adhered polymerised material from the mould; covering the shaped surface of the polymerised first material with a polymerisable further material; and polymerising the further material so as to form the second birefringent material.