WO2004032126A1 - Dispositif de balayage optique pourvu de deux reseaux de diffraction a trois points - Google Patents

Dispositif de balayage optique pourvu de deux reseaux de diffraction a trois points Download PDF

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Publication number
WO2004032126A1
WO2004032126A1 PCT/IB2003/004299 IB0304299W WO2004032126A1 WO 2004032126 A1 WO2004032126 A1 WO 2004032126A1 IB 0304299 W IB0304299 W IB 0304299W WO 2004032126 A1 WO2004032126 A1 WO 2004032126A1
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WIPO (PCT)
Prior art keywords
grating
diffraction
scanning device
optical scanning
radiation
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PCT/IB2003/004299
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English (en)
Inventor
Petrus T. Jutte
Original Assignee
Koninklijke Philips Electronics N.V.
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Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Priority to AU2003263538A priority Critical patent/AU2003263538A1/en
Publication of WO2004032126A1 publication Critical patent/WO2004032126A1/fr

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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
    • G11B7/125—Optical beam sources therefor, e.g. laser control circuitry specially adapted for optical storage devices; Modulators, e.g. means for controlling the size or intensity of optical spots or optical traces
    • G11B7/127—Lasers; Multiple laser arrays
    • 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
    • G11B7/08—Disposition or mounting of heads or light sources relatively to record carriers
    • G11B7/09—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
    • G11B7/0901—Disposition or mounting of heads or light sources relatively to record carriers with provision for moving the light beam or focus plane for the purpose of maintaining alignment of the light beam relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following only
    • G11B7/0903—Multi-beam tracking systems
    • 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
    • G11B7/1353—Diffractive elements, e.g. holograms or gratings
    • 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
    • G11B7/1381—Non-lens elements for altering the properties of the beam, e.g. knife edges, slits, filters or stops
    • 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
    • G11B2007/0006—Recording, reproducing or erasing systems characterised by the structure or type of the carrier adapted for scanning different types of carrier, e.g. CD & DVD

Definitions

  • the invention relates to an optical scanning device for scanning in a first mode of operation a first type of record carrier having a first, HD, information layer and for scanning in a second mode of operation a second type of record carrier having a second, LD, information layer, which device comprises a two-wavelength diode laser for generating a first, HD, radiation beam in the first mode and a second, LD, radiation beam in the second mode, an objective system designed for operation at a first set of conjugates to focus the HD beam on the first information layer in the first mode and for operation at a second, different, set of conjugates to focus the LD beam on at the second information layer in the second mode and a first diffraction element arranged in the radiation path between the two- wavelength diode laser and the objective system for splitting the LD beam into a main and two satellite LD beams.
  • the HD beam and the LD beam are understood to mean the beam used for scanning an information layer with a higher information density and an information layer with a lower information density, respectively.
  • the transparent layer in optical record carriers is intended for protecting the information layer from ambient influences, keeping dust particles, scratches etc at a sufficient distance from the information layer and for providing mechanical support to the information layer.
  • the transparent layer functions as a substrate for the information layer.
  • the thickness of the transparent layer is a compromise between the thickness which is desired to give the record carrier the desired rigidity and the thickness which is desired in connection with the numerical aperture (NA) of the scanning beam incident on the transparent layer.
  • the NA of the objective system on the side of the record carrier is determined by the resolution the scanning device must have to read or write an information layer with a given density.
  • the resolution of the scanning device which resolution is inversely proportional to the minimum scanning spot size that can be formed by the device, is proportional to NA/ ⁇ , wherein ⁇ is the wavelength of scanning beam.
  • a scanning beam hereinafter HD (high density) scanning beam
  • LD (low density) scanning beam used for scanning a record carrier with a lower information density, like the CD (compact disc).
  • the LD scanning beam forms on the LD information layer a scanning spot, which is broader than the scanning spot formed by the HD scanning beam on the HD information layer.
  • the LD scanning beam forms on the LD information layer a scanning spot, which is broader than the scanning spot formed by the HD scanning beam on the HD information layer.
  • a compatible scanning device will have to be able to scan the different types of record carriers, independently of the thickness of the transparent layer.
  • the objective system of a compatible scanning device for two types of record carriers should have a first set of conjugates for scanning the first type of record carrier and a second, different, set of conjugates for scanning the second type of record carrier.
  • an objective system has two conjugates.
  • the first conjugate is the distance between the object plane, in the present device the emitting surface of the radiation source, and the first principal plane of the objective system.
  • the second conjugate is the distance between the second principle plane of the objective system and the image plane, in the present device the plane of the information layer.
  • Scanning a record carrier is understood to mean moving a scanning spot, formed by a scanning beam, and the information layer relative to each other for the purpose of reading, writing and/or erasing information.
  • the HD and LD scanning beams with different wavelengths may be generated by two separate radiation sources, for example laser diodes, emitting different wavelengths. These scanning beams may be combined, i.e. made co-axial, before entering the objective system by a dichroic beam splitting element, for example a prism or a semi-transparent mirror, which transmits a portion of one of the beams and reflects a portion of the other beam in the same direction.
  • a dichroic beam splitting element for example a prism or a semi-transparent mirror, which transmits a portion of one of the beams and reflects a portion of the other beam in the same direction.
  • a so-called two-wavelength laser module in combination with a beam combining element can be used, as shown, for example, in the above-mentioned patent application US2002/0027844.
  • the two-wavelength laser module may be a single laser chip comprising two light emitting elements, which emit different wavelength and are spaced, for example of the order of 100 ⁇ m from each other. If higher laser powers are required, such as for recording information, preferably a two-wavelength laser module in the form of two separate laser chips in a common laser package is used.
  • the beam-combining element is a blazed diffraction grating, arranged close to the laser chip, which diffracts the main portion of the incident LD beam into a first order beam, which is called the main LD beam.
  • the combining diffraction grating is designed such that the chief ray of the main LD beam becomes co-axial with the, non diffracted, HD beam.
  • a second diffraction grating is arranged on second surface of the plate, which carries the first diffraction grating.
  • the second diffraction grating splits the main LD beam into a non-diffracted beam, which forms a main spot on the information plane of the record carrier for reading information from the record carrier, and two first order beams, or satellite beams.
  • the satellite beams form two satellite spots in the information plane of the record carrier, which spots are used for generating a track error signal in a track-servo system to keep the main spot centered on the track momentarily read.
  • This servo system is known as three-spot system.
  • a device comprising a LD three-spot system can be used in a combined CD-recording and DND-reading apparatus, but not in a combined CD-recording and DND-recording apparatus.
  • This device is characterized in that a second diffraction element is arranged in the radiation path between the two-wavelength diode laser and the objective system for splitting the HD beam into a main and two satellite HD beams.
  • the HD satellite spots which are required for HD recording are generated by means of a second diffraction element, which diffracts only the HD beam and not the LD beam, just as the first, LD, diffraction element diffracts only the LD beam and not the HD beam.
  • the axial positions of the first and second diffraction elements can be interchanged.
  • the scanning device wherein the diffraction elements are phase gratings having grating grooves alternating with intermediate grating strips, is further characterized in that the grooves of the first grating have a depth different from that of the grooves of the second grating.
  • the optical scanning device is further characterized in that the direction of grating strips of the first diffraction element makes an angle of the order of one degree with the direction of grating strips of the second diffraction element.
  • a preferred embodiment of the device is characterized in that the first and second diffraction elements are constituted by a first and second diffraction structure arranged at an entrance surface and an exit surface, respectively of a transparent body.
  • the composed diffraction element can be manufactured by well- known pressing or replication techniques.
  • the composed diffraction element can be manufactured in one step.
  • the diffraction element can also be manufactured by plastic molding techniques or by photo lithographic processes comprising an etch step.
  • An embodiment of the device which comprises a beam splitter for separating a beam reflected by an information layer from a beam going to this layer, which beam splitter is arranged between the first and second grating at the one hand and the objective system at the other hand is characterized in that a beam-combining element is arranged between the beam splitter and a radiation-sensitive detection system.
  • this device is further characterized in that the beam splitter is a blazed diffraction grating, which diffracts only one of the HD and LD beam.
  • a blazed diffraction grating is understood to mean a grooved diffraction grating wherein at least one side of the groove is skew with respect to the plane of the grating.
  • a blazed grating can be designed such that a maximum amount of the incident radiation is diffracted in the diffraction order, for example the first order, that is used in the device so that the amount of lost radiation is reduced to the minimum.
  • Such a beam-combining element allows using the same radiation-sensitive detection system for converting reflected radiation of both the HD and LD beam into electrical signals.
  • An alternative embodiment of the scanning device is characterized in that a beam-combining element is arranged between the diode laser at the one hand and the first and second grating at the other hand.
  • the beam-combining element is arranged close to the diode laser chip(s) and combines the paths of the HD and LD beam as soon as they emerge from the diode lasers(s) so that the beams propagate along a common path through the device.
  • the beam- combining element at this position may be single diffraction grating, for example a blazed grating, which diffracts only one of the HD and LD beam
  • the alternative embodiment is characterized in that the beam- combining element comprises two opposed diffraction elements one of which has a lens function for one of the HD beam and LD beam only.
  • the beam-combining element allows adapting the vergence, i.e. convergence or divergence, of either the HD beam or the LD beam before it enters the objective system.
  • the HD beam fills the whole entrance pupil of the objective system and the LD beam only the central portion of this pupil. Consequently, the scanning spot formed on the information plane by the HD beam is smaller than the scanning spot formed by the LD beam.
  • International patent application WO 02/25646 describes the principle of such a beam-combining element and several embodiments thereof, which all can be used in the scanning device of the present invention.
  • the scanning device may be further characterized in that a beam shaper is arranged in front of the two-wavelength diode laser, which beam shaper has a beam vergence changing entrance face and a refractive exit face.
  • a beam shaper which is especially suitable for a dual optical scanning system, is characterized in that it is a lens element having a cylindrical entrance surface and a toroidal exit surface.
  • both the HD and LD diode laser beam having an elliptical cross-section can be converted in a beam having a circular cross- section, without loss of radiation.
  • a toroidal surface is understood to mean a surface which radius of curvature in the lateral plane (of the diode laser) differs from that in the transversal plane.
  • An effective and small beam shaper, in the form of a lens, which can be arranged close to a diode laser, is disclosed in US-A 5,467,335.
  • a dual-scanning device wherein a further integration has been implemented is characterized in that the diffraction structures of the beam-combimng element are arranged on the cylindrical entrance surface and on the toroidal exit surface, respectively of the beam shaper.
  • Both the composed three spot grating and the beam shaper provided with diffraction surfaces may be manufactured by well-known pressing or replication techniques.
  • a first mould having an inner profile which corresponds to the first diffraction structure and a second mould having an inner surface profile corresponding to the second diffraction structure these elements can be manufactured in one step.
  • Fig. 1 shows a dual-scanning device comprising two diode-lasers and a pre- collimator lens in the path of the LD beam;
  • Fig. 2 shows a dual-scanning device comprising a three-spot grating for the HD beam
  • Fig. 3 shows a dual-scanning device comprising a three-spot grating for the
  • Fig. 4 shows schematically a dual three-spot grating for use in this scanning device
  • Fig. 5 shows a top view of an embodiment of the detection system of this device
  • Fig. 6 shows a dual-scanning device having a beam-combining element arranged between the dual three-spot grating and the radiation source;
  • Fig. 7 shows an embodiment of an integrated beam-combining and pre- collimator element for use in the dual-scanning device
  • Fig. 8 shows an embodiment of a beam shaper for use in the scanning device.
  • the same elements are provided with the same reference numbers.
  • Fig. 1 shows a compatible scanning device for reading a first type of record carrier by means of a (HD) beam having a short wavelength and for reading of a second type of record carrier by means of a (LD) beam having long wavelength.
  • the first type of record carrier may be a digital versatile disc (DVD) and the first wavelength, for example 655 nm
  • the second type of record carrier may be a compact disc (CD) and the first wavelength, for example 785 nm.
  • the optical path of the device comprises a radiation source 1 in the form of a two-wavelength diode laser package.
  • a two-wavelength diode laser is a composed semiconductor module, which has two elements 2,3 emitting radiation beams 4,6 at different wavelengths.
  • This module may comprise a single diode laser chip having two emitting elements or two diode laser chips arranged in one package. Although the distance between the emitting element is made as small as possible, the chief rays of the two radiation beams do not coincide. Nevertheless, in Fig. 1 and the following Figs. HD beam 4 and the LD beam 6 are represented by a single radiation beam, for sake of clarity. In case the device should only read the two types of record carriers emitting element 2 emits low power red (HD) radiation and the emitting element 3 emits low power infrared radiation.
  • HD low power red
  • Beam 4 emitted by the two-wavelength laser is incident on a beam splitter, for example a plane transparent plate 8 arranged at an angle of, for example, 45°, with respect to the chief ray of the beam.
  • Plate 8 is provided with a, for example semitransparent, reflective surface 10, which reflects the beam to a collimator lens 14.
  • This lens converts the divergent beam into a collimated beam 16.
  • This beam passes through an objective lens system 18, which changes collimated beam 16 to a converging beam 20 for scanning a record carrier 30.
  • the objective lens system may consist of a single optical element, but it may comprise two or more optical elements, such as shown in the Figure.
  • the record carrier to be scanned by means of the HD beam 4 is of a first, high density, type and comprises a transparent layer 31 having a thickness of e.g. 0,6 mm, and an information layer 32, onto which converging beam 20 comes to a focus, or scanning, spot 22.
  • the radiation reflected from information layer 32 returns along the optical path of beams 20 and 16, passes the beam splitter 8 and is converged by collimator lens 14 to a detector spot 24 on a radiation-sensitive detection system 26.
  • This system converts the beam into an electric detector signal.
  • An information signal representing information stored in information layer 32, and control signals for positioning focus 22 in a direction normal to the information layer 32 (focus control) and in a direction normal to the track direction (tracking control), can be derived from the detector signal.
  • the focus control signal can be generated with the so-called astigmatic method.
  • the beam splitter 8 is positioned at an acute angle relative to the chief ray of the reflected and converged beam, this beam splitter introduces astigmatism in this beam.
  • the detection system 26 comprises a quadrant detector by means of which the shape of the cross- section of the astigmatic beam in the plane of the detection system can be detected. This shape is determined by the position of focus 22 relative to the information layer 32.
  • a lens having a spherical concave surface at its side facing the detection system may be arranged and used as a negative servo lens to set the focus of the beam.
  • the lens surface facing the beam splitter may be shaped cylindrically so that this lens has also a cylindrical lens function. This function can be used if the astigmatism introduced by skew beam splitter 8 is too small.
  • LD beam 6 for scanning the second type of record carrier 36 propagates along the same path as HD beam 4 towards this record carrier, which comprises a substrate 34 having a thickness of, e.g.1,2 mm and an information layer 35..
  • Record carriers 30 and 36 are drawn as a single, two layer record carrier having a semi-transparent information layer 32, but they may also be separate single-layer record carriers having different thickness transparent layers.
  • the LD beam should be brought to a focus, or scanning spot, 28 on the information layer 35.
  • the objective system 18 is designed such as to operate in the first mode at a first set of conjugates, whereby the HD beam from the emitting element 2 is focused on information layer 22.
  • the objective system operates at a second set of conjugates, whereby the LD beam from the emitting element 3 is focused on information layer 35.
  • Radiation reflected from information layer 35 returns along the path of the LD beam 20 and 16 passes the beam splitter 8 and is converged by means of the objective lens system into a detector spot 25 on the radiation-sensitive detection system 26.
  • a beam-combining element 40 may be a ⁇ anged, which make the chief ray of the HD and LD beam co-axial so that the position of spot 24 coincides with the position of spot 25. This allows using the same detection system 26 for the HD beam and the LD beam employed in the different modes, respectively.
  • the beam-combining element may be a wavelength selective grating, which diffracts one of the HD and LD beams and passes the other beam without diffracting it.
  • this grating is a blazed phase grating, for example a grating similar to that described in US 2002/0027844.
  • the device of Fig. 1 is suitable for reading a HD record carrier (DVD type disks)) and reading a LD record carrier (CD type disks) when switched in the HD mode (emitting element 2 switched on) and in the LD mode (emitting element 3 switched on), respectively.
  • a HD record carrier DVD type disks
  • CD type disks LD record carrier
  • Fig. 2 shows a scanning device, which is suitable for reading a LD record carrier and for reading and recording a HD record carrier.
  • This device differs from that of Fig. 1 in that the laser package comprises a high power red radiation emitting element 2' instead of low power element 2.
  • a scanning device comprises an additional detector 42, called forward sense detector 42, arranged at the rear side of the beam splitter 8. This detector supplies an output signal that is proportional to the intensity of the HD beam from element 2' and is used to control the intensity of the recording beam.
  • a diffraction grating 50 called a three-spot grating is arranged between radiation source 1 and beam splitter 8. This grating is provided with a grating structure 52 on one of its sides.
  • Grating structure 52 is wavelength selective, i.e. constitutes a grating structure only for radiation having the wavelength, for example 655 nm, of the HD beam.
  • the LD beam element 50 is transparent plate.
  • Grating 52 splits the incident HD beam 4 into a non-deflected zero order, main, beam and a plus and minus first order satellite beams.
  • Fig. 2 shows only the main beam.
  • the main beam forms in the information plane a main, scanning, spot on a track to be scanned for the purpose of recording or reading this track.
  • the satellite beams form in the information plane two satellite spots (not shown), which are shifted in opposite directions, skew to the track direction, with respect to the main spot.
  • the satellite spots are imaged in additional detector spots (not shown) on the detection system 26 and separate detector elements are provided in this system for these spots.
  • a track error signal i.e. a signal comprising an indication about a deviation between the center of the main spot 22 and the center line of the track being scanned, can be derived.
  • the track error signal can be used in a track servo system to keep the main spot on track. Generating a track error signal and the track servo system per se are well known in the art.
  • the laser package should comprise a high power infrared radiation emitting element 3 'and a three-spot track servo system for the LD beam.
  • a second wavelength selective three-spot grating is arranged between radiation source 1 and beam splitter 8. This grating diffracts only radiation having the wavelength of the LD beam and is a transparent plate for the HD beam.
  • the second three- spot grating may be a separate element, but preferably is integrated with the first three-spot grating in one element.
  • Fig. 3 shows an embodiment of the dual recording device comprising a dual grating element 55 having a HD three-spot grating structure 52 on one side and a LD three- spot grating 54 on the other side.
  • the number of surfaces in the radiation path is reduced so that the chance on false reflections is decreased. Separate alignment of one grating is no longer needed, as will be explained later on.
  • Each of the grating structures should provide a given energy ratio of radiation diffracted in the first order and zero order radiation for the beam it should diffract and the grating should be " invisible" for the beam, which should not be diffracted.
  • Fig. 4 shows very schematically, a portion of the dual grating 55 in cross- section. It comprises a transparent substrate, for example of plastics such as polymethyl methacrylate (PMMA), which is a well-known material for this kind of applications.
  • the grating comprises a first grating structure 52 at a first main surface, which structure shows grooves 56 and intermediate strips 57 of the main surface. Grooves 56 have a first depth d] and the grating pitch is pi.
  • a second grating structure 54 is provided on the other main surface of the substrate, which structure comprises grooves 58 and intermediate strips 59. Grooves 59 have a second depth d 2 and the grating pitch is p 2 .
  • a grating for diffracting a first beam, having wavelength ⁇ lt will not diffract a second beam, having a wavelength ⁇ 2 ⁇ if the groove depth is such that for the second beam the phase difference between beam portions passing the grooves and beam portion passing the intermediate strips is equal to 2 ⁇ radians.
  • This phase difference corresponds to an optical path length difference equal to ⁇ 2 . This means that the depth d of the grooves should satisfies the condition:
  • n the refractive index of the grating substrate.
  • the grating should have a duty cycle of 50%.
  • the duty cycle is understood to mean the ratio of the width w g of the grooves and the pitch, or period, p of the groove structure.
  • a duty cycle of 50% thus means that the groove width is equal to half the grating pitch, or that the groove width w g is equal to the width Wj of the intermediate strips.
  • Io l A.( ⁇ - cos(2 ⁇ .(n ⁇ - l).d/ ⁇ ))
  • Ii Vi (1 - cos(2 ⁇ .(n. ⁇ -l).d/ ⁇ )). [ sin( ⁇ /2).(2/ ⁇ )] 2
  • ⁇ is the wavelength of the beam to be diffracted and n is the refractive index of the substrate material.
  • a three-spot grating may also comprise grating strips having another refractive index than the intermediate strips.
  • the same detection system 26 can be used for the HD beam and the LD beam if for both beams the position of the satellite detector spots with respect to the associated main detector spot is substantially the same.
  • Fig. 5 shows a top view of the radiation-sensitive detection system 26, which comprises a central detector 60, for receiving the main beam and two outer detectors 65,70 for receiving the satellite spots.
  • the main detector spot and the satellite detector spots formed on these detector are denoted by reference numerals 75 and 77, 79, respectively.
  • the central detector is a four-quadrant detector and comprises detector elements 61 , 62, 63 and 64. If S 61 , S 62 , S 63 and S M denote the output signals of these elements, respectively, the information signal Si read from the record carrier is given by:
  • the astigmatic focus error signal Sf is given by:
  • the track error signal S r2 is then given by:
  • ⁇ is the ration of the intensities of the main spot and the satellite spots, respectively.
  • Essential is that the distance D between the main spot and the satellite spots is substantially the same for the HD beam and the LD beam. This distance is dependent on the diffraction angle of the diffraction grating, i.e. the angle between the chief ray of the zero order beam and a first order beam.
  • the diffraction angle ⁇ is proportional to ⁇ / p. So, if for wavelengths ⁇ l and ⁇ 2 the said distance should be equal, a first order requirement is:
  • gi and g 2 is the optical path length between the diode laser and grating 1 (pitch p ) and grating 2 (pitch p 2 ), respectively.
  • the line connecting the two satellite spots on the HD disc should make a smaller angle with the local track direction than the line connecting the two satellite spots on the LD record carrier.
  • the gratings should have a slightly different orientation, or a small difference in azimuth angle ⁇ , i.e. the angle between the grating strips and the virtual local direction of information tracks.
  • the virtual local direction is understood to mean the local direction as projected on the relevant grating.
  • the azimuth angle ⁇ for the two gratings is determined by the distance D between the main spot and the satellite spots on the detection system, the magnification m of the objective system in the direction from the record carrier to the detection system and the information track pitch q on the record carrier.
  • the aximuth angle is given by:
  • the difference ⁇ between the azimuth angles of the two gratings is equal to 0,5°.
  • Fig. 6 shows another embodiment of a dual recording/reading device wherein the invention can be implemented.
  • This embodiment differs from that of Fig. 3 in that the positions of the radiation source 1 and the detection system 26 have been interchanged, that a folding mirror 80 is arranged between the beam splitter 82 and the collimator lens, that an additional lens 84 is arranged between the beam splitter and the detection system and in that a beam combining element 90 is arranged between the dual grating 55 and radiation source 1.
  • Lens 84 may have a concave spherical surface 85 at the side of the detection system and may be used as a negative servo lens to set the focus of the beam. This can be realized by shifting this lens along the optical axis.
  • Surface 86 of the lens 84, at the side of beam splitter 82 may be shaped cylindrically so that this lens also has a cylindrical lens function. This function can be used if the astigmatism introduced by skew beam splitter 82 is too small. It is also possible that lens 84 is only a negative spherical lens or only a cylindrical lens. If necessary, an element which corrects for coma introduced by beam splitter 82 may be arranged instead of, or in addition to such a lens.
  • Beam combining element 90 may be a single diffraction grating, for example a blazed grating similar to the blazed grating shown in US 2002/0027844, which passes the zero order beam of one of the incident beams and a first order beam of the other incident beam.
  • a beam combining function and a pre-collimator function are integrated.
  • spot 22 for scanning the HD information layer 35 is smaller than scanning spot for 28 for scanning the LD information layer 32. If the whole aperture of objective system 18 is used for forming the smaller spot 22, only the central portion of this aperture should be used for forming the larger spot 28. To that end the objective system could be provided with a dichroic ring around the central portion, which rings passes the HD beam, but blocks the outer portion of the LD beam.
  • a better alternative for a dual scanning device which should be able to record information in the LD information layer would be to arrange a positive lens before collimator lens 16 in the path of the LD beam only.
  • the positive lens converts the divergent LD beam from the source 1 in a less divergent beam and may be called a pre-collimator lens.
  • the collimator lens 14 converts the less divergent LD beam into a beam, which fills only the central part of the aperture of the objective system.
  • a refractive pre-collimator lens can not be arranged in the path of the LD beam only.
  • Fig. 7 shows the principle of such a composed diffraction element 90 and the paths of the HD beam 4 and the LD beam 6 from the emitting elements 2' and 3', respectively of the two-wavelength laser to the beam splitter 8 of Fig. 3 and passing through the diffraction element 90.
  • the optical axis of the radiation path portion shown in Fig. 7 coincides with the chief ray 99 of the HD beam 4.
  • the composed diffraction element comprises a substrate 91 that is transparent for the two wavelengths of beam 4 and beam 6.
  • the substrate is provided with a diffraction structure 93, for example a Fresnel lens structure with substantially circular grooves and lands, which acts as a positive lens for the LD beam 6.
  • This diffraction structure converts the diverging beam 6 in a converging beam 95.
  • the cross- section of LD beam 95 is smaller than that of HD beam 4.
  • the substrate 91 is provided with a second diffraction structure 94, which converts the converging beam 95 in a diverging beam 96, the border rays of which are substantially parallel to the corresponding border rays of the HD beam 4.
  • the diffraction structure 94 acts as a negative lens for the LD beam and may also be a Fresnel lens type structure.
  • the depths of the grooves of both the diffraction structures 63 and 64 are chosen such that these structures have no influence on the HD beam 4, i.e. they do not change the direction or the vergence of this beam.
  • the diffraction structures 93 and 94 may be formed as holograms.
  • the original structures for these holograms i.e. the structures used for forming the moulds by means of which the diffraction element 90 is manufactured, are computer- generated structures.
  • Diffraction element 90 shown in Fig. 7 is only one embodiment of the composed pre-collimator and beam-combining element described in WO 02/25646.
  • Another embodiment uses asymmetric diffraction structures 93,94, which do not only change the vergence of the LD beam, but also deflect a portion of this beam to the optical axis. This allows using a symmetric portion of the source beam 6 for forming beam 96.
  • Another embodiment introduces changes in the HD beam, instead of in the LD beam.
  • WO 02/25646 For details about the principle and the different embodiments of the combined pre-collimator and beam- combining diffraction element reference is made to WO 02/25646. Each of these embodiments may be used in the dual recording device
  • a diode laser emits a beam whose angular aperture in a plane parallel to its active layer, known as the lateral plane, is smaller than the angular aperture in a plane perpendicular to the active layer, known as the transversal plane.
  • the beam of such a diode laser has an elliptical cross-section.
  • a round and small, preferably diffraction- limited, scanning spot should be used.
  • the objective system by means of which the scanning spot is formed must be filled with a radiation beam having a circular cross-section. If the objective system is illuminated by a diode laser beam which has an elliptical cross-section size at the entrance aperture of the objective system such that in the direction of the small axis of the ellipse the aperture is filled, then in the direction of the long axis of the ellipse an amount of radiation will fall outside the aperture.
  • a beam shaper which converts the elliptical beam in a round beam, between the diode laser and the objective system.
  • FIG. 8 shows this beam shaper 110, which is a lens element having a cylindrical entrance surface 112 and a toroidal exit surface 113 and can be arranged close to a diode laser 120.
  • This laser comprises a plurality of differently doped layers of which only the strip-shaped active layer 122 is shown.
  • Two partially transparent mirror facets 123 and 124 bound this strip so that laser radiation, which is generated when an electric current from a current source 129 is passed through the laser, can leave the active strip 2.
  • the cross-section, in the XY plane of the three- axis system of co-ordinates XYZ, of the active strip 122 and of the front facet 124 is rectangular.
  • the beam emitted by the diode laser is not symmetrical but has an aperture angle ⁇ t in the XZ plane parallel to the active strip 122, i.e. the lateral plane.
  • This aperture angle is smaller than the aperture angle ⁇ 2 in the YZ plane, i.e. the transversal plane.
  • the border rays of the laser beam in the lateral plane are denoted by the reference numerals 125 and 126 and those in the transversal plane are denoted by the reference numerals 127 and 128.
  • the entrance surface 112 has the shape of a part of a cylinder whose cylindrical axis is parallel to the Y-axis.
  • the entrance surface is a flat interface between, for example, air and the lens medium having a refractive index n. These rays are thus deflected towards the Z-axis to an extent which is determined by n. In other words an angular magnification of 1/n, which is a reduction, occurs in the YZ plane at the entrance surface 112.
  • the entrance surface 112 has a curvature R and this surface introduces a angular magnification of n.
  • the exit surface 113 of the beam shaper 110 has such a radius of curvature Rl in the transversal plane and is arranged at such a Z position that its center of curvature substantially coincides with the image, formed by the surface 112, of the laser facet 124.
  • the surface 113 transmits the rays in the transversal plane in a non- refracted way and the angular magnification in this plane is substantially equal to one.
  • the exit surface has such a radius of curvature R 2 that its center of curvature coincides with the virtual image, formed by the surface 112, of the center of the laser facet 124 so that the angular magnification in this plane is approximately one.
  • the exit surface 113 should have a slightly toroidal shape so as to combine these images to one image.
  • Toroidal is understood to mean that the radius of curvature of the surface in the lateral plane differs from that in the transversal plane. This is shown in Fig. 8 by means of the non-coplanar peripheral curve of the exit surface.
  • the beam shaper of Fig. 8 reference is made to US-A 5,467,335. If such a beam shaper is introduced in the dual scanning device of the present invention both the HD beam and the LD beam are shaped.
  • the beam shaper of Fig. 8 may be integrated with the composed diffraction element of Fig.
  • diffraction structure 93 on the entrance surface 112 and diffraction structure 94 on the exit surface 113 of the beam shaper.
  • the two emitting elements of the two-wavelength diode laser should be correctly positioned with respect to the integrated lens beam shaper. After a first one of these elements has been positioned, the second element can be positioned by rotating the housing of the two-wavelength laser.
  • the diffraction element of Fig. 7, or modifications thereof may also be integrated with a beam shaper of another type than shown in Fig. 8.

Abstract

L'invention concerne un dispositif de balayage optique d'enregistrement et de lecture d'un support d'enregistrement haute densité HD (30) et d'un support d'enregistrement faible densité LD (36), dans lequel est utilisé un laser (1) à diode à deux longueurs d'onde pour générer le faisceau de balayage HD (4) et le faisceau de balayage LD (6). Deux structures (52; 54) de diffraction sont placées dans la trajectoire du faisceau HD et du faisceau LD, l'une formant un réseau de diffraction à trois points pour un seul des faisceaux, et l'autre formant un réseau de diffraction à trois points pour l'autre faisceau seulement. Les structures de diffraction sont, de préférence, intégrées dans un élément.
PCT/IB2003/004299 2002-10-02 2003-09-30 Dispositif de balayage optique pourvu de deux reseaux de diffraction a trois points WO2004032126A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU2003263538A AU2003263538A1 (en) 2002-10-02 2003-09-30 Optical scanning device with two three-spot gratings

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
EP02079098.6 2002-10-02
EP02079098 2002-10-02

Publications (1)

Publication Number Publication Date
WO2004032126A1 true WO2004032126A1 (fr) 2004-04-15

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AU (1) AU2003263538A1 (fr)
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Publication number Priority date Publication date Assignee Title
EP1562186A1 (fr) * 2002-11-13 2005-08-10 Asahi Glass Company Ltd. Unite de source lumineuse a longueur d'onde double et dispositif de tete optique
US7548359B2 (en) 2002-11-13 2009-06-16 Asahi Glass Company, Limited Double-wavelength light source unit and optical head device having four diffraction gratings

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EP1001413A2 (fr) * 1998-10-19 2000-05-17 Victor Company Of Japan Limited Tête de lecture optique et dispositif optique
EP1047052A2 (fr) * 1999-04-20 2000-10-25 Samsung Electronics Co., Ltd. Tête de lecture optique compatible de la reproduction de disque DVD-RAM
US20010021163A1 (en) * 2000-03-10 2001-09-13 Sony Corporation. Optical pickup device and optical disc device
EP1156483A2 (fr) * 2000-05-18 2001-11-21 Samsung Electronics Co., Ltd. Lecteur de disque optique compatible et procédé d'enregistrement et de reproduction de données
US20020027844A1 (en) * 2000-08-22 2002-03-07 Hitoshi Furuhata Optical pickup apparatus
WO2002025646A1 (fr) * 2000-09-25 2002-03-28 Koninklijke Philips Electronics N.V. Dispositif de lecture optique
WO2002065169A1 (fr) * 2001-02-14 2002-08-22 Asahi Glass Company, Limited Element de diffraction selective de longueur d'onde et dispositif a tete optique
US20020196726A1 (en) * 2001-06-21 2002-12-26 Tadashi Takeda Optical pickup device
US20030072047A1 (en) * 2001-09-28 2003-04-17 Hiroyoshi Funato Optical pickup unit and optical disk drive for accurate and stable information recording and reproduction
JP2003162831A (ja) * 2001-11-27 2003-06-06 Sharp Corp 光ピックアップ装置
JP2003196860A (ja) * 2001-12-27 2003-07-11 Ricoh Co Ltd 光ピックアップ装置及び光ディスク装置
US20030179680A1 (en) * 2002-03-19 2003-09-25 Samsung Electronics Co., Ltd. Optical pickup apparatus

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EP1001413A2 (fr) * 1998-10-19 2000-05-17 Victor Company Of Japan Limited Tête de lecture optique et dispositif optique
EP1047052A2 (fr) * 1999-04-20 2000-10-25 Samsung Electronics Co., Ltd. Tête de lecture optique compatible de la reproduction de disque DVD-RAM
US20010021163A1 (en) * 2000-03-10 2001-09-13 Sony Corporation. Optical pickup device and optical disc device
EP1156483A2 (fr) * 2000-05-18 2001-11-21 Samsung Electronics Co., Ltd. Lecteur de disque optique compatible et procédé d'enregistrement et de reproduction de données
US20020027844A1 (en) * 2000-08-22 2002-03-07 Hitoshi Furuhata Optical pickup apparatus
WO2002025646A1 (fr) * 2000-09-25 2002-03-28 Koninklijke Philips Electronics N.V. Dispositif de lecture optique
WO2002065169A1 (fr) * 2001-02-14 2002-08-22 Asahi Glass Company, Limited Element de diffraction selective de longueur d'onde et dispositif a tete optique
EP1361461A1 (fr) * 2001-02-14 2003-11-12 Asahi Glass Company Ltd. Element de diffraction selective de longueur d'onde et dispositif a tete optique
US20020196726A1 (en) * 2001-06-21 2002-12-26 Tadashi Takeda Optical pickup device
US20030072047A1 (en) * 2001-09-28 2003-04-17 Hiroyoshi Funato Optical pickup unit and optical disk drive for accurate and stable information recording and reproduction
JP2003162831A (ja) * 2001-11-27 2003-06-06 Sharp Corp 光ピックアップ装置
JP2003196860A (ja) * 2001-12-27 2003-07-11 Ricoh Co Ltd 光ピックアップ装置及び光ディスク装置
US20030179680A1 (en) * 2002-03-19 2003-09-25 Samsung Electronics Co., Ltd. Optical pickup apparatus

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1562186A1 (fr) * 2002-11-13 2005-08-10 Asahi Glass Company Ltd. Unite de source lumineuse a longueur d'onde double et dispositif de tete optique
EP1562186A4 (fr) * 2002-11-13 2006-07-26 Asahi Glass Co Ltd Unite de source lumineuse a longueur d'onde double et dispositif de tete optique
US7548359B2 (en) 2002-11-13 2009-06-16 Asahi Glass Company, Limited Double-wavelength light source unit and optical head device having four diffraction gratings

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TW200415607A (en) 2004-08-16

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