The invention relates to an optical record carrier having at least one information layer, wherein information is encoded in an information structure comprising track-wise arranged information areas, which alternate in the track direction with intermediate areas.
Such a record carrier is well known in the art and its information structure can be read by means of a dedicated read device. This device comprises a radiation source, usually a diode laser, which generates a read beam having a given wavelength. An objective lens comprising one or more lens element(s) focuses the read beam to a read spot on the information layer. The read spot scans an information track, for example by rotating the disc-shaped record carrier relative to the read spot. Moving the record carrier and the read spot relative to each other in the radial direction allows scanning and thus reading of all information tracks. The size of the read spot is larger than that of the individual information areas, so that these areas diffract the incident read beam, i.e. split this beam into a non-deflected zero-order sub-beam and a number of deflected higher-order sub-beams. Current optical record carriers have a reflective information layer, and the zero-order sub-beam and portion of the first-order sub-beams reflected by the information layer pass through the objective lens. This lens concentrates the radiation portions on a radiation-sensitive detection system, whereby these radiation portions interfere with each other. The radiation-sensitive detection system, which comprises one or more detectors, converts the radiation incident on it in an electrical signal, which represents the information momentarily being read out.
There is a steady demand for ever increasing information density on optical record carriers, i.e. ever decreasing size of the information areas and intermediate areas and decreasing distance between the information tracks. A read spot with a correspondingly decreased size should be used for reading information areas with decreased size, otherwise the information areas cannot be read separately. This means that the resolution of the reading device should be increased. The conventional resolution of a read device is proportional to NA/λ, wherein NA is the numerical aperture of the objective lens and λ is the wavelength of read beam. Increasing NA and/or decreasing λ could increase the resolution. The fact that the depth focus of the objective lens is proportional to λ/(NA)2 sets a limit to the increase of the NA, because the depth of focus will become too small for a large NA. Reading devices with sufficiently small read wavelengths can be realized only when diode lasers emitting such small wavelengths are available.
U.S. Pat. No. 4,242,579 describes a reading device having a resolution which is, for example, twice the conventional resolution. The increased resolution is realized in that the objective lens passes only portions of only one first-order sub-beam and of the zero-order sub-beam of the reflected read radiation to the radiation-sensitive detection system, and in that a detector having a small dimension in the scan direction is used. To that end, the read beam and the record carrier are tilted relative to each other, i.e. the read beam is not incident perpendicularly on the record carrier. As the read beam has to pass the substrate of the record carrier and this substrate has a given thickness, for example 1.2 mm, so as to give the record carrier sufficient mechanical strength, an unacceptable amount of aberration, such as coma and astigmatism, is introduced into the read beam. This results in a read spot on the information which is larger than is acceptable and which causes crosstalk.
It is an object of the invention to realize super resolution, i.e. reading an optical record carrier having an information structure pitch which is smaller than the resolution of the objective lens, without using a skew read beam or tilted record carrier. According to the invention, this object is realized by means of a record carrier as defined in the opening paragraph, which is characterized in that that the information layer comprises means for directing radiation of a read beam, which is perpendicularly incident on the information layer, in a direction at an acute angle to the chief ray of the incident beam.
Providing the record carrier with said means allows reading with super resolution whilst using a read beam which is perpendicularly incident on the record carrier and passes perpendicularly through the carrier substrate so that no coma and astigmatic aberration occurs. Perpendicularly incident is understood to mean that the chief ray of the incident read beam, which is currently a converged beam, is perpendicular to the record carrier. The said means deflect portions of the zero-order sub beam and of one first-order sub-beam such that these portions pass through the objective lens and are focused by this lens on the radiation-sensitive detector to interfere at the location of this detector. This detector may be the same as used in the read device disclosed in U.S. Pat. No. 4,242,579.
A first embodiment of the record carrier according to the invention is characterized in that said means are constituted by a surface profile of the information layer, which profile comprises first surface portions having a first inclination with respect to the normal in the center of the record carrier, said first surface portions alternating with second surface portions having a second inclination opposed to the first inclination.
The first surface portions deflect one of the +1 order and −1 order sub-beams and the zero-order sub-beam, which are generated by the information structure, in a first direction such that radiation of these sub-beams passes the objective lens eccentrically. The second surface portions deflect the other first-order sub-beam and the zero-order sub-beam in a second direction, opposed to the first direction, such that radiation of these sub-beams passes the objective lens eccentrically. In this way it is achieved that during reading, radiation of the zero-order sub-beam and one of the first-order sub-beams will always pass through the objective lens, whilst the incident read beam is perpendicular to the record carrier. As the first-order sub-beams have the same information content a read signal is permanently present during reading.
The surface profile of a disc-shaped record carrier may extend in the tangential direction, i.e. in the track direction. However, in view of the focus servo, the first embodiment is preferably characterized in that the surface profile extends in the radial direction of the disc. in this way it can be avoided that the bandwidth of the focus servo system, which should correct the focus of the read beam for unevenness or inclinations of the information layer, should be enlarged.
Instead of in the radial direction or in the tangential direction, the surface profile may extend in any direction therebetween. The preferred direction is determined by the pattern according to which the information areas are arranged in the information layer and the way in which these areas are read. The information areas may be arranged not only track-wise, i.e. in tracks having a width sufficient to accommodate one information area, but also in a two-dimensional pattern, i.e. a number of information areas are arranged in a two-dimensional block each time, so that the information areas of such a block are read simultaneously, for example by a two-dimensional detector array. The information is then encoded in a lattice structure representing bit positions of the coded information in two dimensions. Said block of information areas may have a hexagonal shape.
The first embodiment of the record carrier may be further characterized in that the surface profile is a sawtooth profile.
Alternatively, the first embodiment of the record carrier is characterized in that the surface profile is a triangular profile.
A second embodiment of the record carrier is characterized in that said means are constituted by a grating having a grating pitch larger that the pitch of the information structure.
This grating, which may be called regular or information-less grating to distinguish it from the diffractive information structure, splits an incident beam in a zero-order sub-beam, a couple of first-order sub-beams, and couples of higher-order sub-beams. As the pitch of the regular grating is larger than the pitch of the information structure, the regular grating deflects the first-order sub-beams through smaller angles than does the information structure. The effect of the regular grating is that first-order sub-beams formed by the information structure are split up into a zero-order sub-beam and deflected secondary first-order sub-beams, some of which enter the objective lens to interfere with the zero-order sub-beam at the detection system. Such secondary first-order sub-beams are, for example the (−1, +1) sub-beam and the (+1, −1) sub-beam, wherein the first number relates to the diffraction by the information structure and the second number to the diffraction by the regular grating.
The strip-shaped regions of the grating also may extend in the radial direction or in the tangential direction or in any direction there between.
If the pitch of the information areas varies across the record carrier, for example decreases from the outer tracks to the inner tracks, the pitch of the grating may show a corresponding variation. In that case the passage “the pitch of the grating is larger than the pitch of the grating structure” is understood to mean that the pitch of the grating at a surface area of the record carrier is larger than the local pitch of the information structure.
The second embodiment of the record carrier may be further characterized in that the grating comprises a structure of alternating first regions having a first refraction coefficient and second regions having a second refraction coefficient different from the first refraction coefficient.
Such a, flat, phase grating may be formed, for example, in a phase change layer of which first regions are in the crystalline state and second regions are in an amorphous state. A phase change layer is well-known layer in the technique of optical recording and is a layer of material that can be switched between a crystalline state and an amorphous state by means of a beam of radiation having sufficient power. In the record carrier according to the invention, the phase change layer covers the information layer and is in its turn covered by a reflective layer.
Alternatively, the second embodiment of the record carrier may be characterized in that the grating comprises a structure of alternating first regions having a first height and second regions having a second height different from the first height A third embodiment of the record carries is characterized in that the grating comprises first surface regions portions having a first inclination with respect to the normal in the center of the record carrier, which first surface portions alternate with second surface portions having a second inclination opposed to the first inclination This embodiment resembles the first embodiment having a triangular or sawtooth surface relief. However, the structure now has a smaller pitch such that it acts as a grating for the wavelength of the read beam.
The invention may also be applied outside the optical record technology, for example in confocal scanning microscopy. The implementation of the invention comprises providing the sample, or in general an information plane of an object, to be viewed or inspected by the microscope with a surface profile or regular grating, for example in the form of a phase plate comprising the profile or grating, which plate covers the information plane during scanning. As the plate forms part of the scanning device, the invention is now implemented in this device. The scanning device according to the invention, which device comprises a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam which is perpendicularly incident on the information plane in a scanning spot, an object holder for holding the object, and a radiation-sensitive detection system for converting radiation from the information plane into an electrical signal, is characterized in that it comprises a plate arranged to cover the information plane during a scanning action, which plate is provided with means for directing scanning beam radiation from the information plane in a direction at an acute angle to the chief ray of the incident beam.
An information plane is understood to mean any plane wherein relevant information is present about an object that is to be inspected of from which information is to be retrieved. Such information may relate to surface conditions, to physical or chemical properties or structure of the object material, indeed any information which can be optically retrieved.
These and other aspects of the invention will be apparent from and elucidated by way of non-limitative example with reference to the embodiments described hereinafter and illustrated in the accompanying drawings.
In the drawings:
The scanning device I comprises a radiation source, preferably in the form of a semiconductor laser 9 emitting a radiation beam 7. The radiation beam, or read beam, is used for scanning the information layer 4 of the optical record carrier 2. A beam splitter 13, for example a semi-transparent mirror, reflects the diverging radiation beam from the radiation source 9 along the optical path towards a collimator lens 14, which converts the diverging beam 7 into a collimated beam 15. The collimated beam is incident on an objective system 18. The objective system, usually called objective lens, may comprise one or more lenses and/or a grating. The objective system of
Radiation reflected by the information layer 4 forms a diverging beam 22, which is transformed into a substantially collimated beam 23 by the objective lens 18 and subsequently into a converging beam 24 by the collimator lens 14. The beam splitter 13 separates the forward beam 12 and the reflected beam 24 by transmitting at least part of the converging beam 24 towards a radiation-sensitive detection system 25. The detection system captures the radiation transmitted by the beam splitter 13 and converts it into electrical output signals 26. A signal processor 27 converts these output signals into various other signals, which are processed by a signal processing circuit 29. The processing circuits 27 and 29 may be 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 a focus error signal and a radial error signal. The focus error signal represents the axial height difference 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 to a servo circuit, which converts these signals into a focus servo signal for controlling a mechanical focus actuator (not shown) in the optical head and a tracking servo signal for controlling the centering of the spot on the track being momentarily scanned. The mechanical focus actuator controls the position of the objective lens 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 (not shown) alters the position of the optical head I in the radial direction 34 of the record carrier 2, thereby 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
The circle 40 having the center 46 represents the cross-section of the zero-order sub-beam in this plane. The circles 42 and 44 having centers 48 and 50 represent the cross-sections of the (+1) order sub-beam and the (−1) order sub-beam, respectively, which are diffracted in the tangential, or track direction 36. In
In the hatched areas in
When the center of the scanning or read spot coincides with the center of an information area, for example a pit, a given phase difference N exists between a first-order sub-beam and the zero-order sub-beam. If the scanning spot moves from a first information area to the next information area, the phase of the first order beam increases by 2π. Therefore, it may be stated that as the scanning spot moves in the tangential direction the phase of the first-order sub-beam relative to the zero-order sub-beam varies by ω.t. Therein ω represents a time frequency, which is determined by the spatial frequency of the information areas and by the scanning speed. The phase φ(+1) of the first-order sub-beam b(+1) relative to the zero-order sub-beam may then be represented by:
φ(+1)=104 =ω.t
The intensity variation caused by interference of the b(+1) sub-beams with the zero-order sub-beam can be detected by a radiation-sensitive detection element 56, represented by broken lines in
Si =A(ψ).cosψ.cosωt
Wherein A(ψ) decreases at decreasing values of ψ. For a given phase depth of the information structure, the amplitude A(ψ)cosψ is constant. The frequency of the signal Si is then determined by the information which is momentarily being scanned.
So far only first-order sub-beams have been discussed. It is obvious that the information structure will diffract radiation in higher diffraction orders. The radiation intensity in these orders is low and the diffraction angles are so large at the high spatial frequencies of the information structure considered here that a negligibly small portion of the higher-order beam will fall within the pupil of the objective lens 18. The influence of the higher-order beams on the detector signal Si may therefore be disregarded.
The optical scanning system discussed above has a given cut-off frequency fc. The distance d between the center 46 of the objective pupil 52 and the centers 48 and 50 of the first-order sub-beams is proportional to λ.f, where f represents the spatial frequency of the information areas in the scan direction and λ the wavelength of the scanning beam 20.
Fc=2.NA/λ
wherein NA is the numerical aperture of the objective lens.
In order to increase the resolution of the scanning device, i.e. to allow reading of spatial frequencies higher than the conventional cut-off frequency, it is proposed in U.S. Pat. No. 4,242,579 to shift the sub-beams relative to the pupil of the objective lens in the tangential direction 36. The shift is such that a portion of a first-order sub-beam and a portion of the zero-order sub-beam still pass through the pupil of the objective lens also if the spatial frequency of the information structure is higher than the cut-off frequency.
As shown in
The width of the detector, in the tangential direction, should be small relative to the period of the intensity pattern. This period is determined by the local spatial frequency of the information areas being scanned. The maximum spatial frequency is known for a specific information structure in a record carrier or of documents or optical representations to be scanned, so that the width of the detector 70 can be adapted accordingly.
The output signal of detector 70 is supplied to a signal processor 27. The signal-to-noise ratio of the read-out signal can be improved by arranging two additional detectors 72 and 74 on both sides of detector 70 and at a distance of approximately half the period of the intensity pattern. The output signals of the detectors are added, and their sum is subtracted from the output signal of detector 30 in a differential amplifier 76 whose output is connected to the signal processor 27 shown in
In the scanning device of U.S. Pat. No. 4,242,579, the shift of the sub-beams with respect to the pupil of the objective lens shown in
As the focused read beam passes the substrate of the record carrier in a skew direction and as this substrate should have a given thickness for the envisaged applications, an unacceptable amount of aberration is introduced into the read beam and thus into the read spot A main aberration is coma, which may cause crosstalk between neighboring tracks of the information structure. Other aberrations are astigmatism and higher-order aberrations. According to the invention, deflection of the sub-beams is realized by adaptation of the information layer surface such that during reading the front surface of the record carrier, i.e. the surface directed to the objective lens, is perpendicular to the chief ray of the read beam. This results in a new type of record carrier.
The triangular shape is superposed on the information structure. If the read beam is incident on a facet 82, the sub-beams formed by the information structure will be deflected upwards, such that a portion of the zero-order sub-beam b(0) and a portion of the first-order sub-beam b(−1) will pass through different halves of the pupil of the objective lens, thus allowing reading of the high-density information at the location of this facet. If the read beam is incident on a facet 84, the sub-beams formed by the information structure will be deflected downwards, such that a portion of the first-order sub-beam b(+1) and a portion of the zero-order sub-beam will pass through different halves of the objective lens pupil. The phase modulation introduced by the information structure in the b(+1) sub-beam is the same as the phase modulation introduced in the b(−1) sub-beam.
During reading of the information structure, either a portion of the b(+1) or of the b(−1) sub-beam passes through the objective lens pupil together with a portion of the zero-order sub-beam at any moment. This means that at each moment the radiation-sensitive detection system supplies an information signal Si (26) at each and every moment. The triangular shape of the information layer thus allows the reading of the same high-density information structure as can be read with a tilted record carrier. However, the read beam does not pass obliquely through the carrier substrate so that unacceptable large aberrations are no longer introduced into this beam and into the read spot formed by this beam. The spherical aberration that may be introduced by the triangular thickness variation of the information layer is so small that it can be disregarded.
The schematic cross-sectional view of the record carrier with a triangular surface profile may be a tangential or a radial cross-section. However, if the succession of the facets 82 and 84 is in the tangential, i.e. the scan, direction, a considerably larger bandwidth is required for the focus servo loop to keep the read beam focused on the information structure. Then, it is preferred to have said succession in the radial direction, in the case in which the information areas are arranged along circular or quasi-circular tracks. The said succession may also be in any direction between the tangential direction and the radial direction. The preferred direction is determined by the arrangement of the information areas and the way these areas are read. The preferred direction for an information structure arranged in tracks may be different from that for an information structure which is arranged otherwise. In a track-wise arranged information structure the information areas succeed each other in the track direction, and the track width suffices to accommodate only one information area. Only one information area is read at any time. An example of a differently arranged information structure is a so-called 2D-OS (two-dimensional optical storage). These structures are divided into a number of blocks which each comprise a number of information areas. These blocks may have a hexagonal shape. The information areas of one block are all read out simultaneously, for example by means of an array of detection elements, the number of which corresponds to the number of information areas in the block. A 2D-OS information structure is described in previously filed co-pending application PHNL020147. For a two-dimensional information structure, the preferred direction of the surface profile or of the grating strips may be diagonal with respect to the blocks.
Instead of a triangular profile, the information layer may also show a sawtooth-shaped rear surface.
For a record carrier wherein the pitch of the information areas is variable, for example decreases from the outer tracks to the inner tracks, the pitch of the surface profile may be made variable, such that this pitch follows that of the pitch of the information structure.
As the regular grating is a flat element for the focus servo system of the read apparatus, the grating strips may extend in the tangential or in the radial direction of a disc-shaped carrier having the information areas arranged in tracks, or in any other direction. Again, the preferred direction is determined by the pattern according to which the information areas are arranged and the way the information areas are read out.
The pitch of the regular grating may again be made variable, if the pitch of the information structure is variable, such that the grating pitch follows the pitch of the information structure.
Provided that it diffracts incident radiation in the required range of deflection angles, the regular grating may be any kind of amplitude or phase grating. In view of radiation efficiency the grating is preferably a phase grating. Such a grating may comprise grating strips at another height than the intermediate strips. Alternatively, the grating may comprise grating strips having an index of refraction different from that of the intermediate strips. For example, the material of the latter grating is a phase change material which has been processed such that the material of the grating strips is in a crystalline state, whilst that of the intermediate strips is in an amorphous state.
The regular grating may also be constituted by a surface shape similar to that shown in
The mastering step, which forms part of the manufacturing process of the record carrier, may be adapted so as to provide a record carrier with the required surface profile or regular grating. In the mastering step, a resist layer on top of a substrate is exposed to a focused beam of radiation whose intensity is modulated in accordance with the information to be written. The modulated scanning across the resist layer forms a pattern of exposed areas alternating with non-exposed areas in the resist layer. Developing the resist and using the resist pattern as an etch mask transfers this pattern into the substrate. From this substrate, which is called a master, different generations of molds are made, which molds are used to make record carriers. To obtain a record carrier having a surface profile or the regular grating according to the invention, the required surface profile or regular grating can be encoded in the signal which controls the modulation of the writing beam so that the profile or grating is inscribed in the master.
Alternatively, the surface profile or grating may be fixed in a separate layer on top of the information layer. For example, the separate layer may be a layer of phase change material into which the surface profile or grating can be written by a beam focused onto a spot that is larger than the write spot used for writing the information structure.
The invention provides a general concept for increasing the information density of an information layer in an optical record carrier that can still be read out satisfactorily and can be used with optical record carriers of different types, such as CD, DVD and record carriers of a higher density type.
The invention may also be used in a multi-layer record carrier, i.e. a record carrier having two or more information layers. Each of the information layers should be provided with a surface profile or regular grating as discussed above.
The invention may also be applied outside optical recording technology, for example in confocal scanning microscopy. The implementation of the invention comprises providing the sample, or in general an information plane, to be viewed or inspected by the microscope with a surface profile or regular grating, for example in the form of a phase plate comprising the profile or grating, which plate covers the information plane during scanning. The pitch of the profile or grating should be larger than the pitch(es) expected to be present in the sample. The said plate forms part of the scanning device so that the invention is now implemented in the device. The scanning device according to the invention differs from conventional confocal scanning microscopes, or scanning devices in general, in that it comprises a plate provided with means for directing the scanning beam radiation from the information plane in a direction at an acute angle to the chief ray of the incident scanning beam. As the different embodiments of the plate means are similar to those for the record carrier means described above, the plate means need not to be described in detail.
Number | Date | Country | Kind |
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03100817.0 | Mar 2003 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/50315 | 3/23/2004 | WO | 9/7/2006 |