WO1999016061A1 - Apparatus and method for differential wax-wane focusing and push-pull tracking for magneto-optical data storage - Google Patents

Apparatus and method for differential wax-wane focusing and push-pull tracking for magneto-optical data storage Download PDF

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Publication number
WO1999016061A1
WO1999016061A1 PCT/US1998/019826 US9819826W WO9916061A1 WO 1999016061 A1 WO1999016061 A1 WO 1999016061A1 US 9819826 W US9819826 W US 9819826W WO 9916061 A1 WO9916061 A1 WO 9916061A1
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Prior art keywords
subdetectors
pair
optical
subdetector
photodetector
Prior art date
Application number
PCT/US1998/019826
Other languages
French (fr)
Inventor
Edward C. Gage
Ronald Evan Gerber
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Seagate Technology, Inc.
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Publication date
Application filed by Seagate Technology, Inc. filed Critical Seagate Technology, Inc.
Publication of WO1999016061A1 publication Critical patent/WO1999016061A1/en

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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
    • G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
    • G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
    • G11B11/1055—Disposition or mounting of transducers relative to record carriers
    • G11B11/10576—Disposition or mounting of transducers relative to record carriers with provision for moving the transducers for maintaining alignment or spacing relative to the carrier
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
    • G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
    • G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
    • G11B11/10532—Heads
    • G11B11/10541—Heads for reproducing
    • G11B11/10543—Heads for reproducing using optical beam of radiation
    • 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
    • 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
    • G11B7/0908—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 focusing only
    • G11B7/0917—Focus-error methods other than those covered by G11B7/0909 - G11B7/0916
    • 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
    • G11B7/0943—Methods and circuits for performing mathematical operations on individual detector segment outputs
    • 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
    • G11B7/13—Optical detectors therefor
    • G11B7/131—Arrangement of detectors in a multiple array
    • 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
    • G11B7/13—Optical detectors therefor
    • G11B7/133—Shape of individual detector elements
    • 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
    • G11B7/1356—Double or multiple prisms, i.e. having two or more prisms in cooperation
    • 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
    • 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
    • G11B7/1384—Fibre optics
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
    • G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
    • G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
    • G11B11/1055—Disposition or mounting of transducers relative to record carriers
    • G11B11/10552—Arrangements of transducers relative to each other, e.g. coupled heads, optical and magnetic head on the same base
    • G11B11/10554—Arrangements of transducers relative to each other, e.g. coupled heads, optical and magnetic head on the same base the transducers being disposed on the same side of the carrier
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
    • G11B11/00—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor
    • G11B11/10—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field
    • G11B11/105—Recording on or reproducing from the same record carrier wherein for these two operations the methods are covered by different main groups of groups G11B3/00 - G11B7/00 or by different subgroups of group G11B9/00; Record carriers therefor using recording by magnetic means or other means for magnetisation or demagnetisation of a record carrier, e.g. light induced spin magnetisation; Demagnetisation by thermal or stress means in the presence or not of an orienting magnetic field using a beam of light or a magnetic field for recording by change of magnetisation and a beam of light for reproducing, i.e. magneto-optical, e.g. light-induced thermomagnetic recording, spin magnetisation recording, Kerr or Faraday effect reproducing
    • G11B11/1055—Disposition or mounting of transducers relative to record carriers
    • G11B11/1058—Flying heads

Definitions

  • the present invention is directed to an improved method and apparatus for determining the focus and tracking of a light beam from a reflective medium, and, in particular, the invention is directed to a method and apparatus for differential wax-wane focusing and push-pull tracking for a magneto-optical data storage device.
  • the diode is focused onto the surface of a rotating optical disk. Data is stored on the disk in a track or tracks.
  • the optical disk may have a single spiral track or
  • sensing information to be included on a part of the data storage surface so that
  • the data storage systems can retrieve information recorded on that surface.
  • Positioning information is typically encoded on the disk surface in a series of pits
  • a focusing servo system generates a focus error signal that controls the positioning of the laser diode with respect to the disk surface.
  • a tracking servo system generates a tracking error signal that controls the positioning of the laser diode with respect to the track centerline.
  • the focus error signal and a track error signal control actuators that are coupled to the laser diode to properly focus and align the laser diode with respect to a desired track.
  • the focus and tracking error signals are
  • Typical magneto-optical systems use up to twelve detector elements to generate the four required signals: the focus error signal (FES), the tracking
  • MODS data signal
  • the present invention includes an apparatus for determining the focus of a light beam from a reflective medium.
  • the apparatus has a prism and a photodetector.
  • the prism receives the light beam and outputs optical beams.
  • the photodetector has subdetectors. Two adjacent subdetectors form a first pair, the other two adjacent subdetectors form a second pair.
  • the prism projects the optical beams onto the photodetector such that one of the optical beams is projected on at least one subdetector of the first pair of the
  • the amount of light projected on the subdetectors indicates the focus of the light beam with respect
  • a disk drive includes the apparatus of the present invention that determines the focus of a light beam from a reflective medium.
  • a method for generating a focus error signal is provided.
  • a light beam is received from a reflective medium and first and second light beams are generated from the received beam.
  • the first light beam is projected onto a first pair of subdetectors in a photodetector; and the second
  • Each subdetector generates a signal in response to the portion of the light beam projected on that subdetector.
  • Two composite signals are determined by adding the signals from diagonally opposite subdetectors in the photodetector.
  • the focus error signal is generated by subtracting one composite signal from the other.
  • Figure 1 is a top plan view of the basic elements of a
  • FIG. 1 illustrates the primary optical paths and some basic electrical elements of a magneto-optical disc drive in which this invention is useful;
  • Figure 3 is a schematic view showing the actuator arm, slider and disc of a magneto-optical disc drive
  • Figures 4A-4F are views from various angles and with varying degrees of detail of a slider and the optics supported on the slider in a magneto-optical disc drive;
  • Figure 5 is a diagrammatic side view of the Wollaston prism and the photodetector of the present invention.
  • Figure 6 is a front view of the photodetector of Figure 5;
  • Figure 7 is a front view of another embodiment of the photodetector of Figure 5;
  • FIG. 8A illustrates the out of focus condition when the
  • magneto-optical head is too far from the disk surface
  • Figure 8B illustrates the in-focus condition when the head
  • magneto-optical head is properly positioned with respect to the disk surface
  • Figures 8C illustrates the out of focus condition when the magneto-optical head is too close to the disk surface
  • Figure 9 is a graph of the FES signal versus the distance
  • Figure 10A illustrates the received beams or spots on the photodetector for an off track condition when the head is positioned too close
  • Figure 10B illustrates the received beams or spots on the photodetector for an on-track position when the head is properly positioned over
  • Figure IOC illustrates the received beams or spots on the photodetector for an off track condition in which the head too close to the right edge of the track or groove; and
  • Figure 11 is a flowchart of the method of generating the focus error signal (FES), tracking error signal (TES), magneto-optical data signal (MODS) and reflectivity data signal (RDS) of the present invention.
  • FES focus error signal
  • TES tracking error signal
  • MODS magneto-optical data signal
  • RDS reflectivity data signal
  • FES focus error signal
  • TES tracking error signal
  • Figure 1 is a plan view illustrating some of the basic elements of a magneto-optical (MO) data storage and retrieval system. Few specific details are identified in this and Figures 2 through 4 as they are intended to portray some of the basic elements of a functional system in which the present invention is useful. The invention is not limited to use in only one specific MO data storage system.
  • the system 100 includes a set of flying heads
  • the disks 107 are double sided.
  • each MO disk surface 107 is provided for each MO disk surface 107.
  • the heads 106 are coupled to a rotary actuator magnet and coil assembly 110 by a suspension 112 and actuator arm 114 to be positioned over the surfaces of the MO discs 107.
  • the MO discs 107 are rotated by a spindle motor 116 to generate aerodynamic lift forces between the flying heads 106 and the rotating disc 107. This maintains each flying MO head 106 in a flying condition above the data recording surface of each MO disc 107.
  • the lift forces are opposed by equal and opposite spring forces supplied by the suspensions 112. During non-operation, each flying MO
  • the MO disc 107 typically on a ramp (not shown) adjacent to the disk surface
  • System 100 further includes a laser-optics assembly 120
  • optical switch 122 which connects to a set of single mode
  • each set of single mode PM optical fibers 124 are coupled through a respective one of the set of actuator arms 114 and suspensions 112 to a respective one of the set of flying MO heads 106.
  • Figure 2 is a diagram showing the laser-optics assembly 120 of the magneto-optical data storage and retrieval system of Figure 1.
  • reading and storing information on the surface of each disc 107 requires both conveying the output of a laser through an optical fiber to the flying head so that the light output is accurately imaged on the surface of the disc, and generating a magnetic field utilizing a coil which is supported on the flying head 106 near to the surface of the disc 107.
  • the data is arranged in a concentric series of tracks 126, with each track being made up of a number of sectors which in turn contain multiple bits of binary data.
  • the servo information is typically recorded in the space 128 between these tracks; typically the
  • the data would be both written and read on the data tracks 126 which would be found between the servo track regions 128 by the MO head 106 supported on an actuator 114 for radial movement in the direction of arrow 132.
  • the movement is caused by a motor
  • the laser-optics assembly 120 is shown to include a linearly polarized diode laser source 231 operating in a visible or near ultraviolet frequency region and emitting an optical
  • the laser diode source may be a RF modulated laser source.
  • the linearly polarized laser source 231 may be a distributed feedback (DFB) laser source.
  • the linearly polarized laser source 231 may be a distributed feedback (DFB) laser source.
  • polarized laser source 231 is selected to operate within a range of 635-685 nm;
  • optics assembly 120 further includes: a collimating optics 234, a low wavelength dispersion leaky beam splitter 232 and a coupling lens 233.
  • the laser-optics assembly 120 directs (from the linearly polarized laser source 231) a linearly
  • the laser-optics assembly 120 further includes a V* wave plate 236 and a mirror
  • an optical isolator 239 is included between the laser source 231 and the collimating lens 234.
  • the VA wave plate 236 and mirror 237 are not used, and the light beam is routed directly to the Wollaston prism 240 from the beam splitter 232.
  • the differential detector 238 includes a Wollaston prism 240 that
  • each subdetector In response to the portion of the optical beam projected upon the subdetector, each subdetector generates a voltage VA,VB,VC,VD, and a current IA, IB,IC,ID, respectively, which are output to summing block 250.
  • an FES generator 252 Inside the summing block 250, an FES generator 252
  • TES generator 254 generates the TES signal using the following relationship:
  • the Reflectivity data signal (RDS) is used for clocking and tracking and is generated by an RDS generator 256 as follows: VA+VB+VC+VD.
  • the Magneto-optical data signal (MODS) represents the stored data and is generated as follows: (VA+VB)-(VC+VD). Note that each of the four signals
  • VA,VB, Vc, VD generated by the photodetector 242 is supplied to each of the generators 252-258.
  • summing amplifiers and subtracters are used to implement the relationships above to generate the FES,
  • Reflectivity variations of the disk can be caused by pits in the disk that cause a loss of return light due to destructive interference.
  • the reflectivity variations can also be caused by differences in material properties that fill the pits. Reflectivity information is used for encoding position information on the disk.
  • Figure 3 is a diagram showing a representative optical path
  • a DFB laser source that includes the use of a DFB laser source.
  • a DFB laser source that includes the use of a DFB laser source.
  • FIG. 3 representative optical path is shown in Figure 3 to include, the optical switch 122, one of the set of single-mode PM optical fibers 124, and one of the set of
  • the optical switch 122 provides sufficient degrees of
  • the outgoing laser beam 191 is further directed by the single
  • modePM optical fiber 124 to exit a respective distal end so as to pass through the flying MO head 106 onto a recording/storage layer 349 of a respective MO disk 107.
  • the outgoing laser beam 191 is provided by a linearly polarized laser source 231 that is a DFB laser source.
  • a DFB diode laser source unlike an RF-modulated Fabry-Perot diode laser,
  • the light exiting the optical fiber includes a polarization state that depends on the relative orientation between the fiber axes and the incident polarization, and moreover, the output polarization state is very stable in time as long as external perturbations which alter the fiber
  • the DFB laser source enables use of the single- mode PM optical fiber 124 for delivery and return of polarized laser light to and from the MO disk 107.
  • the optical intensity of outgoing laser beam 191 is held constant, while a time varying vertical bias magnetic field is used to define a pattern of
  • outgoing laser beam 191 may be modulated in synchronization with the time varying vertical bias magnetic field at the spot of interest 340 in order to better control domain wall locations and reduce domain edge jitter. Subsequently, as the selected spot of interest 340 cools, information is encoded within the
  • the outgoing laser beam 191 (at a lower intensity compared to writing) is selectively routed to the MO disk 107 such that at any given spot of interest 340, the Kerr effect causes (upon
  • a reflected laser beam 192 to have a rotated polarization of either clockwise or
  • the aforementioned optical path is bi-directional in nature. Accordingly, the reflected laser beam 192 is received through the flying MO
  • reflected laser beam 192 propagates along the single-mode PM optical fiber 124
  • Figures 4A-4F are diagrams showing the flying magneto-
  • optical head of the magneto-optical data storage unit in a perspective, a side
  • the flying MO head 106 is shown for use above a recording/storage layer 349 of one of the set of MO discs 107.
  • the flying MO head 106 includes: a slider body 444, an air bearing surface 447, a quarter-wave plate 493, a reflective substrate 400, objective optics 446, a magnetic coil 460, and a yoke 462.
  • the slider body 444 is dimensioned to accommodate the working distances between the objective optics 446, the single-mode PM optical fiber 124 and the reflective substrate 400.
  • the reflective elements 446 is dimensioned to accommodate the working distances between the objective optics 446, the single-mode PM optical fiber 124 and the reflective substrate 400.
  • substrate 400 may include a reflective surface which is aligned so as to direct the outgoing laser beam 191 and 192 to and from the recording/storage layer 349.
  • slider body 444 may include industry standard “mini”, “micro”, “nano” or “pico” sliders, alternatively dimensioned slider bodies 444 may also be used.
  • the slider body 444 comprises a mini-slider height (889 ⁇ m) and a planar footprint area corresponding to that of a nano-slider
  • the single-mode PM optical fiber 124 is coupled to the
  • the cutouts 443 and 411 may be designed as channels, v-grooves, or any other
  • layer 349 of the MO disk 107) that includes: the single-mode PM optical fiber
  • the single-mode PM optical fiber 124 and the objective optics 446 are positioned within their respective cutouts to achieve
  • the single-mode optical fiber 124 and the objective optics 446 may be subsequently secured in place by using ultraviolet curing epoxy or a similar adhesive.
  • the Wollaston prism 240 receives a light beam 502 reflected from the disk.
  • the Wollaston prism 240 outputs or generates two optical beams 504, 506 from the light beam 502.
  • the Wollaston prism 240 is arranged to project the two optical beams 504, 506 onto a sensor surface 508 of the photodetector 242 as shown by spots 510, 512.
  • the sensor surface 508 includes subdetectors A,B,C, and D.
  • the subdetectors A,B,C,D are photo-diodes.
  • Subdetectors A and B form a first pair
  • subdetectors C and D form a second pair.
  • the Wollaston prism 240 projects the optical beams 504, 506 onto the photodetector
  • subdetectors indicates the focus of the light beam with respect to the reflective medium.
  • each subdetector A,B,C,D in response to the amount of light projected on that subdetector, each subdetector A,B,C,D generates a voltage which is output
  • a significant feature of the invention is that one of the subdetector of the first and second pairs of subdetectors receives more light than the other subdetector of that pair when the light beam from the head is properly focused on the reflective medium. As shown in Figure 5, subdetector B receives
  • one of the optical beams 504 is projected asymmetrically onto the subdetectors A,B of the first pair of subdetectors, and the other optical beam 506 is projected asymmetrically onto
  • the subdetectors C, D of the second pair of subdetectors when the light beam is properly focused over the reflective medium.
  • all subdetectors receive a portion of the light from their respective optical beams.
  • distance from the Wollaston prism 240 to the photodetector 242 is predetermined and is a function of the frequency of the laser light, the intensity
  • a Rochon prism is used.
  • the two optical beams are arranged
  • a beam axis 522 is drawn through the center of each optical beam 504, 506 and is aligned with an on-track axis 523. As shown in Figures 5 and 6, the beam axis 522 is aligned with the on- track axis 523 to indicate that the head is properly positioned with respect to a track centerline.
  • the subdetectors A,B,C,D are arranged on the sensor surface 508 in a quadrant which has a vertical axis 524 and a horizontal axis 526.
  • the vertical axis 524 is rotated with respect to the on-track axis 523 by an angle ⁇ . In one embodiment a ranges from approximately 5° to 10° and preferably is approximately 7° . If the beam axis 522 is not aligned with the on-track axis 523,
  • a cross-track axis 528 is substantially perpendicular to the in-track axis 524.
  • the rotation of the vertical axis 524 with respect to the in-track axis 523 causes the FES to have a positive predetermined
  • the TES will be substantially equal to zero when on the track
  • Figures 5-7 show circular spots 510, 512, in an alternate embodiment, the spots 510, 512 may be elliptical.
  • the sensor surface 508 is rotated with respect to the in-track axis of the Wollaston prism.
  • the vertical axis 524 and horizontal axis 526 of the sensor surface 508 are aligned to the edges of the photodetector 242, while the Wollaston prism 240 is rotated
  • photodetector 242 with respect to the Wollaston prism 240 allows the MO data signal to be generated from the voltages output by the photodetector 242 using the equation described above.
  • Figure 7 depicts another embodiment of a photodetector of of the present invention.
  • subdetectors A,B,C,D is substantially perpendicular to the on track axis 522.
  • Subdetectors A,B form a first pair.
  • Subdetectors C, D form a second pair.
  • the first pair A,B is arranged above the horizontal axis 526, and the second pair C,D
  • the subdetectors of the subdetector pairs are
  • the subdetectors are arranged such that regions B and C, diagonally opposite each other, are offset and extend over the
  • this offset causes the FES to have a positive predetermined value or range of values when in focus, and will not be equal to zero.
  • the TES will be substantially equal to zero when on the track centerline.
  • the amount of offset or predetermined distance d is the
  • the amount of offset is such that over 50% but less than 100%, and in particular from over 55% to 95%, and preferably 75% of the optical beams 504, 506 are projected on regions B and D when the head is aligned with the track centerline.
  • the MO data signal to be generated from the voltages output by the photodetector 242 using the equation described above.
  • Figures 8 A, 8B and 8C depict the state of the spots 510, 512 for the near side of focus when the MO head is too far from the disk, the proper
  • Figure 8C will generate an FES with a value that
  • the x-axis represents the disk defocus in microns where the disk defocus is the distance from a predetermined position with respect to the disk surface.
  • the y-axis represents the FES in volts.
  • the FES value is equal to approximately 0.48 volts when the light beam from the head is properly focused on the disk surface.
  • FES curve 900 at the preferred focus value is equal to approximately -0.076 volts/ ⁇ m.
  • the normal operating range is between lines 902 and 904 and is plus or minus one micron.
  • the y-axis of the graph represents volts, other units such as current (amperes) could be used.
  • Figures 10 A, 10B, and 10C depict the received beams or spots 510, 512 on the photodetector 242 from the Wollaston prism for an off track, on track and another off track condition, respectively.
  • the head is positioned to the "left" edge of the groove or track.
  • the spots 510, 512 have a bright area 1002, a gray area 1004 and a dark area 1006.
  • the bright area 1002 represents a region of least light intensity; the gray area represents a region of more light intensity than the bright area; and, the dark area 1006 represents
  • TES has a positive value.
  • a method for generating the focus error signal, tracking error signal, and magneto-optical (MO) read signal is shown.
  • a light beam is received from the reflective medium such as a disk.
  • a prism generates first and second light beams from the received light beam, as described above.
  • the prism projects the first light beam onto a first pair of subdetectors on a photodetector.
  • the prism projects the second light beam onto a second pair of subdetectors on the photodetector.
  • Steps 1106 and 1108 are substantially simultaneous.
  • each subdetector of the photodetector generates a signal in response to the portion of the light beam projected on that subdetector.
  • Steps 1112 and 1114 generate the focus error signal.
  • the focus error signal is generated by subtracting one composite signal from the other, as
  • the focus error signal has a non-zero value.
  • Steps 1116 and 1118 generate the tracking error signal.
  • step 1116 two tracking composite signals are determined by adding the signals
  • a tracking error signal is generated by subtracting one of the tracking composite signals from the other
  • Steps 1120 and 1122 generate the MO read signal.
  • two magneto-optical composite signals are determined by adding signals from the subdetectors of the first pair and the second pair, respectively.
  • the magneto-optical read signal is generated subtracting one of the magneto-optical composite signals from the other magneto-optical composite signal, as described above.
  • a key advantage to using the present invention is that the number of photodetectors and corresponding optical element and circuity is reduced, thereby reducing system complexity.

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Abstract

The present invention is an optical apparatus which comprises a Wollaston prism (240) for splitting a reflected light beam (502) from an optical disk into two beams (504, 506); a photodetector (242) having first pair of subdetectors (A, B) and second pair of subdetectors (C, D) for detecting the two splitted beams (504, 506), one subdetector in each pair collects more light beam than the other, and generating two focus composite signals, two tracking composite signals, and two magneto-optical composite signals; and a focus error signal (FES) is generated by subtracting one focus composite signal from the other, a tracking error signal (TES) is generated by subtracting one tracking composite signal from the other, a magneto-optical data signal (MODS) is generated by subtracting one magneto-optical composite signal from the other, and a reflective data signal (RDS) is generated by adding all signals from both pairs of subdetectors (A, B, C, D).

Description

APPARATUS AND METHOD FOR DIFFERENTIAL WAX-WANE FOCUSING AND PUSH-PULL TRACKING FOR MAGNETO-OPTICAL
DATA STORAGE
CROSS-REFERENCE TO RELATED APPLICATION
This invention is based on U. S. Provisional Patent Application, Serial No. 60/059,439, filed September 22, 1997, assigned to the
assignee of this application and incorporated herein by reference.
FIELD OF THE INVENTION
The present invention is directed to an improved method and apparatus for determining the focus and tracking of a light beam from a reflective medium, and, in particular, the invention is directed to a method and apparatus for differential wax-wane focusing and push-pull tracking for a magneto-optical data storage device.
BACKGROUND OF THE INVENTION In an optical disk data storage system, light from a laser
diode is focused onto the surface of a rotating optical disk. Data is stored on the disk in a track or tracks. The optical disk may have a single spiral track or
multiple concentric tracks. Data storage on rotating media requires position
sensing information to be included on a part of the data storage surface so that
the data storage systems can retrieve information recorded on that surface.
Positioning information is typically encoded on the disk surface in a series of pits
and grooves. As the disk rotates, the light from the laser diode should be focused in the plane of the disk and on the track centerline. A focusing servo system generates a focus error signal that controls the positioning of the laser diode with respect to the disk surface. A tracking servo system generates a tracking error signal that controls the positioning of the laser diode with respect to the track centerline. The focus error signal and a track error signal control actuators that are coupled to the laser diode to properly focus and align the laser diode with respect to a desired track. The focus and tracking error signals are
typically generated by combining the electrical signals from a series of photodetectors that capture the light reflected off the optical disk surface. In addition, a reflectivity data signal which has clock and positioning information, and magneto-optical (MO) data signal which represents the encoded data are generated. Typical magneto-optical systems use up to twelve detector elements to generate the four required signals: the focus error signal (FES), the tracking
error signal (TES), the reflectivity data signal (RDS) and the magneto-optical
data signal (MODS). Other designs place additional optical elements, such as astigmatic lenses or diffraction gratings, in the light path to generate the four
signals.
Therefore, it is desirable that a simpler method and apparatus
generate the focus error signal, tracking error signal, the reflectivity data signal
and the magneto-optical data signal. SUMMARY OF THE INVENTION
It is therefore a primary object of the invention to provide a less complex apparatus and method for generating the focus error signal.
It is a related objective to provide a less complex apparatus and method for generating the tracking error signal.
It is also an object of the invention to generate the reflectivity data signal and the magneto-optical data signal using the less complex apparatus and method.
The present invention includes an apparatus for determining the focus of a light beam from a reflective medium. The apparatus has a prism and a photodetector. The prism receives the light beam and outputs optical beams. The photodetector has subdetectors. Two adjacent subdetectors form a first pair, the other two adjacent subdetectors form a second pair. The prism projects the optical beams onto the photodetector such that one of the optical beams is projected on at least one subdetector of the first pair of the
subdetectors, and another of the optical beams is projected on at least one
subdetector of the second pair of the subdetectors. The amount of light projected on the subdetectors indicates the focus of the light beam with respect
to the reflective medium.
In another embodiment, one of subdetectors of each pair
receives more light than the other subdetector of that pair when in focus. In yet another embodiment, a disk drive includes the apparatus of the present invention that determines the focus of a light beam from a reflective medium.
In addition, a method for generating a focus error signal is provided. A light beam is received from a reflective medium and first and second light beams are generated from the received beam. The first light beam is projected onto a first pair of subdetectors in a photodetector; and the second
light beam is projected onto a second pair of subdetectors in the photodetector. Each subdetector generates a signal in response to the portion of the light beam projected on that subdetector. Two composite signals are determined by adding the signals from diagonally opposite subdetectors in the photodetector. The focus error signal is generated by subtracting one composite signal from the other.
Other features and advantages of the present invention will become apparent to a person of skill in the art who studies the following
invention disclosure given with respect to the following disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention can be better understood by reference to and review of the following drawings:
Figure 1 is a top plan view of the basic elements of a
magneto-optical disc drive; Figure 2 illustrates the primary optical paths and some basic electrical elements of a magneto-optical disc drive in which this invention is useful;
Figure 3 is a schematic view showing the actuator arm, slider and disc of a magneto-optical disc drive;
Figures 4A-4F are views from various angles and with varying degrees of detail of a slider and the optics supported on the slider in a magneto-optical disc drive;
Figure 5 is a diagrammatic side view of the Wollaston prism and the photodetector of the present invention;
Figure 6 is a front view of the photodetector of Figure 5;
Figure 7 is a front view of another embodiment of the photodetector of Figure 5;
Figure 8A illustrates the out of focus condition when the
magneto-optical head is too far from the disk surface;
Figure 8B illustrates the in-focus condition when the head
magneto-optical head is properly positioned with respect to the disk surface;
Figures 8C illustrates the out of focus condition when the magneto-optical head is too close to the disk surface; Figure 9 is a graph of the FES signal versus the distance
from an in-focus position of the magneto-optical head with respect to the disk
surface; Figure 10A illustrates the received beams or spots on the photodetector for an off track condition when the head is positioned too close
to a left edge of track or groove;
Figure 10B illustrates the received beams or spots on the photodetector for an on-track position when the head is properly positioned over
the track centerline;
Figure IOC illustrates the received beams or spots on the photodetector for an off track condition in which the head too close to the right edge of the track or groove; and Figure 11 is a flowchart of the method of generating the focus error signal (FES), tracking error signal (TES), magneto-optical data signal (MODS) and reflectivity data signal (RDS) of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is an apparatus and method for
providing a focus error signal (FES) and a tracking error signal (TES) in a
magneto-optical storage system. Therefore a brief description of the basic
elements of the magneto-optical storage system will be provided. Since the
necessary servo information is a series of pits formed on or readable through the disk surface and requires an optical reader, a brief description of the basic
elements of an optical reading system is also included.
Referring now to the drawings, Figure 1 is a plan view illustrating some of the basic elements of a magneto-optical (MO) data storage and retrieval system. Few specific details are identified in this and Figures 2 through 4 as they are intended to portray some of the basic elements of a functional system in which the present invention is useful. The invention is not limited to use in only one specific MO data storage system. In Figure 1, the system 100 includes a set of flying heads
106, whose details will be described below, that are adapted for use with MO discs 107. In the example, the disks 107 are double sided. One flying head 106
is provided for each MO disk surface 107. The heads 106 are coupled to a rotary actuator magnet and coil assembly 110 by a suspension 112 and actuator arm 114 to be positioned over the surfaces of the MO discs 107. In operation, the MO discs 107 are rotated by a spindle motor 116 to generate aerodynamic lift forces between the flying heads 106 and the rotating disc 107. This maintains each flying MO head 106 in a flying condition above the data recording surface of each MO disc 107. The lift forces are opposed by equal and opposite spring forces supplied by the suspensions 112. During non-operation, each flying MO
head 106 is maintained statically in a storage condition away from the surface of
the MO disc 107, typically on a ramp (not shown) adjacent to the disk surface
107. It is of course possible that the heads could be landed on the surface of the disc in a non-data storage region; however, such an approach would not be the
optimum approach.
System 100 further includes a laser-optics assembly 120
coupled to an optical switch 122 which connects to a set of single mode
polarization maintaining (PM) optical fibers 124. The laser optics assembly 120 can be coupled with optically with beams 191, 192 or with single mode PM optical fiber. In the exemplary embodiment, each set of single mode PM optical fibers 124 are coupled through a respective one of the set of actuator arms 114 and suspensions 112 to a respective one of the set of flying MO heads 106. Figure 2 is a diagram showing the laser-optics assembly 120 of the magneto-optical data storage and retrieval system of Figure 1. As will
now be explained with respect to Figures 2 and 3, reading and storing information on the surface of each disc 107 requires both conveying the output of a laser through an optical fiber to the flying head so that the light output is accurately imaged on the surface of the disc, and generating a magnetic field utilizing a coil which is supported on the flying head 106 near to the surface of the disc 107.
Referring also to Figure 1, typically the data is arranged in a concentric series of tracks 126, with each track being made up of a number of sectors which in turn contain multiple bits of binary data. The servo information is typically recorded in the space 128 between these tracks; typically the
information is embossed on the surface of the disc or otherwise formed as a
series of pits and grooves in the data recording surface. In forming a magnetic
storage system disc, the pits and grooves for each track 126 of servo data could
be created as part of the forming or molding process which is used to form the disc. The type of materials of which such magnetic storage discs are currently
formed have been well identified and described in a number of published
documents, and need not be further described herein. The data would be both written and read on the data tracks 126 which would be found between the servo track regions 128 by the MO head 106 supported on an actuator 114 for radial movement in the direction of arrow 132. The movement is caused by a motor
generally indicated at 110 which causes rotation of the actuator 110 about an actuator bearing and pivot 134.
The description of Figures 2 and 3 will briefly summarize the reasons for providing both a light source and a magnetic field to selectively access the data on the surface of the disc 107. In Figure 2, the laser-optics assembly 120 is shown to include a linearly polarized diode laser source 231 operating in a visible or near ultraviolet frequency region and emitting an optical
power sufficient for reading and writing using the set of MO discs 107. In a first embodiment, the laser diode source may be a RF modulated laser source. In a second embodiment, the linearly polarized laser source 231 may be a distributed feedback (DFB) laser source. In an exemplary embodiment, the linearly
polarized laser source 231 is selected to operate within a range of 635-685 nm;
however, a laser source of other wavelengths could also be used. The laser-
optics assembly 120 further includes: a collimating optics 234, a low wavelength dispersion leaky beam splitter 232 and a coupling lens 233. The laser-optics assembly 120 directs (from the linearly polarized laser source 231) a linearly
polarized outgoing laser beam (shown in Figure 1) to the optical switch 122.
The laser-optics assembly 120 further includes a V* wave plate 236 and a mirror
237. In the first embodiment, a linearly polarized reflected laser beam 192
(shown in Figure 1) is directed by the optical switch 122 to the coupling lens 233, and is routed by the leaky beam splitter 232 to the V* wave plate 236 and mirror 237. The light is reflected off the mirror 237 into a differential detector
238. In the second embodiment, an optical isolator 239 is included between the laser source 231 and the collimating lens 234. In an alternate embodiment, the VA wave plate 236 and mirror 237 are not used, and the light beam is routed directly to the Wollaston prism 240 from the beam splitter 232.
In a noteworthy aspect of the invention, in both embodiments, the differential detector 238 includes a Wollaston prism 240 that
is placed between the mirror 237 and a photodetector 242. The Wollaston prism 240 receives the incoming light beam 244 and projects two optical beams onto the photodetector 242. In a preferred embodiment, the photodetector 242 has four subdetectors arranged to receive the optical beams. In response to the portion of the optical beam projected upon the subdetector, each subdetector generates a voltage VA,VB,VC,VD, and a current IA, IB,IC,ID, respectively, which are output to summing block 250. The arrangement of the subdetectors
A,B,C,D and the orientation of the optical beams from Wollaston prism with
respect to the subdetectors A,B,C,D affects the generated voltage
Figure imgf000012_0001
Inside the summing block 250, an FES generator 252
generates the FES signal using the following relationship: (VB+VC)-(VA+VD). A
TES generator 254 generates the TES signal using the following relationship:
(VA+VC)-(VB+VD). The Reflectivity data signal (RDS) is used for clocking and tracking and is generated by an RDS generator 256 as follows: VA+VB+VC+VD. The Magneto-optical data signal (MODS) represents the stored data and is generated as follows: (VA+VB)-(VC+VD). Note that each of the four signals
VA,VB, Vc, VD generated by the photodetector 242 is supplied to each of the generators 252-258. As is well-established in the art, summing amplifiers and subtracters are used to implement the relationships above to generate the FES,
TES, RDS and MODS in the respective generator 252-258.
With respect to the reflectivity data signal, the sum of the signals from the subdetectors produces a signal whose amplitude is proportional to the amount of light reflected from the disk. Reflectivity variations of the disk can be caused by pits in the disk that cause a loss of return light due to destructive interference. The reflectivity variations can also be caused by differences in material properties that fill the pits. Reflectivity information is used for encoding position information on the disk.
Figure 3 is a diagram showing a representative optical path
that includes the use of a DFB laser source. In a preferred embodiment, a
representative optical path is shown in Figure 3 to include, the optical switch 122, one of the set of single-mode PM optical fibers 124, and one of the set of
flying MO heads 106. The optical switch 122 provides sufficient degrees of
selectivity for directing the outgoing laser beam 191 (with reference to laser
source 231) to enter a respective proximal end of a respective single-mode PM optical fiber 124. The outgoing laser beam 191 is further directed by the single
modePM optical fiber 124 to exit a respective distal end so as to pass through the flying MO head 106 onto a recording/storage layer 349 of a respective MO disk 107.
In the preferred embodiment, the outgoing laser beam 191 is provided by a linearly polarized laser source 231 that is a DFB laser source. A DFB diode laser source, unlike an RF-modulated Fabry-Perot diode laser,
produces a very narrowband single-frequency output due to the use of a wavelength selective grating element inside the laser cavity. When linearly polarized light from a laser source 231 that is a DFB laser source is launched into a single-mode PM optical fiber 124, the light exiting the optical fiber includes a polarization state that depends on the relative orientation between the fiber axes and the incident polarization, and moreover, the output polarization state is very stable in time as long as external perturbations which alter the fiber
birefringence are negligible. The DFB laser source enables use of the single- mode PM optical fiber 124 for delivery and return of polarized laser light to and from the MO disk 107.
During writing of information, the outgoing laser beam 191
is selectively routed by the optical switch 122 to the MO disk 107 so as to lower
a coercivity of the recording storage layer 249 by heating a selected spot of
interest 350 to approximately the Curie point of the recording/storage layer 349.
Preferably, the optical intensity of outgoing laser beam 191 is held constant, while a time varying vertical bias magnetic field is used to define a pattern of
"up" or "down" magnetic domains perpendicular to the MO disk 107. This
technique is known as magnetic field modulation (MFM). Alternatively, outgoing laser beam 191 may be modulated in synchronization with the time varying vertical bias magnetic field at the spot of interest 340 in order to better control domain wall locations and reduce domain edge jitter. Subsequently, as the selected spot of interest 340 cools, information is encoded within the
recording/storage layer 249 of the respective rotating disk.
During readout of information, the outgoing laser beam 191 (at a lower intensity compared to writing) is selectively routed to the MO disk 107 such that at any given spot of interest 340, the Kerr effect causes (upon
reflection of the outgoing laser beam 191 from the recording/storage layer 349) a reflected laser beam 192 to have a rotated polarization of either clockwise or
counter clockwise sense 363 that depends on the magnetic domain polarity at the spot of interest 340.
The aforementioned optical path is bi-directional in nature. Accordingly, the reflected laser beam 192 is received through the flying MO
head 106 and enters the distal end of the single-mode PM optical fiber 124. The
reflected laser beam 192 propagates along the single-mode PM optical fiber 124
to exit at its proximal end and is selectively routed by the optical switch 122 for transmission to laser-optics assembly 120 for subsequent input to the summing
block to generate the FES, TES, RDS, and MODS.
Figures 4A-4F are diagrams showing the flying magneto-
optical head of the magneto-optical data storage unit in a perspective, a side
cross-sectional, an expanded cross-section, a side, a front, a bottom, and a rear view, respectively. In Figure 4 A, the flying MO head 106 is shown for use above a recording/storage layer 349 of one of the set of MO discs 107. The flying MO head 106 includes: a slider body 444, an air bearing surface 447, a quarter-wave plate 493, a reflective substrate 400, objective optics 446, a magnetic coil 460, and a yoke 462. The slider body 444 is dimensioned to accommodate the working distances between the objective optics 446, the single-mode PM optical fiber 124 and the reflective substrate 400. The reflective
substrate 400 may include a reflective surface which is aligned so as to direct the outgoing laser beam 191 and 192 to and from the recording/storage layer 349. Although slider body 444 may include industry standard "mini", "micro", "nano" or "pico" sliders, alternatively dimensioned slider bodies 444 may also be used.
In the preferred embodiment, the slider body 444 comprises a mini-slider height (889 μm) and a planar footprint area corresponding to that of a nano-slider
(1,600 x 2,032 μm).
The single-mode PM optical fiber 124 is coupled to the
slider body 444 along an axial cutout 443, and the objective optics 446 is
coupled to the slider body 444 along a vertical corner cutout 411. Preferably, the cutouts 443 and 411 may be designed as channels, v-grooves, or any other
suitable means for coupling and aligning the single-mode optical fiber 124 and
objective optics 446 to the flying MO head 106. In this embodiment, the laser
beams 191 and 192 traverse an optical path (to and from the recording/storage
layer 349 of the MO disk 107) that includes: the single-mode PM optical fiber
124, the reflective substrate 400, the quarter-wave plate 493, and the objective optics 446. In this embodiment, the single-mode PM optical fiber 124 and the objective optics 446 are positioned within their respective cutouts to achieve
focus of the outgoing laser beam 191 within the spot of interest 350 (See Figure
3) as a focused optical spot 448. The single-mode optical fiber 124 and the objective optics 446 may be subsequently secured in place by using ultraviolet curing epoxy or a similar adhesive.
Referring now to Figure 5, in a noteworthy aspect of the invention, the Wollaston prism 240 receives a light beam 502 reflected from the disk. The Wollaston prism 240 outputs or generates two optical beams 504, 506 from the light beam 502. The Wollaston prism 240 is arranged to project the two optical beams 504, 506 onto a sensor surface 508 of the photodetector 242 as shown by spots 510, 512. The sensor surface 508 includes subdetectors A,B,C, and D. The subdetectors A,B,C,D are photo-diodes. Subdetectors A and B form a first pair, and subdetectors C and D form a second pair. The Wollaston prism 240 projects the optical beams 504, 506 onto the photodetector
242 such that one of the optical beams is projected on at least one subdetector of the first pair of subdetectors, and another of the optical beams is projected on
at least one subdetector of the second pair. The amount of light projected on the
subdetectors indicates the focus of the light beam with respect to the reflective medium.
In particular, in response to the amount of light projected on that subdetector, each subdetector A,B,C,D generates a voltage which is output
on wires as VA,VB,Vc,VD,to the summing block, which was described above, to
generate the FES, TES, RDS and MODS signals. A significant feature of the invention is that one of the subdetector of the first and second pairs of subdetectors receives more light than the other subdetector of that pair when the light beam from the head is properly focused on the reflective medium. As shown in Figure 5, subdetector B receives
more light than subdetector A, and subdetector C receives more light than subdetector D. The spot 510 covers a larger portion of the surface area of subdetector B than subdetector A, and the spot 512 covers a larger portion of the surface area of subdetector C than D. In other words, one of the optical beams 504 is projected asymmetrically onto the subdetectors A,B of the first pair of subdetectors, and the other optical beam 506 is projected asymmetrically onto
the subdetectors C, D of the second pair of subdetectors when the light beam is properly focused over the reflective medium.
In a preferred embodiment, all subdetectors receive a portion of the light from their respective optical beams. The two optical beams 504, 506 output by the Wollaston
240 prism are both deviated, and the distance between the optical beams 504,
506 increases as the distance from the Wollaston prism 240 increases. The
distance from the Wollaston prism 240 to the photodetector 242 is predetermined and is a function of the frequency of the laser light, the intensity
of the laser light, and the size and arrangement of the subdetectors A,B,C,D of the photodetector 242.
In an alternate embodiment, other types of prisms that
separate the light beam into two optical beams according to the polarization state of the received light beam are used. In another embodiment, a Rochon prism is used.
In Figure 5, the orientation of the subdetectors A,B,C, and D with respect to the spots 508, 510 from the optical beams 504, 506 is another
noteworthy feature of the invention. In one embodiment, the two optical beams
504, 506 are projected onto the subdetecters A,B,C,D to indicate that the head is properly positioned over a track centerline. A beam axis 522 is drawn through the center of each optical beam 504, 506 and is aligned with an on-track axis 523. As shown in Figures 5 and 6, the beam axis 522 is aligned with the on- track axis 523 to indicate that the head is properly positioned with respect to a track centerline. The subdetectors A,B,C,D are arranged on the sensor surface 508 in a quadrant which has a vertical axis 524 and a horizontal axis 526. The vertical axis 524 is rotated with respect to the on-track axis 523 by an angle α. In one embodiment a ranges from approximately 5° to 10° and preferably is approximately 7° . If the beam axis 522 is not aligned with the on-track axis 523,
then the prism is not properly positioned in the detection system. In Figure 6,
a cross-track axis 528 is substantially perpendicular to the in-track axis 524.
Significantly, the rotation of the vertical axis 524 with respect to the in-track axis 523 causes the FES to have a positive predetermined
value or range of values when in focus, and will not be equal to zero. In
contrast, the TES will be substantially equal to zero when on the track
centerline. Although Figures 5-7 show circular spots 510, 512, in an alternate embodiment, the spots 510, 512 may be elliptical.
In Figure 5, the sensor surface 508 is rotated with respect to the in-track axis of the Wollaston prism. In an alternate embodiment, the vertical axis 524 and horizontal axis 526 of the sensor surface 508 are aligned to the edges of the photodetector 242, while the Wollaston prism 240 is rotated
by the angle α with respect to the vertical axis 524 thereby causing the on-track axis 523 to be rotated by the angle α with respect to the vertical axis 524 of the subdetectors. Significantly, this dual beam configuration of the
photodetector 242 with respect to the Wollaston prism 240 allows the MO data signal to be generated from the voltages output by the photodetector 242 using the equation described above.
Figure 7 depicts another embodiment of a photodetector of of the present invention. In this embodiment, the horizontal axis 526 of the four
subdetectors A,B,C,D is substantially perpendicular to the on track axis 522.
Subdetectors A,B form a first pair. Subdetectors C, D form a second pair. The first pair A,B is arranged above the horizontal axis 526, and the second pair C,D
is below the horizontal axis 526. The subdetectors of the subdetector pairs are
not divided along the on-track axis 522. The subdetectors are arranged such that regions B and C, diagonally opposite each other, are offset and extend over the
on-track axis 522 for a predetermined distance d. Significantly, this offset causes the FES to have a positive predetermined value or range of values when in focus, and will not be equal to zero. The TES will be substantially equal to zero when on the track centerline. The amount of offset or predetermined distance d is the
same for subdetectors B and C. The amount of offset is such that over 50% but less than 100%, and in particular from over 55% to 95%, and preferably 75% of the optical beams 504, 506 are projected on regions B and D when the head is aligned with the track centerline.
Significantly, the dual beam configuration of the photodetector 242 of Figure 7 with respect to the Wollaston prism 240 allows
the MO data signal to be generated from the voltages output by the photodetector 242 using the equation described above.
Figures 8 A, 8B and 8C depict the state of the spots 510, 512 for the near side of focus when the MO head is too far from the disk, the proper
or best focus, and the far side of focus when the MO head is too close to the
disk, respectively. As the optical disk moves away from the best focus position, the spots 510, 512 on the subdetectors A, B, C, D either grow or shrink with respect to the best focus. In Figure 8B, when properly focused and the FES is generated by applying the relationship, (VB+VC)-(VA+VD) , VB+VC will exceed
VA+VD and the FES will have a positive value. However, Figure 8A will
generate an FES that is positive but the value of the FES will be less than that of Figure 8B. In contrast, Figure 8C will generate an FES with a value that
exceeds the FES value of Figure 8B.
In Figure 9, an exemplary FES curve 900 showing the
preferred range of operation is shown. The x-axis represents the disk defocus in microns where the disk defocus is the distance from a predetermined position with respect to the disk surface. The y-axis represents the FES in volts. Preferably, the FES value is equal to approximately 0.48 volts when the light beam from the head is properly focused on the disk surface. The slope of the
FES curve 900 at the preferred focus value is equal to approximately -0.076 volts/μm. The normal operating range is between lines 902 and 904 and is plus or minus one micron. Although the y-axis of the graph represents volts, other units such as current (amperes) could be used.
Figures 10 A, 10B, and 10C depict the received beams or spots 510, 512 on the photodetector 242 from the Wollaston prism for an off track, on track and another off track condition, respectively. In Figure 10 A, the head is positioned to the "left" edge of the groove or track. The spots 510, 512 have a bright area 1002, a gray area 1004 and a dark area 1006. The bright area 1002 represents a region of least light intensity; the gray area represents a region of more light intensity than the bright area; and, the dark area 1006 represents
a region of the greatest light intensity. Applying the TES equation to the spots
510, 512 of Figure 10A results in VA+VC being greater than VB+VD, and the
TES has a positive value.
In Figure 10B, the head is properly positioned over the track
and each spot 510, 512 is grey 1006. Applying the TES equation to the spots
510, 512 of Figure 10B results in VA+VC being equal to VB+VD, and the TES is
equal to zero. As the head drifts offtrack in the opposite direction of
Figure 10 A, the spots 510, 512 of Figure IOC have a pattern similar to that shown in Figure 10A, but reversed. Applying the TES equation to the spots of
Figure IOC results in VA+VC being less than VB+VD, and the TES has a negative value.
In Figure 11, a method for generating the focus error signal, tracking error signal, and magneto-optical (MO) read signal is shown. In step 1102, a light beam is received from the reflective medium such as a disk. In step 1104, a prism generates first and second light beams from the received light beam, as described above. In step 1106, the prism projects the first light beam onto a first pair of subdetectors on a photodetector. In step 1108, the prism projects the second light beam onto a second pair of subdetectors on the photodetector. Steps 1106 and 1108 are substantially simultaneous. In step 1110, each subdetector of the photodetector generates a signal in response to the portion of the light beam projected on that subdetector.
Steps 1112 and 1114 generate the focus error signal. In step
1112, two composite signals are determined by adding the signals from
diagonally opposite subdetectors in the photodetector. In step 1114, the focus error signal is generated by subtracting one composite signal from the other, as
described above. The focus error signal has a non-zero value.
Steps 1116 and 1118 generate the tracking error signal. In
step 1116, two tracking composite signals are determined by adding the signals
from adjacent subdetectors such that one subdetector is in the first pair and the other subdetector is in the second pair. In step 1118, a tracking error signal is generated by subtracting one of the tracking composite signals from the other,
as described above.
Steps 1120 and 1122 generate the MO read signal. In step 1118, two magneto-optical composite signals are determined by adding signals from the subdetectors of the first pair and the second pair, respectively. In step
1122, the magneto-optical read signal is generated subtracting one of the magneto-optical composite signals from the other magneto-optical composite signal, as described above.
A key advantage to using the present invention is that the number of photodetectors and corresponding optical element and circuity is reduced, thereby reducing system complexity.
Other features and advantages of the present invention will become apparent to a person of skill in the art who studies the present invention disclosure. Therefore, the scope of the present invention is to be limited only by
the following claims.

Claims

CLAIMS WHAT IS CLAIMED IS:
1. Apparatus for determining the focus of a light beam from a reflective medium, comprising: a prism for receiving the light beam and outputting optical beams; and a photodetector having subdetectors, two adjacent subdetectors forming a first pair, the other two adjacent subdetectors forming a second pair, wherein the prism projects the optical beams onto the photodetector such that one of the optical beams is projected on at least one subdetector of the first pair of the subdetectors, and another of the optical beams is projected on at least one subdetector of the second pair of the subdetectors, wherein the amount of light projected on the subdetectors indicates the focus of the light beam with respect to the reflective medium.
2. The apparatus as claimed in claim 1 wherein one of
subdetectors of each pair receives more light than the other subdetector of that
pair when in focus.
3. The apparatus as claimed in claim 1 wherein one of the
optical beams is projected asymmetrically onto the subdetectors of the first pair of subdetectors, and the other optical beam is projected asymmetrically onto the subdetectors of the second pair of subdetectors.
4. The apparatus as claimed in claim 1 wherein the optical beam is projected over a larger portion of the surface of one of the subdetectors than
the other subdetector of each pair of subdetectors when in focus.
5. The apparatus as claimed in claim 1 wherein the photodetector has four subdetectors arranged in quadrants, the quadrants being aligned with an in-track axis and a cross-track axis, wherein the optical beams from the prism are substantially vertically aligned along a beam axis, wherein the
beam axis of the prism is rotated with respect to the in-track axis.
6. The apparatus as claimed in claim 1 wherein the photodetector has four subdetectors arranged in quadrants, the quadrants being
aligned along a vertical axis and a horizontal axis, the optical beams from the
prism are substantially aligned with an in-track axis, wherein the vertical axis of
the photodetector is rotated with respect to the in-track axis of the prism.
7. The apparatus as claimed in claim 1 wherein the amount of
light from the optical beams projected on the subdetectors indicates a
polarization state of the light beam from the reflective medium.
8. The apparatus as claimed in claim 1 wherein the reflective medium has at least one track, and the amount of light projected on the
subdetectors indicates the position of the light beam with respect to one of the at least one tracks of the reflective medium.
9. The apparatus of claim 1 wherein the prism separates the light beam into two optical beams according to the polarization state of the received light beam.
10. The apparatus as claimed in claim 1 wherein the prism is a
Wollaston prism.
11. The apparatus as claimed in claim 1 wherein the prism is a Rochon prism.
12. The apparatus as claimed in claim 1 wherein the prism
projects the optical beams onto both subdetectors of the first and second pairs of subdetectors substantially simultaneously.
13. The apparatus as claimed in claim 1 wherein all subdetectors
of the first and second pairs of subdetectors receive a portion of the respective optical beam.
14. The apparatus as claimed in claim 1 wherein the photodetector has a horizontal axis aligned with a cross track direction, the first
pair of subdetectors are on one side of the horizontal axis, the second pair of subdetectors are on the other side of the horizontal axis, one subdetector from each pair of subdetectors being arranged to receive a larger portion of the
respective optical beam when in focus.
15. The apparatus as claimed in claim 1 wherein the photodetector has four substantially rectangular subdetectors, the first pair of
subdetectors being on one side of a horizontal axis aligned to a cross-track direction, the second pair of subdetectors being on the other side of the horizontal axis, each subdetector being adjacent the horizontal axis, one subdetector of each pair of subdetectors extending along the horizontal axis for
a greater distance than the other subdetector of that pair.
16. A disk drive comprising:
a motor coupled to an optical disk for causing the disk to rotate;
a head optically coupled to the optical disk for generating a
light beam in response to information stored on the optical disk;
an actuator coupled to the head for positioning the head with
respect to the optical disk; a prism for receiving the light beam from the head and outputting optical beams; and
a photodetector having subdetectors, two adjacent subdetectors forming a first pair, the other two adjacent subdetectors forming a second pair,
wherein the prism projects the optical beams onto the photodetector such that one of the optical beams is projected on at least one
subdetector of the first pair of the subdetectors, and another of the optical beams is projected on at least one subdetector of the second pair of the subdetectors, each subdetector generating a voltage in response to the optical beam, a subtractor for generating a focus error signal by adding the voltage from diagonally opposite subdetectors to generate first and second diagonal sums, then subtracting the first and second diagonal sums to generate
the focus error signal.
17. A method for generating a focus error signal, comprising the
steps of: receiving a light beam from a reflective medium;
generating first and second light beams from the received light beam; projecting the first light beam onto a first pair of subdetectors on a photodetector; projecting the second light beam onto a second pair of subdetectors on the photodetector; generating a signal from each subdetector in response to the portion of the light beam projected on that subdetector; determining two composite signals by adding the signals
from diagonally opposite subdetectors in the photodetector; and generating the focus error signal by subtracting one
composite signal from the other.
18. The method as claimed in claim 17 wherein the generated focus error signal is not equal to zero.
19. The method as claimed in claim 17 further comprising the
steps of:
determining two tracking composite signals by adding the signals from adjacent subdetectors such that one subdetector is in the first pair
and the other subdetector is in the second pair; and generating a tracking error signal by subtracting one of the
tracking composite signals from the other.
20. The method as claimed in claim 17 further comprising the
steps of: determining two magneto-optical composite signals by adding signals from the subdetectors of the first pair and the second pair, respectively; and generating a magneto-optical read signal subtracting one of the magneto-optical composite signals from the other magneto-optical composite
signal.
PCT/US1998/019826 1997-09-22 1998-09-21 Apparatus and method for differential wax-wane focusing and push-pull tracking for magneto-optical data storage WO1999016061A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
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US60/059,439 1997-09-22

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