Classifications

• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
  • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
  • G11B7/1392—Means for controlling the beam wavefront, e.g. for correction of aberration
  • G11B7/13925—Means for controlling the beam wavefront, e.g. for correction of aberration active, e.g. controlled by electrical or mechanical means
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
  • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
  • G11B7/1365—Separate or integrated refractive elements, e.g. wave plates
  • G11B7/1369—Active plates, e.g. liquid crystal panels or electrostrictive elements
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
  • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
  • G11B7/1365—Separate or integrated refractive elements, e.g. wave plates
  • G11B7/1367—Stepped phase plates
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B7/12—Heads, e.g. forming of the optical beam spot or modulation of the optical beam
  • G11B7/135—Means for guiding the beam from the source to the record carrier or from the record carrier to the detector
  • G11B7/1392—Means for controlling the beam wavefront, e.g. for correction of aberration
  • G11B7/13922—Means for controlling the beam wavefront, e.g. for correction of aberration passive
• • G—PHYSICS
  • G11—INFORMATION STORAGE
  • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
  • G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
  • G11B2007/0003—Recording, reproducing or erasing systems characterised by the structure or type of the carrier
  • G11B2007/0009—Recording, reproducing or erasing systems characterised by the structure or type of the carrier for carriers having data stored in three dimensions, e.g. volume storage
  • G11B2007/0013—Recording, reproducing or erasing systems characterised by the structure or type of the carrier for carriers having data stored in three dimensions, e.g. volume storage for carriers having multiple discrete layers

Definitions

• This invention relates to an optical scanning device, and an optical element for use therein, for scanning an optical record carrier, such as an optical disk, comprising an information layer, the device comprising a radiation source for generating a radiation beam and an objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer.
• the invention relates to an optical scanning device including an optical element to compensate for spherical aberrations generated by different optical path lengths (referred to herein as information layer depths) through which the beam travels in an optical disk to reach an information layer in the disk.
• NA numerical aperture
• a further known optical scanning device is described in WO-A-124174, in which a radiation beam is passed through a twisted nematic (TN) liquid crystal cell which selectively rotates the polarisation of incident light by 90°. The beam is then passed, when in a convergent state, through a birefringent plate to produce spherical aberration therein.
• the birefringent plate produces different amounts of spherical aberration depending on the state of the TN cell, to compensate for the different information layer thicknesses.
• phase structure which is used to make an objective lens designed for scanning DVD record carriers also suitable for scanning CD record carriers.
• DVD record carriers are designed for being scanned with a radiation beam of a wavelength and numerical aperture different from that used for scanning a previous generation of record carrier such as CD.
• the phase structure consists of stepped non-periodic annular zones, such that each zone gives rise to a phase step which is equal to a multiple of 2 ⁇ for the DVD wavelength (660 nm), so that the phase structure has no effect at this wavelength. For CD read out, however a different wavelength is used (785 nm).
• the stepped phase profile results now in phase steps which no longer are equal to a multiple of 2 ⁇ .
• the phase introduced by the phase structure in the CD case reduces the wavefront aberration caused by the disk thickness difference to below the diffraction limit.
• the structure is capable of reducing the wavefront aberration for two discrete wavelengths.
• JP-A-2001 -174614 describes a diffractive device for an optical head for an optical scanning device capable of operating at two different wavelengths.
• the diffractive device includes a grating formed of a birefringent material embedded in a material having a uniform refractive index.
• the birefringent material is arranged in a periodic structure, i.e. one which regularly repeats across the element.
• the device transmits light at one polarisation without diffraction and diffracts light at the orthogonal polarisation.
• For the second wavelength light is transmitted without diffraction at both polarisations.
• One drawback is that the production of the periodic phase structure, i.e. the grating, is relatively complex due to the large number of elements within the structure. Furthermore, due to the diffraction, a certain amount of the input light is wasted, which is undesirable.
• PPC polarisation phase compensator
• the PPC includes a birefringent material sandwiched between glass plates and including annular phase structures which doe not affect a first beam which has a wavelength of 405nm and a first polarisation, but which effects spherical aberration compensation in a second beam which has a wavelength of 650nm and a second, orthogonal polarisation. How the PPC is to be integrated into an optical scanning device is not described, however two different radiation sources are presumably used to provide the different wavelength beams.
• an optical scanning device for scanning an optical record carrier comprising an information layer, the device comprising a radiation source for generating a radiation beam, an objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer, an electro-optical element switchable between a first state, in which the polarisation of light exiting the electro-optical element has a first orientation relative to a predetermined polarisation of light entering the electro-optical element, and a second state in which the polarisation of light exiting the electro-optical element has a second, different orientation relative to said predetermined polarisation, so as to provide a first beam of radiation primarily having a first polarisation and a second beam of radiation primarily having a second, orthogonal polarisation in dependence on the state of the electro-optical element; and an optical element for providing wavefront
• Wavefront aberration compensation may be provided for information layers at various depths within an optical disk, or between optical disks, even in a relatively high numerical aperture device, without the need for a mechanical system to provide such wavefront aberration compensation. There is also no need for more than one radiation source.
• birefringent lenses instead of a step-like structure; however the arrangement of the present invention has an advantage in that astigmatism can be reduced; birefringent lenses in general give rise to astigmatism wavefront aberration, because rays do not travel parallel to the optical axis of the lens.
• phase structure according to the invention has a non-periodic pattern, i.e. one which does not regularly repeat in the radial direction, and, therefore, does not form diffraction orders.
• the phase structure does not have the inherent losses of a grating.
• the optical element introduces the required wavefront changes without appreciable loss of radiation energy.
• an optical scanning device for scanning an optical record carrier comprising an information layer
• the device comprising a radiation source for generating a radiation beam, an objective lens, located in an optical path between the radiation source and the information layer, for converging the radiation beam to a spot on the information layer, the objective lens being mounted in mechanical actuation means for performing servo-based correction of the position of the objective lens during scanning of the optical record carrier, and an optical element for providing wavefront aberration compensation with a first radiation beam comprising radiation of a first polarisation and a second radiation beam comprising radiation of a second, orthogonal polarisation, the optical element including a part formed of polarisation sensitive material and having a surface to be located in the path of the first and second radiation beams, said surface comprising a phase structure in the form of stepped annular zones, the zones forming a non-periodic pattern of optical paths of different lengths, differences in the optical paths for the first radiation beam and the optical paths for the second radiation beam providing a difference
• centring of the optical element in relation to the objective lens is provided during scanning, without dependence on the state of actuation of the objective lens.
• the optical element is preferably provided in which the surface comprising the phase structure interfaces with air. This allows the weight of the optical element to be reduced, thereby aiding operation of the mechanical actuation means, which generally operates at high frequencies.
• an optical element for providing wavefront aberration compensation with a first radiation beam comprising radiation of a first polarisation and a second radiation beam comprising radiation of a second, orthogonal polarisation
• the optical element including a part formed of polarisation sensitive material and having a surface to be located in the path of the first and second radiation beams, said surface comprising a phase structure in the form of stepped annular zones, the zones forming a non-periodic pattern of optical paths of different lengths, differences in the optical paths for the first radiation beam and the optical paths for the second radiation beam providing a difference in wavefront aberration compensation in the first and second beams respectively, wherein said surface interfaces with air.
• the weight of the optical element may be reduced. Furthermore, the step heights of the annular zones can generally be reduced, in comparison with that required when the surface interfaces with a covering material.
• an optical element for providing wavefront aberration compensation with a first radiation beam comprising radiation of a first polarisation and a second radiation beam comprising radiation of a second, orthogonal polarisation
• the optical element including a part formed of polarisation sensitive material and having a surface to be located in the path of the first and second radiation beams, said surface comprising a phase structure in the form of stepped annular zones, the zones forming a non-periodic pattern of optical paths of different lengths, differences in the optical paths for the first radiation beam and the optical paths for the second radiation beam providing a difference in wavefront aberration compensation in the first and second beams respectively, and a part formed of non-polarisation sensitive material, the non-polarisation sensitive part interfacing with the polarisation sensitive part along said surface, wherein the polarisation-sensitive material exhibits a first refractive index for radiation of said first polarisation and a second refractive index for radiation of said second polarisation, and wherein the refractive index of the
• Figure 1 is a schematic illustration of an optical scanning device arranged in accordance with embodiments of the invention
• Figure 2 is a schematic illustration of optical components used in the arrangement of Figure 1;
• Figure 3 is a cross-sectional view of a phase modifying element in accordance with a first embodiment of the invention;
• Figure 4 shows wavefront aberrations before and after compensation
• Figure 5 is a cross-sectional view of a phase modifying element in accordance with a second embodiment of the invention
• Figure 6 is a cross-sectional view of a phase modifying element in accordance with a third embodiment of the invention.
• Figure 7 is a cross-sectional view of a phase modifying element in accordance with a fourth embodiment of the invention.
• Figure 1 is a schematic illustration of components common to a device, in accordance with each of the embodiments of the invention described by way of example below, for scanning an optical record carrier.
• the record carrier is for example an optical disk as will be described, by way of example, below.
• the optical disk OD comprises a substrate 1 and a transparent layer 2, between which at least one information layer 3 is arranged.
• a transparent layer 2 between which at least one information layer 3 is arranged.
• two information layers 3, 4 are arranged behind the transparent layer 2, at different depths within the disk, separated by 20 ⁇ m ( ⁇ 10 ⁇ m).
• a further transparent layer 5 separates the two information layers.
• the transparent layer 2, having a thickness of approximate 80 ⁇ m ( ⁇ 30 ⁇ m), has the function of protecting the uppermost information layer 3, while the mechanical support is provided by the substrate 1.
• Information may be stored in the information layers 3, 4 of the optical disk in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in Figure 1.
• the marks may be in any optically readable form, e. g. in the form of pits, or areas with a reflection coefficient or a direction of magnetization different from their surroundings, or a combination of these forms.
• the scanning device includes an optical pickup unit (OPU) mounted on a radially-movable arm.
• the OPU includes all components illustrated in Figure 1, other than the disk OD.
• the lens system includes a collimator lens 9, an objective lens 12 and a condenser lens 11.
• the objective lens 12 is rigidly mounted on a movable mounting 13 held within mechanical actuators (not shown) for performing radial tracking servo and focus servo adjustment of the position of the objective lens 12.
• the device also includes a twisted nematic (TN) liquid crystal cell 10 and a phase modifier 14, to be discussed in further detail below.
• the modifier 14 is rigidly mounted to mounting 13.
• the collimator lens 9 refracts the diverging radiation beam 7 to form a collimated beam 15.
• collimated we intend to mean a substantially parallel beam, for which the compound objective lens has a transverse magnification substantially equal to zero.
• the need for a collimated beam arises when the birefringent phase modifier 14, and other optical elements in the collimated beam path are designed for use with an ideally collimated (parallel) beam, as in this embodiment.
• a collimated beam is however not necessary when the elements in the beam path are designed for use with a divergent or convergent beam. Even using elements designed for use with an ideally collimated beam, a certain tolerance as to vergence of the beam is acceptable, depending on the efficiency required of the optical system.
• the collimated beam preferably has a vergence resulting in an absolute magnification of the objective lens smaller than 0.02.
• the objective lens 12 transforms the collimated radiation beam 15 into a converging beam 16 having a high numerical aperture (NA), in this example 0.85, which comes to a spot 18 on the information layer, 3 or 4, being scanned.
• NA numerical aperture
• Radiation of the converging beam 16 reflected by the information layer 3 or 4 forms a diverging reflected beam 20, which returns along the optical path of the forward converging beam.
• the objective lens 12 transforms the reflected beam 20 to a substantially collimated reflected beam 21, and the beam splitter 8 separates the forward and reflected beams by transmitting at least part of the reflected beam 21 towards the condenser lens 11.
• the condenser lens 11 transforms the incident beam into a convergent reflected beam 22 focused on detection systems, generally indicated by a single element 23 although a plurality of detector elements are used.
• the detection systems capture the radiation and convert it into electrical signals.
• One of these signals is an information signal 24, the value of which represents the information read from the information layer being scanned.
• Another signal is a focus error signal 25, the value of which represents the axial difference in height between the spot 18 and the respective information layer 3, 4 being scanned.
• Another signal is a tracking error signal 26, the value of which represents a radial deviation of the spot from the track being scanned.
• Each of the signals 25, 26 are input to the focus servo and tracking servo mechanical actuators controlling the position of mounting 13 during scanning.
• Another signal input into TN cell 10 is a spherical aberration control signal 30.
• the spherical aberration control signal 30 represents the selected information layer 3 or 4 in the optical disk currently being scanned.
• FIGS 2(A) and 2(B) schematically illustrate an embodiment of the invention, including those components forming part of a spherical aberration compensation optical subsystem.
• the TN cell 10 is a planar cell, consisting of a liquid crystal layer interposed between two transparent planar plates having conductive transparent layers formed on the inner surfaces thereof, forming the electrodes of the TN liquid crystal cell 10.
• the surfaces of the electrodes adjacent the liquid crystal layer are coated with an alignment material.
• the material at one side of the liquid crystal cell 10 aligns the liquid crystal molecules in an orientation which is perpendicular to the direction of orientation with which the material on the other side of the liquid crystal cell aligns the liquid crystal molecules.
• a 90° twist is formed in the bulk of the liquid crystal layer, between the two sides of the liquid crystal cell, when the cell 10 is in an off-state.
• the liquid crystal cell 10 is connected to a voltage source which is controlled by the spherical aberration control signal 30.
• the voltage source switches the liquid crystal cell 10 into an on-state, in which the liquid crystal molecules are aligned generally parallel to the optical axis of the objective lens 12.
• the polarisation of the incident radiation is rotated through 90° when passing through the liquid crystal cell 10.
• the liquid crystal cell 10 has no effect on the polarisation of the radiation passing through the cell 10.
• the liquid crystal layer in the TN liquid crystal cell 10 is relatively thin, typically at 4-6 ⁇ m.
• the response speed of the spherical aberration compensation optical subsystem is correspondingly fast, with the cell switching between on and off states within 10-50 ms.
• a further component of the spherical aberration compensation optical subsystem is a passive phase modifier 14 A.
• the phase modifier 14A is formed of a linear birefringent material, such as a cured liquid crystal compound having its molecules aligned along an optic axis.
• a spherical aberration compensating wavefront aberration generated in the phase modifier 14A which is dependent upon the optical path length in the birefringent material, is variable between two discrete states, by switching the TN liquid crystal cell 10.
• the refractive index of the birefringent phase modifier is n o when the polarisation of the incident radiation is perpendicular to its optic axis, whereas the refractive index is ng when the polarisation of the incident radiation is parallel to its optic axis.
• a polarisation rotating element 14B such as a quarter wavelength retarder plate, is interposed between the birefringent phase modifier 14A and the optical disk OD, in order to improve the optical efficiency of the device in combination with the polarising beam splitter 8, by creating a 90° rotation in polarisation between the reflected and incident beams at the polarising beam splitter 8.
• an incident beam indicated at 104, generated by the radiation source 6, first passes through the polarising beam splitter 8 with a P-type polarisation.
• the polarisation of the incident beam is rotated to an S-type polarisation.
• the beam passes through the birefringent phase modifier 14 A, the birefringent phase modifier 14A exhibiting a refractive index of no in this case since the optic axis of the birefringent phase modifier 14A is arranged in the P-direction.
• the polarisation of the incident beam is modified to a right- handed circular polarisation, and the incident beam is reflected from the information layer 3 or 4 being scanned in the optical disk 1.
• the polarisation of the reflected beam is thereby modified to a left-handed circular polarisation, which on passing through the quarter wavelength plate 14B is modified to a P-type polarisation.
• the reflected beam experiences a refractive index of ng, and on passing through the off-state TN liquid crystal cell 10 the P-type polarisation is modified to an S-type polarisation by the 90° rotation effect of the TN liquid crystal cell 10.
• the polarising beam splitter 8 reflects the majority of the reflected beam, in its S-type polarisation state, towards the detector 23 in the beam indicated at 106.
• the discussion relating to Figure 2(A) applies, however in this case the TN liquid crystal cell 10 is switched to an on-state by the spherical aberration control signal 30.
• the polarisation of the radiation incident on the TN liquid crystal cell 10 is unaffected by its passage through the cell 10. Accordingly, the beam when passing through the birefringent phase modifier 14A remains in a P-type polarisation state and experiences a refractive index of ri e , thereby generating a different pattern of wavefront aberration to the pattern of wavefront aberration generated in the off-state of the TN liquid crystal cell 10 described in relation to Figure 2(A).
• the reflected beam passes through the birefringent phase modifier 14A, the beam is in an S-type polarisation state, and experiences a refractive index of n o , again generating a different pattern of wavefront aberration to that generated when the TN liquid crystal cell 10 is in an off-state.
• the reflected beam meets the polarising beam splitter 8 the reflected beam is in an S-type polarisation, and the polarising beam splitter 8 reflects the majority of the beam towards the detector 23.
• Switching the TN liquid crystal cell 10 thus creates a difference in the shape of the wavefront of the beam incident upon the optical disk.
• the different wavefront aberrations generated in the phase modifying element 14A are used to compensate spherical aberrations generated by the passage of the beam through the transparent layers 2, 5.
• An improved resolution can thus be obtained at each of the information layers 3, 4 of the disk, even though two different depths of information layer are required to be read by the scanning device, in this example in a dual-layer optical disk.
• Figure 3 shows an example of a non-periodic phase structure (NPS) surface 100 formed of stepped annular zones on a phase modifier 114A for use in the arrangement of Figure 1, scanning each of the two information layers 3, 4 of a dual layer disk at approximately 400 nm.
• the phase modifier is formed of a physically uniform birefringent material interfacing, at least on the side of the NPS, with air. The opposite side may interface with air, or may be formed on a planar substrate, such as a glass plate.
• the disk has a transparent cover layer 2 of 80 ⁇ m and a layer separation of 20 ⁇ m.
• 2 ⁇ f(p) is the wavefront phase introduced by the variation in information layer depth.
• the root mean square of the optical path difference (OPD (rms)) of the above wavefront aberration is 194m ⁇ .
• NPS having zones generating wavefront deviations approximating the spherical aberration.
• the zones are planar annular regions arranged concentric to a perpendicular to the optical axis of the element.
• the wavefront deviation generated corresponds most closely with the phase aberration being compensated; greater (but still reduced) aberrations remain in the other parts of each zone.
• the step heights are chosen so that, for the other polarisation state, a phase of a multiple of 2 ⁇ is generated by each of the zones.
• n 0 l.5
• the number of zones used is selected on the basis of two competing criteria, the first being to increase the number of zones to provide a desired amount of wavefront aberration compensation, and the second being to reduce the number of zones so as to increase the efficiency of production of the device.
• the number of zones is preferably selected at between 5 and 25 zones, more preferably between 10 and 20 zones; in this example the number of zones chosen is selected to be 13, each zone having a radial extent as illustrated in Table 1. Table 1
• Table 1 gives the relative pupil coordinates of the outermost points ( to r 13 ) of each of the thirteen zones of the NPS together with the number of step height increments m used in each zone and the corresponding relative phase.
• the number of step height increments m used for each zone respectively is selected such that a zone generating the largest phase deviations is the smallest in thickness, and a zone generated the smallest phase variation is the largest in thickness. Note that, by such selection, the variation in thickness of the zones of the NPS can be arranged to be less than half the number of zones multiplied by the step height increment, in this case only 4.3 ⁇ m.
• Figure 4(B) shows the compensated wavefront aberration with the above structure present is shown.
• the wavefront aberration is reduced from 194m ⁇ (rms) to 37m ⁇ (rms), which is well below the diffraction limit at the operation wavelength, so that a diffraction limited spot size is obtained.
• Figure 5 illustrates a further embodiment of the invention, in which the birefringent phase modifying element 214A is integrated with a quarter wave retarder plate 214B, the two elements being bonded together at a planar interface.
• the phase modifier 214A includes a non-periodic phase structure 200 similar to that illustrated in Figure 3.
• Figure 6 illustrates a further embodiment of the invention, in which the wavefront aberration is generated by means of a phase modifier including a part formed of birefringent material 314A integrated with a part 302 formed of a non-polarisation sensitive material, i.e. one having only a single refractive index.
• the non-periodic phase structure surface 300 forms an interface between the part 314A and 302.
• the zones occupy the radial extents set out in Table 1, however the step heights are increased, in that the step height increments are defined by:
• phase modifier in this embodiment is also integrated with a quarter wave retarder plate 314B, the parts 314A and 314B being bonded via a planar interface.
• Figure 7 illustrates a yet further embodiment of the invention, in which the birefringent phase modifier 414A is integrated with a part 402 formed of non-polarisation sensitive material having a refractive index matching the ordinary refractive index, n o of the birefringent material of part 414A. Due to the refractive index matching at one polarisation the interface 400 of the non-periodic phase structure surface is invisible to the light at one polarisation, independent of the step heights.
• ni n o
• ni the refractive index of the non-polarisation sensitive material
• p s is the relative pupil coordinate at a selected part (e.g. outer periphery, centre or inner periphery) of the zone, and q is an integer (..., -2, -1, 0, 1, 2 ...), in this embodiment zero to reduce the step heights.
• the zones are arranged to have a step height increment which is constant; however this is not necessary. Due to the matching of the refractive indices irregular variations in step height increments may also be used.
• the number and radial arrangement of zones is the same as that of the previous embodiments.
• the number of step height increments, n, selected in each zone are given in Table 2 below.
• step height increments in this embodiment are significantly smaller than that of the embodiment shown in Figure 6, and furthermore the total step heights used are smaller than the embodiment shown in Figure 3.
• the largest phase deviations are generated by the thickest zones.
• a quarter wave retarder 414B is also integrated with the phase modifier 414A, the two parts being bonded via a planar interface.
• phase modifier 314A, 414A may also be integrated with the TN cell; in this case the liquid crystal layer of the TN cell is preferably sandwiched between the planar outer surface of non-polarisation sensitive part 302, 402, and a transparent planar cover plate.
• the present invention allows for the reading of an optical disk of high capacity, using a relatively low wavelength radiation beam, for example a radiation beam of approximately 400nm wavelength, using a high numerical aperture beam at the optical disk, and without using a mechanical actuator or a birefringent lens to effect spherical aberration compensation, even for varying depths of information layer in the optical disks being read.
• an NPS pattern may be used capable of correcting coma wavefront aberration.
• switching the TN cell results in a different amount of wavefront deviation being generated approximating to coma, for example in response to detection of excessive tilt of the disk in one state of the TN cell.
• an NPS pattern may be used to correct for astigmatism, wherein different degrees of wavefront deviation may be generated to correct for astigmatism in two different states of the TN cell.
• a TN liquid crystal cell is used to selectively rotate the polarisation of the incident radiation through 90°
• similar (but less optimal in terms of complexity and the efficiency of the optical system) functionality could be provided by dispensing with the polarisation rotating element and instead using either a single radiation source emitting radiation at an orientation of 45° to the axis of birefringence and/or the beam splitter, or two separate radiation sources emitting orthogonally-polarised radiation at each of the required polarisations.
• the required spherical aberration compensation can then be selected, in accordance with a selection control signal, for example by a switchable polarisation-selective filter at the detector.
• the sources may be selectively energized in accordance with the selection control signal.
• the objective lens is indicated in Figure 1 as having one planoconvex lens element, it may comprise more elements, and other lens types such as convex- convex or convex-concave lenses may also be used.
• the objective may also comprise a hologram operating in transmission or reflection, or a grating for coupling radiation out of a waveguide carrying the radiation beam.
• annular is not intended to be limited to an area exhibiting perfect circular symmetry; in one embodiment the annular zones of the NPS exhibit such symmetry; in other embodiments the zones may deviate from perfect circular symmetry.

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  • 2002-12-05 EP EP02804330A patent/EP1459307A2/de not_active Withdrawn
  • 2002-12-05 WO PCT/IB2002/005238 patent/WO2003049095A2/en active Application Filing
  • 2002-12-05 JP JP2003550209A patent/JP2005512255A/ja active Pending
  • 2002-12-05 KR KR10-2004-7008627A patent/KR20040071703A/ko not_active Application Discontinuation
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