WO2001045095A2

WO2001045095A2 - Multi-element detector for use reading multiple tracks of optical disks having diverse formats

Info

Publication number: WO2001045095A2
Application number: PCT/EP2000/012826
Authority: WO
Inventors: Tania Kosoburd; Jacob Finkelstein; Eliyahu Chachamov; Itzhak Katz; Michael Naor; Steven R. Rogers
Original assignee: Zen Research Ireland Ltd
Current assignee: Zen Research Ireland Ltd
Priority date: 1999-12-15
Filing date: 2000-12-15
Publication date: 2001-06-21
Anticipated expiration: 2002-06-15
Also published as: TW479247B; AU3159801A; WO2001045095A3

Abstract

Apparatus and methods are provided for using a single multi-element detector to simultaneously read multiple tracks from either a CD disk or a DVD disk. The multi-element detector has a staggered arrangement and includes elongated detector elements, so that multiple reading beams reflected from multiple tracks of a CD disk (X) are projected onto the detector, as are multiple reading beams reflected from multiple tracks of a DVD disk (Y). An optical pickup using the multi-element detector of the present invention also is described.

Description

MULTI -ELEMENT DETECTOR FOR USE

READING MULTIPLE TRACKS OF OPTICAL

DISKS HAVING DIVERSE FORMATS

Field of the Invention The present invention relates to methods and apparatus for simultaneously reading multiple tracks of an optical disk, and more specifically to a multielement detector array for use in an optical drive that reads multiple tracks simultaneously from CD and DVD format optical disks.

Background of the Invention

Due to their high storage density, long data retention life, and relatively low cost, optical disks have become the predominant media format for distributing information. For example, the compact disk (CD) format, developed and marketed for the distribution of musical recordings, has replaced vinyl records. Similarly, high-capacity, read-only data storage media, such as CD-ROMs have become prevalent in the personal computer field for the distribution of software and databases. The DVD format may soon replace videotape as the distribution medium of choice for video information. Additionally, drives capable of reading DVD disks are becoming increasingly prevalent in personal computers, and the DVD format is starting to be used for distribution of software.

Although DVD disks have a much larger data storage capacity than CD disks, CD disks are currently far more common as a distribution medium for software and other computer-readable data. Thus, to best ensure success in the marketplace, optical drives preferably should be capable of reading both DVD disks and CD disks. Physical differences between these formats, however, such as the spacing between tracks on the disk (track pitch) , the size of the data features (pits) , the depth of the clear substrate that covers the reflective surface of the disks, and the wavelength of the light used to read the disks, may require drives capable of reading both types of disk to be more complex than drives capable of reading only a single type of disk. For example, U.S. Patent Number 5,696,750, to Katayama, describes a system having a common re lected-light optical path for reading CD and DVD disks.

Physically, the information bearing portion of an optical disk consists of a series of pits, or bumps, arranged to form a spiral track. Data is encoded in the length of individual pits and the length of the space between pits. An optical pickup assembly reads the data by reflecting a laser beam off of the optical disk. Because the disk is rotated, the laser beam alternately reflects from the pits and the spacing between the pits. This causes discernable changes in the reflected laser beam which are detected and decoded to recover data stored on the optical disk.

As used herein, a data track refers to a portion of the spiral data track corresponding to a single rotation of an optical disk. A drive capable of reading multiple data tracks simultaneously reads multiple such portions of the spiral track at once. For disks having multiple concentric spiral tracks, a data track refers to one revolution of one of the concentric spiral tracks. For optical disks having concentric circular tracks, a data track refers to one such circular track.

U.S. Patent No. 5,793,549 to Alon et al . , describes an optical disk reader that reads multiple data tracks simultaneously, for example, using multiple laser beams. The multiple laser beams, which may be obtained by splitting a single beam using a diffraction grating or by providing multiple laser sources, are focused cr. and aligned with corresponding tracks of the optical disk. The reflected beams are then detected and decoded. Thus, a disk rotated at 6χ the standard speed in a disk drive reading ten tracks at a time provides a data rate equivalent to a 60χ single beam drive, buz without the complications associated with high rotational speeds.

In addition to being aligned with the data tracks, the beams in a multi -beam optical pickup must be maintained at specified distances from each other to avoid crosstalk and to properly align the beams with the detectors. These distances are determined by the spacing cf the tracks (i.e., the track pitch), the magnification of the optics, and the size and spacing of the detectors used to read the information. Typically, the minimum spacing is greater than the track pitch, requiring the multiple laser beams to be spaced circumferentially as well as radially with respect to the optical disk.

The track pitch of a CD type disk is approximately 1.6 microns, while the track pitch of a DVD type disk is approximately 0.74 microns. For a multi-bear system, it is necessary to arrange and align the beams so that each beam focuses on a track, and to arrange the detectors so that each beam reflected from an optical disk is projected onto a detector. Since the track pitch of DVD and CD type disks are different, the spacing of the beams and the spacing of the detectors for a system that simultaneously reads multiple tracks of a DVD type disk is different than the spacing of the beams and detectors for a system that simultaneously reads multiple tracks of a CD type disk This presents unique difficulties m building a single optical drive that simultaneously reads multiple tracks of ooth CD and DVD dιsκs . Thus, for example, one cannot simply multiply the number of detectors employed m such devices as shown m the aforementioned Katayama patent, because the spacmgs for the non-central beams differ for each of the two formats.

Due to differences m the wavelength of light that is used to read DVD and CD disks, a system that reads both formats will typically have two laser diodes (one for each wavelength) , and combine the beams using a beamsplitter. Thus, arranging the spacing of the beams may be handled before the separate optical paths of the two laser diodes are combined by the beamsplitter. However, it is difficult and costly to provide separate optical patns for light reflected from the optical disk, and to use two different sets of detectors having different spacmgs between detector elements.

It would therefore be desirable to provide methods and apparatus for simultaneously reading multiple tracks of both CD and DVD type optical disks using a single multi-element detector. It would further be desirable to provide a multi -element detector that can be used to simultaneously read multiple tracks of both CD and DVD type disks.

Summary of the Invention

In view of the foregoing it is an object of the present invention to provide methods and apparatus for simultaneously reading multiple tracks of both CD and DVD type optical disks using a single multi -element detector.

It is a further object of the present invention to provide a multi-element detector that can be used to simultaneously read multiple tracks of both CD and DVD type disks. These and other objects of the present invention are achieved by providing a multi -element detector comprising an array of elongated detector elements wherein adjacent detector elements are staggered by predetermined distances and may be offset from each other by predetermined distances. Multiple reading beams reflected from multiple tracks of a CD disk are projected onto the mul i -element detector ^■ with a spacing and angle such that each of the beams is projected onto one of the detector elements of the multi-element detector. Similarly, multiple reading beams reflected from multiple tracks of a DVD disk are projected onto the same multi -element detector with a spacing and angle such that each of the reading beams corresponds to one of the detector elements. A central detector element of the multielement detector may be divided into four detector segments for use as a quad detector, generating astigmatic focus error signals and tracking signals. Additionally, two outermost detector elements may each be divided into two segments, for use in providing magnification error signals for the multiple reading beams used to read multiple tracks of a DVD disk, or for crosstalk correction.

The multi -element detector of the present invention is used with an optical pickup having two separate optical paths for generating reading beams having wavelengths, spacing, and angles appropriate for reading either a CD disk or a DVD disk. A beamsplitter is used to combine these optical paths into a single optical path after the reading beams have been generated. A holographic optical element permits an objective to focus reading beams having a wavelength for reading CD disks onto a CD disk, and to focus reading beams having a wavelength appropriate for reading DVD disks onto a DVD disk.

Brief Description of the Drawings

The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawings, in which like characters refer to like parts throughout, and in which: FIG. 1 is a simplified representation of a multi-beam optical pickup suitable for use in the present invention;

FIG. 2 shows a holographic optical element used in the multi-beam optical pickup of FIG. 1 ; FIGS. 3A and 3B show multiple reading beams projected on a portion of a CD disk and a DVD disk, respectively; FIG. 3C shows spots produced by multiple reading beams reflected from a surface of a CD disk and a DVD disk, as well as the orientation of a detector element of the multi-element detector of the present invention;

FIG. 4 shows a multi -element detector constructed in accordance with the principles of the present invention;

FIG. 5 shows spots reflected from a CD disk and a DVD disk projected onto the multi -element detector of FIG. 4; and

FIGS. 6 and 7 show alternative embodiments of a multi-element detector constructed in accordance with the principles of the present invention, with spots reflected from a CD and a DVD disk.

Detailed Description of the Invention

Referring to FIG. 1, a simplified diagram of illustrative dual-path multi-beam optical pickup 10, built in accordance with the principles of the present invention, is described. Optical pickup 10 may be used for reading optical disk 20, which may be either a CD format or a DVD format disk. Except for detector 22, the individual components of optical pickup 10 may comprise elements used in previously known optical disk readers.

Light source 11, typically a laser diode, selectively generates a beam of light having a first wavelength, suitable for reading a first type of optical media. For reading a CD disk, light source 11 preferably generates a beam having a wavelength of 785 nm. Similarly, light source 12, typically a laser diode, selectively generates a beam of light having a second wavelength, suitable for reading a second type of optical media. For reading a DVD disk, light source 12 preferably generates a beam having a wavelength of 658 nm.

Light from light source 11 passes through diffractive element 13, which splits the beam of light into multiple reading beams, spaced at a first preselected distance between adjacent beams, and aligned at a first angle with respect to the radial direction of an optical disk, so that each of the multiple reading beams will be incident on a corresponding track on the first type of optical disk (e.g. a CD disk) . Similarly, the beam generated by light source 12 is split into multiple reading beams by diffractive element 14. The reading beams generated by diffractive element 14 are spaced apart by a second preselected distance, and are aligned at a second angle with respect to the radial direction of an optical disk, so that each of the reading beams will be incident on a corresponding track on the second type of optical disk (e.g. a DVD disk) .

After the beam from light source 11 or 12 has been split into multiple reading beams by diffractive element 13 or 14, respectively, the multiple reading beams pass through beamsplitter 15, which combines the separate optical paths of the light generated by light sources 11 and 12 into a single optical path. Beamsplitter 15 passes the beams from light source 11 through the beamsplitter, while reflecting the beams from light source 12, so that light from either source shares a common optical path. Beamsplitter 15 preferably comprises a dichoric beamsplitter, that passes light having the first wavelength, and reflects light having the second wavelength. Alternatively, beamsplitter 16 may comprise a typical half-silvered mirror beamsplitter, or a polarizing beamsplitter, assuming that the light from light source 11 is polarized differently than the light from light source 12. Next, the reading beams pass through beamsplitter 16, and collimator 17, and are focused onto a surface of optical disk 20 by objective 18 to project diffraction limited spots onto the surface of optical disk 20. If multibeam optical system 10 is required to read two different types of optical media that have transparent substrates of different thickness, such as DVD and CD disks, the reading beams must pass through holographic optical element (HOE) 19 before passing through objective 18. HOE 19 is divided into two parts -- an inner and an outer part. The inner part of HOE 19 is designed so that light having the first wavelength may be focused on the first type of optical media by objective 18, while light having the second wavelength may be focused on the second type of optical media by the same objective 18. Thus, the inner part of HOE 19 effectively forms a dichoric lens, which has different optical properties for light having the first wavelength than for light having the second wavelength. A holographic optical element having the properties of the inner part of HOE 19 is described in the aforementioned patent to Katayama, which is incorporated herein by reference .

The outer part of HOE 19 is designed so that it has no effect on light having the second wavelength (for reading a DVD disk) , and restricts the numerical aperture of the objective lens for light at the first wavelength (for reading a CD disk) . Any portion of the light at the first wavelength that passes through the outer portion of HOE 19 is directed out of the optical axis, so that it forms a large diameter ring. As a result, the outer part of objective 18 has no effect on the spots projected on the first type of optical media (CD disks) . Optical disk 20 contains a reflective layer in which data is recorded in the form of pits (or bumps) in the reflective layer. Alternatively, some recordable optical disks use physical or chemical properties of the reflective layer material, such as its magnetic properties, or its ability to polarize incident light, to record the data.

The reading beams focused on optical disk 20 are reflected by the reflective layer and modulated by the data recorded therein. The reflected beams travel back through objective lens 18, HOΞ 19, and collimator 17, and are directed by beamsplitter 16 toward focus element 21 and multi-element detector 22. Focus element 21 is a holographic element that introduces astigmatism into at least a central reading beam, so that an astigmatic focus detector may be used. In accordance with the principles of the present invention, multi-element detector 22, which will be described in greater detail hereinbelow, comprises multiple optical detector elements, each of which detects the intensity of light reflected from a corresponding track of optical disk 20. One of the optical detector elements of multi-element detector 22, preferably a central element, may comprise a quadrant detector, for use in detecting focus and tracking errors.

Multi-element detector 22 provides electrical signals corresponding to the light beams impinging thereon. Processing circuitry, as described, for example, in commonly assigned U.S. Patent No. 5,627,805, which is incorporated herein by reference, decodes and processes the electrical signals to recover the data recorded on the optical disk. Additional circuitry converts the data to a format suitable for use by a computer or other processing device, and acts as an interface between the optical disk reader and computer or other processing device.

It will be understood by one skilled in the art that diffractive elements 13 and 14 alternatively may comprise holographic elements. Additionally, beamsplitter 16 may comprise a half -silvered mirror or a polarizing beam splitter. In addition, many other changes may be made to the physical arrangement of the optical components of multibeam optical system 10 without departing from the present invention.

The beams reflected from optical disk 20 are directed toward multi-element detector 22 regardless of the format of the disk from which they are reflected. In accordance with the principles of the present invention, multi-element detector 22 is designed to handle different formats of optical media, wherein the first type of optical media and the second type of optical media have different spacing between tracks, as is the case for CD and DVD disks. FIG. 2 shows a more detailed view of HOE 19.

As described above, HOE 19 includes inner part 26 and outer part 28. Inner part 26 is designed to have different optical properties for light having the first wavelength than for light having the second wavelength, so that light having the first wavelength may be focused on the first type of optical media by objective 18, while light having the second wavelength may be focused on the second type of optical media by the same objective 18. Outer part 28 has no effect on light having the second wavelength, and restricts the numerical aperture of objective 18 for light having the first wavelength by directing such light off of the optical axis, so that the outer part of objective 18 has no effect on illumination spots projected onto the first type of optical media.

The difference in beam spacing for DVD disks and CD disks is demonstrated in FIGS. 3A and 3B. In FIG. 3A, spots 30a - 30g are projected onto the tracks of a CD disk having a track pitch of 1.6 microns. In FIG. 3B, spots 32a - 32g are projected onto the tracks of a DVD disk, having a track pitch of 0.74 microns.

The angles of the rows of spots shown in FIGS . 3A and 3B are meant only for the purpose of illustrating that the rows of spots projected onto a surface of optical disk 20 may have different angles relative to a radial direction of the disk, depending on the disk type. In an actual system, the angles would be much larger -- typically above 80° from a radial direction, making the rows of spots nearly tangential on optical disk 20. The angle that should be used depends on the track pitch of the media to be read, the size of the spots projected onto the disk, the minimal distance between the spots necessary to avoid crosstalk and other interference between the beams, and the spacing of the detectors.

The angles of the reading beams may be adjusted so that when the reflected beams are imaged on multi-element detector 22, the spots projected on a CD disk and the spots projected on a DVD disk have the same spacing along one axis, and variable spacing along another axis, while aligning with two different track pitches. This is demonstrated in FIG. 3C, which shows row P of multiple reading beams projected onto a DVD disk, row Q of multiple beams projected onto a CD disk, and Detector element R, showing the alignment of the detector elements of multi-element detector 22. As can be seen, spots in row P and row Q align with two different track pitches, but when projected onto detector element R of multi-element detector 22, the beams align along an axis parallel to a long direction of detector element R. The beams are equally spaced along an axis perpendicular to the long direction of detector element R. This is achieved by proper design of diffractive elements 13 and 14, and of multi-element detector 22, and by their alignment or rotation.

FIG. 4 shows a multi-element detector for a seven beam system built in accordance with the principles of the present invention. Multi-element detector 22 comprises detector elements 40a - 40g, each of which detects light reflected from a corresponding track of an optical disk. Each of elements 40a, 40b, 40c, 40e, 40f and 40g has an elongated shape with height h, and width w. Adjacent elements of multielement detector 22 are separated from each other by a predetermined spacing s, and are staggered and offset relative to adjacent elements by a predetermined distance v. In a preferred embodiment, w is approximately 50 microns, h is approximately 120 microns, v is approximately 4.2 microns, and s is approximately 9.8 microns.

As used herein, a staggered arrangement of detector elements is one in which a top or bottom edge of an element is differently positioned along an axis than the top or bottom edge of an adjacent element. Elements are offset from each other along an axis if their centers are differently positioned along that axis. Thus, detector elements 40a - 40g are both staggered, since their top and bottom edges have differing vertical positions, and offset, since they are centered at varying vertical positions . Element 40d, the central element of multielement detector 22, preferably has both height and width , and preferably comprises a quadrant detector with four quadrants, A, B, C, and D. The quadrants are separated by a distance t, which is approximately 3 microns in a preferred embodiment. As is well-known in the art, signals generated by these quadrants may be used in astigmatic focus error detection, and for detecting errors in tracking. For both CD and DVD disks, the central reading beam reflected from the disk will be projected onto the center of element 40d.

Elements 40a and 40g, the outermost elements of multi-element detector 22, are preferably split into two segments each, labeled J, K, L, and M. Segments J and K, and segments L and M, also are separated by distance t. These segments are used to generate a signal indicative of variations in track pitch or magnification error for DVD disks. In use, the outermost reading beams reflected from a DVD disk will be projected so that they illuminate each of segments J, K, L, and M equally when the magnification of the system is correctly adjusted, and when the track pitch of the disk is correct. When the magnification is too high or the track pitch is slightly too wide, the spacing between images projected onto the detectors will increase, and segments J and M will receive more illumination than segments K and L. Conversely, when the magnification of the system is too low or the track pitch is too small, segments K and L will receive more illumination than segments J and M. By calculating (J+M) - (K+L) , the system may produce a magnification error signal for use with a magnification correction system such as is described m commonly assigned U.S. Patent Number 5,729,512, which is incorporated herein by reference. Segments J, K, L, and M may also be used to estimate parameters for crosstalk cancellation.

In FIG. 5, multi-element detector 22 is shown with spots X, representing incident light reflected from a CD disk when optical pickup 10 is used to read a CD disk, and spots Y, formed by incident light reflected from a DVD disk when optical pickup 10 is alternatively used to read a DVD disk As can be seen, each of the spots projected onto multi-element detector 22 from either type of disk corresponds to one of the detector elements. Both a central one of spots X and a central one of spots Y are incident on the same region of the central detector element. For non-central detector elements, spots X are incident on different regions of the detector elements than spots Y.

The angle at which the beams are projected onto the surface of the disk, and at which the reflected beams are projected onto multi -element detector 22 varies according to the type of disk. This variation is introduced by diffractive elements 13 and 14, which may be oriented at different angles to produce lines of beams having spacing and orientation necessary to align with multiple tracks of an optical disk, and to be projected onto elements of multi- element detector 22. The angle at which both sets of beams are projected onto multi-element detector 22 also depends on the orientation of multi-element detector 22. For a CD disk, the track pitch is approximately twice the track pitch of a DVD disk, and tne spacing between the beams is approximately the same as for a DVD disk, so the angle at which the beams are projected onto multi-element detector 22 must be greater than the angle at which the beams reflected from a DVD disk are projected onto multi -element detector 22. Adjustment of this angle permits the spacing of the beams to be different for each of the two types of disks, while permitting beams projected onto a surface of multi-element detector 22 to be equally spaced in a horizontal direction, and spaced differently only m a vertical direction.

Elements 40a, 40b, 40c, 40e, 40f, and 40g are elongated m a vertical direction (i.e. height h is greater than width ) , and have a staggered and offset arrangement, permitting them to detect light projected onto multi-element detector 22 over a large vertical area. This ability to detect light over a large vertical area, combined with the ability to project spots at different angles for two different types of optical media, provides an ability to use a single set of detectors to detect multiple reading beams reflected from two types of optical media having different spacing between tracks.

Referring now to FIG. 6, an alternative embodiment of the multi -element detector of the present invention is described. Multi-element detector 50, which may be used in optical pickup 10 as a replacement for multi-element detector 22, comprises detector elements 52a - 52g. Detector elements 52a - 52c and 52e - 52g have an elongated shape. As before, central detector element 52d has a square shape, and is divided into four segments, which may be used to detect focus and tracking errors. Outermost detector elements 52a and 52g are each divided into two segments, which may be used in the manner described hereinabove to detect magnification errors in the DVD beams (spots Y) . Detector elements 52c and 52e are staggered and offset vertically in opposite directions with respect to central detector element 52d, and detector elements 52a and 52b are aligned with detector 52c, while detector elements 52f and 52g are aligned with detector element 52e. As before, the detector elements are spaced horizontally at equal distances from each other.

As can be seen, both spots X, which represent the spots projected when the system is reading a CD disk, as well as spots Y, representing the spots projected when the system is reading a DVD disk, are incident on the detectors of multi-element detector 50. For non-central detector elements, spots X are incident on different regions of the detector elements than spots Y, while on the central detector element, the spots are incident on the same region. Due to the angles of the spots, and the elongated shapes and staggered and offset arrangement of the detector elements of multi-element detector 50, multiple tracks may be simultaneously read from either a CD or a DVD disk.

FIG. 7 shows another alternative embodiment of a multi-element detector built in accordance with the principles of the present invention. Multi-element detector 60 comprises detector elements 62a - 62g. Central detector element 62d comprises four segments that form a quad detector for use detecting tracking and focus errors, and outermost detector elements 62a and 62g each comprise two segments which may be used to detect magnification errors in the DVD beams.

Detector elements 62a - 62c and 62e - 62g have elongated shapes, with the height of a detector element varying according to the distance of that detector element from central detector element 62d. Thus, detector elements 62a - 62g have a staggered arrangement, with each detector element staggered relative to an adjacent element by a predetermined distance. The centers of detector elements 62a - 62g are aligned, so there is no offset between the elements .

Multi-element detector 60 covers a vertical area that becomes larger with distance from central detector element 62d, matching the increasing vertical separation of the spots from the two different media types as the distance from the central spot grows larger. As can be seen, both spots X, projected when the system is reading a CD disk, and spots Y, projected when the system is reading a DVD disk are incident on the detector elements of multi-element detector 60. As in other embodiments, for non-central detector elements, spots X are incident on different regions of the detector elements than spots Y, while on the central detector element, the spots are incident on the same region.

While preferred illustrative embodiments of the present invention are described above, it will be evident to one skilled in the art that various changes and modifications may be made without departing from the invention. For example, the multi -element detector of the present invention could be adapted to handle more or fewer beams, other staggered arrangements of detector elements could be used, or the dimensions, positions and spacing of the individual detector elements could be altered. It is intended in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.

Claims

C laims :

1. A multi-element detector for use in simultaneously reading a plurality of data tracks from either one of first and second optical disks, the first and second optical disks having different track pitches and data densities, the multi-element detector comprising : a plurality of detector elements, each one of the plurality of detector elements spaced apart equidistant from adjacent ones of the plurality of detector elements along a first axis and, for at least a subset of the plurality of detector elements, each one of the subset of detector elements has a portion that is staggered by a predetermined distance relative to an adjacent detector element in a direction perpendicular to the first axis.

2. The multi-element detector of claim 1, wherein the plurality of detector elements comprises a plurality of elongated detector elements.

3. The multi-element detector of claim 2 wherein, for at least a subset of the plurality of detector elements, each one of the plurality of detector elements is offset a predetermined distance relative to an adjacent detector element in a direction perpendicular to the first axis.

4. The multi-element detector of claim 2 or 3, wherein each one of the plurality of elongated detector elements has a predetermined height greater than a predetermined width.

5. The multi-element detector of claim 2, 3 or 4 wherein the plurality of detector elements comprises a central detector element having a height equal to its width.

6. The multi-element detector of claim 5, wherein the central detector element comprises four detector segments arranged to form a quadrant detector.

7. The multi-element detector of claim 6, wherein signals from the four detector segments of the central detector element are used to generate a focus error signal.

8. The multi-element detector of claim 6 or 7 , wherein signals from the four detector segments of the central detector element are used to generate a tracking error signal.

9. The raulri-element detector of any one of claims 1 to 8 , wherein the plurality of detector elements comprises first and second outermost detector element, each one of the first and second outermost detector elements having two detector segments.

10. The multi-element detector of claim 9, wherein signals generated by the two detector segments of the first outermost detector element and signals generated by the two detector segments of the second outermost detector element are used to compute a magnification error signal.

11. The multi-element detector of any one of claims 1 to 10, wherein the first optical disk is a CD format optical disk and the second optical disk is a DVD format optical disk.

12. A method of simultaneously reading a plurality of data tracks from either one of first and second optical disks having different formats, the method comprising: providing a central detector element and a plurality of non-central detector elements; generating a plurality of reading beams including a plurality of non-central reading beams; focusing the plurality of non-central reading beams onto a surface of one of the first or second optical disks to generate a plurality of non-central reflected beams, each one of the plurality of non- central reading beams being focused onto a corresponding one of plurality of data tracks to generate a corresponding one of the plurality of non- central reflected beams; and if the first optical disk is read, projecting each one of the plurality of non-central reflected beams onto a first region of each one of the plurality of non-central detector elements, and if the second optical disk is read, projecting each one of the plurality of reflected beams onto a second region of each one of the plurality of non-central detector elements, the first region of each one of the plurality of non-central detector elements is spaced apart from the second region of that non-central detector element.

13. The method of claim 12, wherein providing a central detector element comprises providing a central detector element having four detector segments arranged to form a quadrant detector, each one of the four detector segments producing a signal indicative of an amount of light incident on that detector segment, and wherein the method further comprises : adding astigmatism to at least a central reading beam of the plurality of reading beams; projecting a reflected beam corresponding to the central reading beam onto the central detector element; and generating a focus error signal responsive to the signals produced by the four detector segments.

14. The method of claim 13, further comprising generating a tracking signal responsive to the signals produced by the four detector segments.

15. The method of claim 12 or 13, wherein providing a plurality of non-central detector elements comprises providing first and second outermost detector elements, each one of the first and second outermost detector elements having two detector segments.

16. The method of claim 15, further comprising generating a magnification error signal responsive to signals generated by the two detector segments of the first outermost detector element and signals generated by the two detector segments of the second outermost detector element .

17. The method of claim 12, 13, 14, 15 or 16, wherein generating a plurality of reading beams comprises selectively generating a plurality of reading beams having a first wavelength suitable for reading the first optical disk or a second wavelength suitable for reading the second optical disk.

18. The method of claim 17, further comprising providing a holographic optical element that permits an objective to produce diffraction limited spots on a surface of the first optical disk with light having the first wavelength, and to produce diffraction limited spots on a surface of the second optical disk with light having the second wavelength.

19. The method of claim 18, wherein providing a holographic element further comprises providing a holographic element having an inner part and an outer part, the outer part restricting a numerical aperture of the objective for light having the first wavelength.

20. An optical pickup that simultaneously reads a plurality of data tracks from either first or second optical disks, the first and second optical disks having different formats, the optical pickup comprising: a first light source selectively generating a first illumination beam having a first wavelength; a first diffractive element that splits the first illumination beam into a first plurality of reading beams, the first plurality of reading beams having a first spacing and aligned at a first angle; a second light source selectively generating a second illumination beam having a second wavelength; a second diffractive element that splits the second illumination beam into a second plurality of reading beams having a second spacing and aligned at a second angle; a beamsplitter that redirects the second plurality of reading beams so that the first plurality of reading beams and the second plurality of reading beams share a common optical path following the beamsplitter; a holographic optical element that permits an objective to focus the first plurality of reading beams onto multiple tracks of the first optical disk to produce a first plurality of diffraction limited spots, and to focus the second plurality of reading beams onto multiple tracks of the second optical disk to produce a second plurality of diffraction limited spots; and a plurality of elongated detector elements, each one of the plurality of elongated detector elements having a first region for receiving an image of corresponding one of the first plurality of diffraction limited spots reflected from the first optical disk and a second region, spaced apart from the first region, for receiving an image of a corresponding one of the second plurality of diffraction limited spots reflected from the second optical disk.

21. The optical pickup of claim 20, wherein the holographic optical element comprises an inner part and an outer part, the outer part directing incident light having the first wavelength off of the optical axis to restrict a numerical aperture of the objective for the first plurality of reading beams.

22. The optical pickup of claim 20 or 21, further comprising a central detector element having four detector segments arranged to form a quad detector.

23. The optical pickup of claim 22, further comprising a holographic element that introduces astigmatism into a reading beam projected onto the central detector element.

24. The optical pickup of claim 23, wherein signals generated by the four detector segments of the central detector element are used to generate a focus error signal.

25. The optical pickup of claim 22, 23 or 24, wherein signals produced by the four detector segments are used to generate a tracking signal.

26. The optical pickup of any one of claims 20 to 25, wherein each one of first and second outermost detector elements of the plurality of elongated detector elements have two detector segments.

27. The optical pickup of claim 26, wherein signals produced by the two detector segments of the first outermost detector element and signals produced by the two detector segments of the second outermost detector element are used to generate a magnification error signal.