EP1776695A1

EP1776695A1 - Optical disk and reader therefor

Info

Publication number: EP1776695A1
Application number: EP05758625A
Authority: EP
Inventors: Peter Imperial College London TOROK; Martin Heptagon Oy SALT; E.E Aristotle University of Thessaloniki KRIEZIS; Peter R.T. Imperial College of London MUNRO; Hans Peter Institute of Microtechnology HERZIG; Carsten Institute of Condensed Matter ROCKSTUHL
Original assignee: Imperial Innovations Ltd
Current assignee: Ip2ipo Innovations Ltd
Priority date: 2004-07-26
Filing date: 2005-07-06
Publication date: 2007-04-25
Also published as: WO2006010882A1; TW200606878A; JP2008507806A

Abstract

A readable disk and an element for a reader therefore encode information in the orientation of a pit element on the disk modulo π/4 with respect to a track on the disk providing an increased storage capacity. Storage capacity can be further increased by multiplexing with a number of further signals.

Description

Optical Disk and Reader Therefor

The invention relates to an optical disk and an optical disk reader, and in particular to an optical disk having a high storage capacity and a reader therefore.

In known optical disks such as compact disks, information is stored in pits indented in a surface of the optical disk, whereby different pit lengths encode different bit sequences. Since information is encoded in the length of the pits, only one bit is stored in the smallest resolvable pit element. The pits are arranged on circular tracks centred on a disk and separated by a pitch and are read out by a laser beam. The amount of information that can be stored on such an optical disk is thus limited by the size of the smallest resolvable pit element and the smallest pitch such that when the laser beam scans a given track, no appreciable cross-talk is picked up from the adjacent track.

So far, progress in increasing the storage capacity of an optical disk has focused on increasing the resolution of the pick-up system and thus allowing for high information density by decreasing the size of the smallest resolvable pit element. However, this approach is limited by the need to use increasingly high numeral aperture optics for decreasing wavelengths optical systems in order to increase resolution. Furthermore, higher storage density achieved by smaller pit sizes requires higher data transfer rates and novel mastering techniques.

An alternative approach that has been suggested for increasing the storage capacity, is to increase the number of bits that can be stored in a single pit element. One approach to this is to encode sequences of bits in the orientation of a pit element with respect to the track.

EP-A-0376673 discloses a system that is arranged to resolve four orientations of an elongated pit with respect to the track. This allows for a sequence of two bits to be encoded in each element. EP-A-0553548 discloses a pit element and corresponding reader that can resolve eight orientations of an anisotropically shaped pit element by detecting a change in the direction of light reflected from the pit element using an octant detector. With eight resolvable orientations, a sequence of three bits can be encoded. However this approach necessitates increasing the pit dimension and provides limited resolution.

EP0553548 discloses an optical disk reproducing apparatus in which pit elements are either sloped at different orientations at multiples of π/4 or have step elements at those orientations. The reading beam spot overlaps more than one readable element and the beam reflected and diffracted from the readable elements is received on a spatial position detector which is able to resolve the individual readable elements by virtue of a complex algorithm.

The invention is set out in the independent claims, additional optional features being defined in the dependent claims.

Because the orientation of a pit element is determined by detecting for example the amount of polarisation conversion for a reading beam of electro-magnetic radiation (for example a laser beam) reflected from the pit element the orientation of an anisotropically shaped pit element can be obtained on a finer scale. Also, there is no need for highly increased date transmissions, at least with respect to the analogue orientation signals and the present approach avoids the difficulties of mass manufacturing disks with ever smaller pit elements. In addition, since information is not encoded in the pit length, all pit elements can be made equal in size and evenly spaced, making it easier to manufacture an accurate master. In turn, this results in much reduced jitter and thus better signals.

Polarisation conversion can be a result of scattering by objects having "shape birefringence", whereby a polarisation of electro-magnetic radiation is rotated when the radiation is incident on an anisotropic shape, the rotation angle depending on the relative orientation of an axis of the shape and the plane of polarisation of the incident radiation. The reading beam can be any polarisable reading beam.

In the remainder of this description, the terms electro-magnetic radiation and light will be used interchangeably. While customarily optical disk readers use light in the visible spectrum produced by a laser, it is understood that the use of the terms light and electro-magnetic radiation encompasses the entire spectrum of electro-magnetic radiation. Although reference is made to light reflected from a readable element it will be appreciated that this term embraces more complex optical phenomeua by which incident light is returned or scattered from the element.

Embodiments of the invention are now described by way of example only and with reference to the accompanying figures wherein:

Fig. 1 is a schematic representation of an optical disk according to an embodiment of the invention;

Fig. 2 is a block diagram of an optical disk reader comprising an optical disk reading element according to an embodiment of the invention; Fig. 3 is a schematic representation of an optical disk reading element according to the invention;

Fig. 4 is a perspective view of a pit element according to an embodiment of the invention;

Fig. 5 is a schematic side sectional view of the pit element of Fig 4;

Fig. 6 is a plot of intensity against pit element angle showing the angular dependency of polarisation conversion;

Fig. 7 shows a schematic representation of a detector detecting measured signals according to a multiplexing scheme according to an embodiment of the invention;

Figs. 8A, 8B and 9 illustrate alternative pit elements according to embodiments of the invention;

Fig. 10 is a block diagram illustrating a further embodiment of the invention; and

Fig. 11 is a block diagram illustrating a further embodiment of the invention.

With reference to Fig. 1 an optical disk 100 according to an embodiment of the invention comprises a plurality of tracks 110 centered on a centre hole 120. An enlarged section 130 of track 110 shows that the track 110 comprises a plurality of pit elements 140 (i.e. readable elements) aligned along the track, the pit elements having an anisotropic shape that defines an axis or direction indicated by an arrow. As discussed in more detail below the orientations of the pit elements can vary on a fine scale and still be resolvable by detecting shape birefringence induced polarisation conversion by the pits. By digitising the analogue value of orientation of each pit element, a sequence of bits describing the orientation of each pit element can be derived. For example, in an embodiment described below, angles can be resolved over a range from 0 to π/4 at a resolution of 1.2% or 0.01 radian. As many as 83 signal levels are therefore resolvable over the interval of 0 to π/4, corresponding to more than 6 bits per pit element. An optical disk reader arranged to provide this degree of resolution and thus information density, using polarisation conversion, will now be described.

An optical disk reader according to an embodiment of the invention is shown schematically in Fig. 2 and comprises a source 210 of plane polarised electro¬ magnetic radiation, for example plane polarised light from a laser, providing a reading beam along a path 230 illuminating an optical disk of the type shown in Fig. 1 mounted on a drive 220. Intersecting the light path 230 between the source 210 and the drive 220 is an optical disk reading element 300. A central processor 240 comprising a plurality of logic circuits controls the drive 220 and the source 210. Central processor 240 also controls optical reading elements 300 and receives signals therefrom.

Referring also to Fig 3 the source of plane polarised light 210 comprises a laser 212 arranged to generate a reading beam to illuminate a surface 222 of the optical disk mounted on drive 220. In a particular embodiment, a laser reading beam of wavelength 405 nanometre is used, the same as for BluRay ™ systems. The source 210 further comprises a beam shaper 216 and a first polariser 214, for example a Glan-Thomson polariser which has an extinction ratio of 10⁵ intersecting the light path 230 between the laser 212 and the surface 222.

The set-up defines a coordinate system with a z-direction lying along the light path 230 of the reading beam, an x-direction perpendicular to the z-direction and defined by the plane of polarisation of light from the source 210, (lying in the plane of the paper in the drawing), and a y-direction perpendicular to both the x and z-direction and thus perpendicular to the plane of polarisation of the source. Polarised light from the source 210 is referred to as x-polarised, and polarised light polarised in a plane perpendicular to the plane of polarisation of the source 210 (i.e. polarised in the z-y plane) is referred to as y-polarised.

The optical disk is arranged inside the drive 220 such that its tracks have a tangent at focal point 224 of light path 230, which is parallel to the x-direction.

The optical disk reading element 300 comprises, in the direction of the reading beam, a first and second non-polarising beam splitter 310 and 320, a beam expander 330 with a magnification of, for example unity and comprising two lenses 332 and a pinhole aperture 334 in a plane intermediate the two lenses, and an objective lens 340 which focuses the illuminating light onto the surface 222.

The pin-hole diameter is chosen as small as possible in practice, for example 75 μm. The pin-hole acts as a spatial filter, blocking higher diffractive orders of the returned light and thus increasing resolution. Even more advantageously, a small pin-hole effectively filters out most intrinsic depolarisation of the optical components between the pin-hole 334 and the disk surface 222. Since the orientation of the elements is measured using a change of the polarisation in the reflected light, any such intrinsic depolarisation would otherwise distort the measured signals.

Light emitted from the source 210 passes through the first and second beam splitter 310 and 320, the telescopic system 330 and the objective lens 340 and is then reflected from the surface 222. At the second beam splitter 320 a fraction of light is diverted to a regulating photodiode 250, which thus detects a quantity proportional to the intensity of light emitted from source 210. This measurement is used to regulate the intensity of the source 210 to a constant level. Reflected light travels back through the system and is split at the second beam splitter 320 to pass through a second Glan- Thompson polariser 350 to first sensor 352 via a focusing lens 356. The remaining reflected light along light path 230 is split by the first beam splitter 310 to reach an autofocus unit 240 for controlling the objective lens such that the beam remains in focus on the disk, as in conventional optical disk drives and in an optical data storage system it also provides the control signal for radial tracking..

In the coordinate system described above, the axis of the first polariser 214 is oriented such that it blocks y-polarised light and transmits x-polarised light. The second polariser 350 is rotated by π/2 (in the frame of reference of the light path 230) and hence transmits y-polarised light, blocking x-polarised light. Since the first and second polarisers are crossed with respect to each other, when light from the source 210 is reflected from a plane surface, the intensity detected by the first sensor 352 should be practically zero, not accounting for intrinsic depolarisation due to the optical components and the finite extinction ratio of the polarisers. However if the incident light is reflected from an anisotropic pit element on the surface 222, this result in a change in the direction of polarisation of the reflected light with respect to the incident light (polarisation conversion), which change depends on the orientation of the pit element and is described in more detail below. Any change in the plane of polarisation of the reflected light is detected by a non-zero intensity reading at sensor 352 as the light with a rotated plane of polarisation now has a y- component of polarisation and is thus partially transmitted by the second polariser 350. The signal 354 of detector 352 thus provides a measure of the y- component of polarisation of the reflected light and thus of the amount of polarisation conversion. It will be appreciated that although discussion is made of plane polarised light in practice the light includes non-plane polarised components introduced by the optics in the system.

The signal 354 depends not only on the state of polarisation of the reflected light, but also on the intensity of incident light from source 210. In order to achieve reliable measurements of the plane of polarisation, signal 354 can be normalised with respect to a signal 364 at a second sensor 362 representing an intensity proportional to the intensity of light rejected by the Glan-Thompson polariser 350, that is, essentially, X-polarised light.

The functioning of the optical disk reader element described above is now described in detail for a specific embodiment of the pit element described with reference to Figs. 4 and 5 in which a pit element comprises a cut cylinder defined by a cylindrical depression or cavity 400 bisected by a first base portion 410 at a first depth from the surface and a second base portion 420 adjacent to the first base portion at a second depth with respect to the surface such as to define a step inside the cavity. The first and second base portions thus each comprise a semi-circular surface which are joined at their rectilinear ends 412, 422 by a riser 430. The cavity defines an inner cylindrical wall 440 which extends perpendicular to the surface 222. The depth of the first base portion and the depth of the second base portion are such that the first base portion is a distance a away from the surface 222 and the second base portion is a distance 2a away from the surface 222. Of course, the height of the step is not limited for half the pit depth and other depths of the base portions are envisaged. Given the anisotropic shape of the cylindrical step pit element, an axis 450 can be defined for it, for example perpendicular to the step direction defined by riser 430. Such a structure displays a form of "shape birefringence" as discussed above. In particular a plane of polarisation of light reflected from the structure is turned with respect to the plane of polarisation of incident light when the plane of polarisation of the incident light is not aligned parallel or perpendicular with the axis of the shape. The amount of rotation depends on the relative angle. In the present embodiment, the plane of polarisation of the incident light is aligned in the x direction along the tangential direction of a track on the optical disk and therefore, if the axis 450 is parallel or perpendicular to the direction of the track, both incident and returned lights are polarised in the x direction and the signal detected at sensor 352 is, ideally, zero. However, if the axis 450 is turned out of alignment with the track, the interaction between the pit element and incident light results in a rotation of the plane of polarisation of the reflected light. This effect increases as the angle between axis 450 and the track is increased, increasing the intensity of light detected at the first sensor 352. The effect reaches its maximum at an angle π/4 between axis 450 and the track and drops off symmetrically as the axis 450 reaches perpendicular alignment with the track.

The result of simulations of the output 354 of the first sensor 352 as a function of pit element orientation is now described with reference to Fig. 6. The simulation assumes a light source at wavelengths λ = 405 nanometres, a pit depth of 2a = λ/2 and a pit diameter of 2.2μm.

As described above, the simulations shown in Fig. 6 confirm that the intensity of y polarised light measured by first sensor 352 increases as the element is rotated away from a parallel alignment with the x or track direction, peaks at π/4 and decreases as the pit element is rotated towards perpendicular alignment at π/2. The same pattern is repeated for continuing rotations through angles π/2 to π. As becomes clear from Fig. 6 the angle between the axis 450 of the pit element and the x or track direction can be measured uniquely from the detected intensity in any interval of width π/4. However, over the interval from zero to π, there are four possible values of angle corresponding to each measured intensity, and the angle can thus be determined modulo π/4. Further measurements are required to extend the range of measurable angles beyond π/4, as will be described now in more detail.

The structure of first sensor 352 is arranged not only to detect the intensity of light transmitted by the second polariser (a measure of polarisation conversion and thus the angle of the pit element modulo π/4) but also which of the π/4 sector from the range of zero to π the angle occupies allowing extension of the range of measurable angles. In an alternative view, this additional angular information can be seen as an additional signal which is independent or orthogonal in information theory sense, to the polarisation conversion signals. With reference to Fig 7, the first sensor 352 is an octant detector 700 comprising eight equiangular sectors 710 arranged around a centre 715. In practice the detectors may be a composite of two quadrant detectors rotated by 45° (π/4) relative to one another in conjunction with a beam splitter directing a portion of the reading beam to each detector.

The octant detector 352 is arranged such that the tangent of the track of an optical disk at a focal point of the light path 230 is aligned with and projected onto a line passing between two sectors. In Fig 7, the sectors are labelled anti¬ clockwise from I to VIII and the track tangent is projected on the line passing between octants II and III and octants VI and VII.

In order to extend the range of measurable angles two signals I₁= I₁ + I_n + Iv + Ivi - (Im + Irv + I_VII + Ivm) are constructed during detection and I₂ = In + Im + L_/i + I_VII - (I₁ + I₁V + Iv + IV_IIIX where the roman numeral subscripts indicate the measured intensities for each of the octants of the detector. Accordingly the π/4 sector in which the pit orientation falls from 0 to π can be determined uniquely according to the following table:

0 - π/4 π/4 - π/2 ^■all - ³A n ³A π - π

Ii + + - -

I₂ + - - +

In the case of two quadrant detectors assuming that the first quadrant detector has outputs I_alj I_a2j I₅₃, I_a4 and that the second quadrant detector has outputs I_bi, I_b2, I_b3 and I_b4) an equivalence can be made between the outputs of the quadrant detector and combinations of the output of the octant detector described with reference to Fig 7. For example, signals I₁ and I₂ can be rewritten in terms of the outputs of the two octant detectors as Ii= I_a4 + 1₃₂- (I_ai + 1^) and I₂ = I_bi + I_b3 - (I_b2 + I_b4). In effect, the octant detector is thus implemented by combining the outputs of two quadrant detectors, rotated by π/4 with respect to each other, in quadrature.

The octant detector allows use of the spatial distinction of detected intensity to determine the sector of orientation by virtue of lens 356 forming an image of the field of the electromagnetic radiation return from the pit element on the surface of the sensor 352. For the pit element described above with reference to figures 4 and 5, this image comprises two distinct areas of intensity aligned on the axis 450 of a pit element. This intensity distribution at 720 is shown schematically in figure 7, assuming that the reading beam is centred on the pit element. This image thus contains information on the orientation of the pit element modulo π (due to the π rotational symmetry of the intensity image) and can thus be used to determine the orientation of the pit element between zero and π. The spatial resolution of the octant detector described above is not sufficiently high to determine a precise angle of orientation, however, it is sufficiently high to determine the sector of orientation by analysing the signs signals and I₁ and I₂ according to the above table. The exact angle of orientation is then obtained from the signal 354, measuring the intensity of light transmitted by the second polariser and thus the amount of polarisation conversion is formed by summing all eight intensities I₁ to I_Vm. Of course the intensity and angular resolution signals could be measured by separate detectors.

The signal from the sensors 352 and normalising sensor 362 are processed by electronic circuits in the processing unit 240. The polarisation conversion signal (signal 354 normalised by signal 369), which is representative of the orientation of the pit element modulo π/4 is digitised using any convenient algorithm, for example level detection. Based on empirical results for the signal to noise ratio, at least 83 levels can be discriminated and these digitised version can thus provide a bit sequence of at least 6 bit lengths, but longer sequence lengths are possible by optimising the reader element to achieve higher resolution and hence more levels, for example by using high precision optical alignment and an improved object lens that combines high numerical aperture with small polarisation aberration. Of course, sequences smaller than 6 bits are equally envisaged. The octant detector allows the angle to be determined from zero to π and thus increases the number of available levels to 332. Thus using both the depolarisation conversion signal and the signal relating to the sector of orientation determined by the octant detector a fourfold increase in storage capacity over polarisation conversion alone is achieved corresponding to an additional two bits in the pit sequence that can be stored for each pit element It will be evident to the skilled person that the use of the effect of polarisation conversion through "shape birefringence", that is a change in the plane of polarisation of reflected light as a result of the orientation of an anisotropic feature with respect to the plane or state of polarisation of incident light, or other forms of polarisation change or rotation is not restricted to the specific shape described above. Any shape that has, for example, a clearly defined edge, or defines an axis otherwise, may also give rise to "shape birefringence". Examples of such alternatively shaped pit elements are shown in Figs. 8 and 9. In addition in some instances isotropic shapes can cause changes in the state of polarisation of the returning light and hence be detected as described herein.

The pit element shown in Fig. 8A comprises an elongated rectangular trench, the "shape birefringence" effect relying on the presence of an oriented edge. A particular example of a data cell 800 with a rectangular pit 802 having rounded ends 804 is depicted schematically in Fig. 8B. The data cell has overall dimensions of 340nm metres by 250 mn, including a 90nm gap between adjacent pits, resulting in a length of the pit of 250nm. The data cell comprises a pit element which has two straight opposed side walls and two curved opposed end walls, forming the rectangular pit. The depth of the pit is preferably λ/4 (λ being the wavelength in the cover layer).

The pit element shown in Fig. 9 does not comprise an edge such as in Fig. 4 or 8 elements, but defines an axis by virtue of two hemispherical protrusions from the surface of the optical disk through each of the respective centres. In addition to the size of the hemispherical protrusions, this pit element is also defined by the distance between the two respective centres of the hemispherical protrusions along the axis. While there is a certain amount of design freedom in choosing the dimension and separation of the hemispherical protrusions, for "shape birefringence" the size of the hemispheres and their separation must be smaller than the resolution of the optical disk reader element 300.

The Fig 9 pit element offers a further possibility for encoding additional information by making use of the resonant phenomenon of surface plasmon coupling. For the Fig 9 pit element geometry, a predetermined distance between the centre of two hemispherical protrusions results in a strong return signal of light of a corresponding wavelength owing to surface plasmon coupling. The difference in returned intensity between a "resonant wavelength" and an adjacent wavelength maybe several orders of magnitude allowing distinction between pit elements having different distances between the hemisphere. For example, existing BluRay optical disks readers already provide three different lasers with different frequencies. The three different lasers could thus be used to distinguish three different types of pit elements having different distances between the hemispheres. Of course, systems with more or less than three different lasers are also envisaged. Alternatively an LED (Light Emitting Diode) can be used providing a broader spectrum. By detecting which of the different wavelength laser result in a surface plasmon resonance at a given pit element, further information may be incurred at a single pit element, thus providing further orthogonal infonnation. It will be understood that the application of this concept goes beyond the use in conjunction with a system which detects the orientation of pit elements and may be generally useful to provide a further dimension of information encoding on an optical disk.

Even though the pit element embodiments described above constitute a novel solution to high capacity optical disk storage by multiplexing several bits of information in a single pit element, the corresponding optical disk can be manufactured readily using existing BluRay disk manufacturing techniques which will be well known to the skilled reader. The master for a disk comprising the Fig 4 and 5, as well as the Fig 8 embodiments can be manufactured using Phase Transition Mastering of any appropriate type. An optical disk comprising the figure 9 embodiment can be manufactured by deep UV mastering (Samsung™).

The overall structure of the optical disk according to the embodiments of the invention is similar to a BluRay disk, and that as comprises a protective top layer to ensure scratch resistance and finger print protection. As in a BluRay disk, the hard coating is followed by a cover layer which covers the data layer comprising the data pits and the data layer is supported on a substrate. Because the top layer and the cover layer possess ultra low birefringence (< 1 nanometre at λ = 405 nanometre) it is possible to largely avoid unwanted depolarisation effects. The disk may be provided with one or more data layers and the data layers can be formed separately and then be bonded by low birefringence UV curable resins.

In addition to the distribution of pit elements on the optical disk that encode digital information, a calibration track or section of a track may be provided on a disk. This track contains one pit element of each of the orientations of pit elements that are used on the optical disk. For example, if information was encoded by 300 different orientations of the pit elements, the calibration track would comprise 300 elements, one for each orientation. It is readily seen, that compared to the overall storage capacity of the disk, the provision of the calibration track has a negligible effect on the overall storage capacity, but it highly advantageous in terms of increasing the accuracy of orientation detection. When an optical disk comprising a calibration track as described above is inserted into an appropriately configured optical disk the reader reads the pit elements on the calibration track and thus establishes an up to date calibration of the orientation of the elements present on the disk from the measured signals each time a new disk is inserted. This calibration data is then used subsequently during playback to convert the measured signals into orientation values and/or the corresponding digitization levels. This simple calibration procedure makes the system robust with respect to changes over time and the system components and/or the use of disks having different pit orientations encoded thereon. Furthermore, if the processor 240 detects inaccuracies in the interpretation of the measured signals, the calibration track may be reread in order to obtain an updated calibration.

It will be appreciated that other embodiments of the invention are contemplated which similarly achieve high levels of resolution.

In a first further embodiment shown schematically hi Fig. 10 and in which only the fundamental components of the system are shown, it being clear that the additional components shown for example in Fig. 3 can be incorporated as appropriate, a source 1010 passes a reading beam through a linear polariser 1020 and then onto a readable disk 1030. The light returned by the readable elements on the disk 1030 passes through a beam dividing component 1040 which can be, for example, a half silvered mirror beam splitter or a diffractive element or hologram. A first portion of the beam passes through a second polariser 1050 which is crossed with the first polariser 1020 and is incident upon an intensity detector 1060. The measured intensity is representative of the change in polarisation and hence the orientation of each individual pit element being read, as described in more detail above. The remaining portion of the beam is incident on an octant detector 1070 of the type described above with reference of Fig. 7 to provide additional information about the orientation of the readable element as described in more detail above. As a result any possibility of extinguishing of the signal at the octant detector by the second polariser 1050 is removed, providing an improved signal to noise ratio. In addition the signal on the octant detector 1070 which is effectively modulated by the reading beam passing over reading elements can be used as a synchronisation or gating signal to identify which portion of the intensity signal on intensity detector 1060 corresponds to alignment of the reading beam with a readable element, providing additional accuracy in decoding of the angular orientation of the readable element. It will be appreciated that the intensity detector and octant detector can be in either portion of the beam path with the polariser appropriately positioned and that the beam divider element 1040 can also be a polariser for one of the portions as appropriate, for example a Glan- Thompson polariser which rejects any light not polarised along its axis of polarisation.

Referring now to Fig. 11, a second further embodiment is shown in which the orientation of the reader element is detected directly from the angular orientation of the reading being returned from the disk. The general form of the returned signal can be seen at 720 in Fig. 7 and comprises two symmetrically distributed intensity regions either side of an axis of symmetry. It is believed that this form of the signal is caused by polarisation and interference effects occurring at the readable element such that the axis of symmetry dividing the intensity regions, ie their angular orientation as detected at the detector, corresponds to the angular orientation of the step dividing the two regions of the pit element. As a result, by implementing the intensity measurement approach described with reference to Fig. 7 then the exact angular position of the intensity regions and hence the position of the axis of symmetry and orientation of the pit element can be derived with high levels of resolution. In particular as the intensity region will typically overlap adjacent octants of the detector then, from the intensity detected in each octant the specific spatial position of the intensity region can be identified. Once this has been performed for both intensity regions then their respective centres can be identified and the perpendicular bisector thereof derived using any appropriate mathematical or computational technique, for example using the full value of I, and I₂ described above (as opposed to only the sign) The orientation of the perpendicular bisector corresponds to the orientation of the readable element step and hence provides full encoding information.

A particular preferred embodiment is shown in Fig. 11 in which a source 1110 emits a reading beam via a circular polariser such as a quarter wave plate 1120 onto a readable disk 1130. The beam returned from the readable disk 1130 is received by an octant detector of the type described above 1140 and the orientation of each readable element step is derived as described above. It is found that circularly polarising the reading beam prior to incidence on the readable disk 1130 provides particularly good modulation/returned reading beam quality onto the detector 1140 such that the spatial position of the detected reading beam can be determined to a high degree of accuracy hence providing an correspondingly high level of resolution of the angular oreintation of the detected reading beam and hence the readable element orientations.

In the second further embodiment it will be appreciated that it is preferable to add circular polarisation to the reading beam, but linearly or otherwise polarised light or unpolarised light can alternatively be used for example in conjunction with additional signal filtering components for processing the received signal on the detector 1140. It will be understood by those skilled in the art that a reader element as described above can be used in conjunction with a reader having known means for mounting and rotating the optical disk and aligning and focusing the reading beam. Such a reader may employ conventional reading technology. The readable elements in the description have been referred to as pits, but bumps are equally envisaged. An optical disk, as described above, and the optical reader element therefore may be fabricated using conventional techniques known to those skilled in the art. It is understood that any suitable materials optical elements and detectors such as CCD detectors may be employed and that any suitable encoding scheme or pit shape can be used.

It will be further appreciated that where other spatial characteristics of the pit elements are used to encode information then the corresponding spatial characteristics of the returned reading beam is detected to decode the information.

It will further be recognised that any appropriate polarisation of incident light for example plane polarised or rotationally polarised light can be used, and that features from respective embodiments can be interchanged or juxtaposed as appropriate

It will further be recognised that the skilled person will know how to optimise the parameters and dimensions of the pit elements described above. For example, a good trade-off between signal quality of the polarisation conversion and signal representing the sector of orientation was found by simulation to be a pit depth of ¹A λ for the cut cylinder pit element.

While detailed descriptions of specific embodiments of the present invention have been provided, it would be apparent to those skilled in the art that numerous variations of the apparatus and methods disclosed are possible which would not deviate from the scope of the present invention.

Claims

1. A readable disk reader element comprising a first detector arranged to produce a first detector output representative of a change in polarisation state of a reading beam.

2. An element as claimed in claim 1, further comprising an incident reading beam source arranged to generate a reading beam having a first polarisation state and in which the change in polarisation state is a change of polarisation of a returned reading beam with respect to the first state.

3. An element as claimed in claims 1 or 2 in which the first detector is arranged to detect an intensity signal representative of change in polarisation state.

4. An element as claimed in claim 3 in which the first detector further comprises a spatial distribution detector.

5. An element as claimed in claim 4 in which the first detector comprises an octant detector.

6. An element as claimed in claim 5 in which the octant detector comprises two quadrant detectors rotated with respect to each other by π/4.

7. An element as claimed in any of claims 1 to 3 further including a second, spatial distribution detector.

8. A method of reading a readable disk comprising generating a reading beam having a first polarisation state and detecting a change in the state of polarisation of the reading beam returned from the readable disk.

9. A method as claimed in claim 8 including:

a) illuminating a readable element on the disk with incident reading beam polarised in a first state; b) detecting a change in the state of polarisation of the reading beam returned from the readable element with respect to the first state; and c) deteraiining a bit sequence represented by the change.

10. A method as claimed in claim 9, the method further comprising d) detecting a sector of orientation of the readable element; and e) determining a bit sequence represented by the sector.

11. A method as claimed in any of claims 8 to 10 in which a first detector detects a change in the state of polarisation of the reading beam and a second detector detects the intensity of the reading beam returned from the readable element comprising the steps of detecting intensity at the second detector and deriving a synchronisation signal therefrom for the first detector.

12. A method of reading a readable disk comprising a) detecting a signal representing independent information from a readable element of the disk; b) decoupling the independent components; and c) determining a bit sequence represented by the decoupled components

13. A method as claimed in claim 12, the first signal component comprising a change in the state of polarisation of a returned reading being returned from the readable element from an incident reading being incident on the readable element, and the second signal comprising a sector of orientation of the readable element or a signal representing a resonance wavelength of the readable element.

14. A method as claimed in claim 12 or 13 comprising using a first of the decoupled components as a synchronisation signal for a second decoupled component.

15. A readable disk comprising a plurality of readable elements, the readable elements being shaped such that an orientation is defined; the readable elements being arranged to encode a bit sequence representing digital data, each readable element encoding at least three bits.

16. A disk as claimed in claim 15, each readable element encoding at least six bits.

17. A disk as claimed in claim 15 or 16, the disk further comprising a calibration track or track section.

18. A disk as claimed in claim 17, wherein the calibration track or track section comprises a set of readable elements such that each orientation of readable element present on a disk occurs at least once within the said calibration track or track section.

19. A disk as claimed in any of claims 15 to 18, the readable elements each comprising an elongated formation.

20. A disk as claimed in any of claims 15 to 19, the readable elements each comprising an elongated pit having opposed substantially straight sidewalls and opposed curved end walls, cooperatively defining the pit; the pit preferably having a depth of λ/4.

21. A disk as claimed in any of claims 15 to 19, the readable element comprising first and second protrusions from the surface of the disk.

22. A disk as claimed in claim 21 in which the protrusions each define a centre separated from each other by a distance substantially smaller than the separation between two readable elements on a track.

23. A disk as claimed in claim 20 or 22, each protrusion comprising a hemisphere.

24. A disk as claimed in any of claims 15 to 18, the readable element comprising a cylindrical formation in a surface of the optical disk, the formation having a first portion at a first distance from the surface and a second base portion adjacent to the first at a second distance such as to define a step.

25. A disk as claimed in claim 24, the first and second portions each comprising a semi-circular surface such that the projected cross- sectional area of the element comprises a circular disk.

26. A method of reading a readable disk comprising generating a reading beam having a plurality of frequency components and detecting a dominant frequency of the reading beam returned from the readable disk.

27. A method as claimed in claim 26, in which the dominant frequency is a resonant frequency of a readable element on the disk.

28. A method as claimed in claim 27 in which the resonant frequency results from surface plasmon coupling with the readable element.

29. A readable disk comprising a plurality of readable elements, each readable element comprising a first and second protrusion from the surface of the disk and arranged to return a signal at a resonant frequency owing to surface plasmon coupling.

30. A readable disk reader element comprising an incident reading beam source arranged to generate a reading beam having a plurality of frequency components.

31. An element claimed in claim 30 in which the reading beam source comprises one of a laser or an LED.

32. A readable disk reader element comprising a detector arranged to detect a spatial characteristic of a reading beam returned from a readable element corresponding to a spatial characteristic of the readable element.

33. A readable disk reader as claimed in claim 32 in which the spatial characteristic is angular orientation.

34. A readable disk reader including a reader element as claimed in claim 32 or 33, comprising a reading beam source and an optical element for imparting circular polarisation of the reading beam.

35. A method of reading a readable disk comprising generating a reading beam and detecting a spatial characteristic of the reading beam returned from the readable disk to determine a corresponding spatial characteristic of a readable element on the readable disk.

36. A method as claimed in claim 35 in which the spatial characteristic is angular orientation.

37. A readable disk reader including a reader element as claimed in any of claims 1 to 7 and/or implementing a method to read a disk as claimed in any of claims 8 to 14 or 25 to 27 or 34 to 35.

38. A disk reading element, disk reader, method of reading an optical disk or an optical disk substantially as described herein with reference to the accompanying drawings.