Abstract:
A multi-track optical disk reader is provided having an optical pickup including multiple reading beams arranged in a pattern, such that each beam is directed towards a track to be read. The optical pickup is at least partly mounted on a swing-arm mechanism that orients the reading beam pattern with respect to the tracks being read. Methods and apparatus are provided for maintaining a desired position of the reading beams with respect to the tracks on an optical disk.

Description:
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 methods and apparatus for use with a swing-arm type optical pickup. 
     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-ROM have become prevalent in the personal computer field for the distribution of software and databases. And the DVD format may soon replace videotape as the distribution medium of choice for video information. 
     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, 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, data track refers to one revolution of one of the concentric spiral tracks. For optical disks having concentric circular tracks, data track refers to one such circular track. 
     U.S. Pat. No. 5,793,549 to Alon et al., describes as 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 on and aligned with corresponding tracks of the optical disk. The reflected beams are then detected and decoded. Thus, a disk rotated at 6× the standard speed in a disk drive reading ten tracks at a time provides a data rate equivalent to a 60× single beam drive, but 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 of 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 necessary spacing between beams may be decreased either by increasing the magnification of the optics or by decreasing the size and spacing of the detectors as described in allowed U.S. patent application Ser. No. 09/042,185, “METHODS AND APPARATUS FOR PERFORMING CROSS-TALK CORRECTION IN A MULTI-TRACK OPTICAL DISK READER BASED ON MAGNIFICATION ERROR” now U.S. Pat. No. 5959953. Increasing the magnification of the optics reduces the optical efficiency of the system, and reducing the size of the detectors reduces efficiency and increases manufacturing cost. The spacing of the beams in a multi-beam system represents a tradeoff between these factors. When the size, sensitivity, and cost of photo detectors improve, it may be possible to reduce the spacing between the beams. 
     An exemplary multi-beam optical disk reader is described in commonly-assigned U.S. patent application Ser. No. 08/911,815, entitled “INTEGRATED MULTI-BEAM PICKUP ASSEMBLY,” which is incorporated herein by reference. The optical disk reader described therein includes a plurality of reading beams arranged in a single row. Co-pending, commonly-assigned U.S. patent application Ser. No. 08/912,881, entitled “MULTI-BEAM OPTICAL PICKUP ASSEMBLY AND METHODS USING A COMPACT TWO-DIMENSIONAL ARRANGEMENT OF BEAMS,” which is incorporated herein by reference, describes an optical disk reader including a plurality of reading beams arranged in a two dimensional pattern. To maintain the needed distances between spots projected onto the surface of the disk as determined by the beam spacing, the pattern of laser beam spots must have a specific orientation with respect to the radial direction of the disk. 
     To read different portions of an optical disk a mechanism is provided for positioning the optical pickup adjacent to the portion to be read. Swing arm, rack-and-pinion, screw drive, and linear motor systems for positioning the optical pickup are known in the art, and described, for example, in  Compact Disk Technology,  Nakajima, H. and Ogawa, H., translated by Aschmann, C., Ohmsha, Ltd., Japan, 1992, and  The Compact Disk Handbook,  Pohlmann, K., 2nd. Ed., A-R Editions, 1992. 
     Selection of a positioning mechanism involves tradeoffs between access speed, design complexity, and manufacturing expense. For example, rack-and-pinion and screw drives are relatively slow at positioning the optical pickup. However, because they are also inexpensive and robust, they are often used in consumer level applications. By comparison, linear motors provide faster positioning, but are complex and more expensive than rack-and-pinion mechanisms. A swing-arm type positioning mechanism, such as that shown in U.S. Pat. No. 5,828,644, provides rapid positioning and is less complex than a linear motor systems. 
     While it would be desirable to use a pivoting arm such as described in the foregoing patent, several drawbacks arise from attempting to use such technology in a multi-beam optical disk reader. For example, in an optical disk reader that uses multiple laser beams, the orientation of laser beam spot pattern would change when the swing-arm pivots. Consequently, the laser beam spots may not align with the respective tracks at some radial positions of the optical pickup. This effect may increase the number of read errors or reduce the number of tracks that may be read simultaneously. 
     It would therefore be desirable to provide methods and apparatus for keeping multiple reading beams aligned with respective tracks of an optical disk when employing a swing-arm mechanism for positioning an optical pickup. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing it is an object of the present invention to provide methods and apparatus that keep multiple reading beams aligned with respective tracks of an optical disk when employing a swing-arm mechanism for positioning an optical pickup. 
     These and other objects of the present invention are achieved by providing methods and apparatus for compensating for the rotation of the laser beam pattern projected onto an optical disk caused by motion of a swing-arm. In a first embodiment, when the swing-arm is pivoted, the laser beam spot pattern is pivoted, or rotated, in an opposite direction to offset rotation caused by motion of the swing-arm. In a second embodiment, the swing-arm and optical pickup are constructed so that the orientation of the laser beam spot pattern does not change when the swing-arm is pivoted. And in a third embodiment, the spacing between the laser beams spots is adjusted to compensate for the rotation of the line of laser beam pattern. 
    
    
     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 schematic representation of a multi-beam optical pickup suitable for use in practicing the present invention; 
     FIG. 2 is a plan view of a swing-arm mechanism positioned adjacent to a portion of an optical disk; 
     FIGS. 3A through 3C are, respectively, simplified illustrative embodiments of optical pickups constructed in accordance with the principles of the present invention; 
     FIG. 4 is a schematic of illustrative circuitry for providing a rotation error signal in accordance with the principles of the present invention; 
     FIG. 5 is a simplified representation of another illustrative embodiment of an optical pickup in accordance with the principles of the present invention; and 
     FIGS. 6-10 depict various alternative means for adjusting spacing between the multiple laser beams in the pickup assembly of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a simplified diagram of illustrative multi-beam optical pickup  10 , suitable for use in the present invention, is described. Individual components of optical pickup assembly  10  may comprise elements used in previously known optical disk readers. Light from light source  11 , typically a laser diode, is directed by prism mirror  12  to grating  13  which splits the light into multiple beams. The multiple beams pass through beam splitter  14 , are collimated by lens  15  and directed to optical disk  16  by prism mirror  17 . Objective lens  18  is adjusted by a servo mechanism to keep the light beams focused on the surface of optical disk  16 . 
     Optical disk  16  contains a reflective layer in which the data is recorded. Typically the 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 light beams focused on optical disk  16  are reflected by the reflective layer and modulated by the data recorded therein. The reflected light travels back through objective lens  18  and is reflected by prism mirror  17  to collimator  15 . The reflected light beams are then directed toward optical sensor  20  by beam splitter  14 . Lens  19  may be provided to further focus the reflected light beams onto optical sensor  20 . 
     Optical sensor  20  provides electrical signals corresponding to the light beams impinging thereon. Processing circuitry, as described, for example, in commonly owned U.S. Pat. No. 5,627,805, 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. 
     Diffraction grating  13  also may comprise a hologram, and fiber optic wave guides may be used in place of one or more of mirrors  12  and  17 , as well as lenses  15 ,  18 , and  19 . Beam splitter  14  may comprise a half-silvered mirror or a polarizing beam splitter. In addition, one or both of prism mirrors  12  and  17  may be omitted by changing the physical arrangement of the optical components. 
     The multiple laser beams are used to simultaneously read multiple tracks of an optical disk. When projected onto an optical disk, the laser beam spots are arranged in a specific pattern, such as a line or array of spots, to ensure each laser spot illuminates the correct track. If a swing-arm type of mechanism is used to position an optical pickup assembly, however, the angular orientation of the laser beam spot pattern changes as the swing-arm pivots to read different portions of an optical disk. 
     For example, in FIG. 2, swing-arm  22  is positioned near an inner circumference of optical disk  16 , so that laser beam spots  26  are focused on, and aligned with, corresponding ones of tracks  25 . When swing-arm  22  is moved to read tracks near the outer circumference of optical disk  16 , as indicated by phantom pickup arm  22 ′, the orientation of laser beam spots  26 ′ with respect to optical disk  16  changes so that the laser beam spots are no longer aligned with tracks  25 ′. This may in turn affect the number of tracks the optical disk reader is able to accurately read simultaneously. 
     One method of compensating for the change in angular orientation of the laser beam spot pattern is to rotate, or pivot, the array of laser beam spots in a direction opposite to the rotation of the swing-arm. Illustrative compensation mechanisms are shown, for example, by optical pickups  30  and  31  of FIGS. 3A and 3B, respectively. 
     Optical pickup  30  of FIG. 3A generally includes the same optical components as optical pickup  10  of FIG.  1 . It differs from optical pickup  10  in that it is primarily oriented perpendicular to the surface of optical disk  16 . Some of the components of optical pickup are mounted to form assembly  33 . At a minimum, laser  11 , grating  13 , beam splitter  14 , lens  19 , and detector  20  are mounted on assembly  33 . Collimator  15  and objective lens  18  may also be mounted on assembly  33  if desired. 
     Assembly  33  is mounted on the end of swing-arm  22  of FIG. 2, in such a way that assembly  33  rotates about axis  35 , shown in FIG.  3 A. Preferably, axis  35  is aligned with the optical axis of objective lens  18 . Rotation of assembly  33  about axis  35  changes the angular orientation of the laser beam spot pattern relative to the radius of optical disk  16 , thereby enabling the optical disk reader to maintain the laser beam spots in alignment with the corresponding tracks of optical disk  16 . 
     To fully compensate for the motion of swing-arm  22 , assembly  33  should be rotated through the same angle as swing-arm  22 , but in an opposite direction. For example, in FIG. 2, swing-arm  22  is rotated clockwise through an angle of approximately 25 degrees in moving from position  22  to position  22 ′. To compensate, assembly  33  of FIG. 3 is rotated approximately 25 degrees in a counter-clockwise direction. 
     The proper rotation of assembly  33  required to compensate for movement of swing-arm  22  may be specified statically, or determined dynamically. In a statically compensated system, a look up table is created containing the proper rotation of assembly  33  needed to compensate for various angular positions of swing-arm  22 . In using such a system, an optical disk reader obtains the angular position of swing-arm  22  directly through a position sensing device, such as a shaft encoder, or indirectly from the number of a track being read, i.e. a block number. The position of swing-arm  22  is then used as an-entry into the lookup table to find the required rotation of assembly  33  to provide the required compensation. 
     In a dynamically determined system, a servo system is used to continuously and automatically adjust the rotation of assembly  33  to compensate for the angular position of swing-arm  22 . With respect to FIG. 4, circuitry for providing a rotational error signal, is described. Optical pickup  20  includes an array of sensors  41 - 45 , including one sensor for each track to be read from the optical disk. Each sensor is electronically biased so that it outputs a signal proportional to the intensity of the reflected laser beams incident thereon. The output of each sensor is processed to recover the data stored in the corresponding tracks. In addition, the outputs of sensors  41  and  45 , which are split into halves,  41   a  and  41   b,  and  45   a  and  45   b,  respectively, are used to provide a rotational error signal. Although not shown in FIG. 4, other sensors may be split to provide tracking and focus error signals as is known in the art. 
     Circuitry  40 , comprising summing circuits  46 - 47  and difference circuit  48 , uses signals from sensors  41  and  45  to calculate error signals indicative of rotational errors. In particular, the output of sensor  41   a  is summed with the output of sensor  45   b,  and the output of sensor  41   b  is summed with that of sensor  45   a.  In the absence of a rotational error, each half of sensors  41  and  45  receive approximately equal illumination and no rotation error signal is provided. That is the illumination on sensor  41   a  is approximately equal to that on sensor  41   b,  and the illumination on sensor  45   a  is approximately equal to that on  45   b,  the output signals provided by summing circuits  46  and  47  are substantially the same, and the output of difference circuitry  48  is nearly zero. 
     However, in the presence of a rotational error, the outputs of the halves of sensors  41  and  45  are unequal. For example, in FIG. 2, the leftmost sensor on phantom swing-arm  22 ′ is misaligned with the corresponding track, causing the signal provided by sensor  41   a  to differ from the signal provided by sensor  41   b.  However, the rightmost sensor is aligned with its corresponding track, so that the signals from sensors  45   a  and  45   b  are substantially the same. 
     Thus, the output of summing circuitry  46  (i.e.,  41   b + 45   a ) differs from the output of summing circuitry  47  (i.e.,  41   a + 45   b ) and difference circuitry  48  provides a signal indicative of the rotational error. Although not shown in FIG. 4, the rotational error signal is preferably low-pass filtered to remove unwanted high frequency components and provide a more stable error signal. The filtered error signal then may be used by a servo system to rotate assembly  33  to compensate for any rotational error. 
     Referring now to FIG. 3B, optical pickup  31  includes prism mirrors  36  and  37  arranged to form a periscope. Prism mirrors  36  and  37 , together with objective lens  18 , are mounted on a structure to form objective assembly  38 . Objective assembly  38  is mounted on swing-arm  22  so that objective assembly  38  may be rotated, or pivoted, about axis  39  to keep the laser beam spots aligned with the tracks being read. 
     To reduce any radial movement of the laser beam spot pattern associated with rotation of objective assembly  38 , an optical axis of objective lens  18  and axis  39  are preferably close together,. Small radial motions of the laser beam spot pattern appear to the optical disk reader control circuitry as a tracking error, and are compensated for by the tracking system. 
     Optical pickups  30  (FIG. 3A) and  31  (FIG. 3B) compensate for rotational errors caused by movement of the swing-arm by rotating the laser beam spot pattern. Alternatively, the swing-arm and optical pickup may be configured so that the orientation of the laser beam spot pattern does not change due to movement of the swing-arm. One such configuration is described with respect to optical pickup  32  of FIG.  3 C. 
     In FIG. 3C, prism mirror  17  and objective lens  18  are disposed at the end of swing-arm  22 , and prism mirror  12  is disposed at the pivot point of swing-arm  22 , such that prism mirror  12  rotates with swing-arm  22  about axis  35 ′. Prism mirrors  12  and  17 , collimator lens  15 , and objective lens  18  make up objective assembly  38 ′. Together, prism mirrors  12  and  17  form a periscope for directing laser beams to and from the surface of optical disk  16 . Collimator lens  15  may be located either between prism mirrors  12  and  17 , or between beam splitter  14  and prism mirror  12 . 
     Laser  11 , grating  13 , beam splitter  14 , lens  19 , and sensor  20  are not mounted on swing-arm  22 , and, therefore, do not pivot or rotate as swing-arm  22  is pivoted to access different portions of optical disk  16 . Because these components are not mounted on the swing-arm, the angular orientation of the laser beam spot pattern does not change as the swing-arm  22  is rotated. 
     In a fourth embodiment of the apparatus of the present invention, the optical power, or magnification, of optical pickup  10  of FIG. 1 is adjusted to compensate for rotational errors. Referring again to FIG. 2, the span of laser beams spots  26 ′ is greater than the span of corresponding tracks  25 ′. One means of reducing the span of laser beam spots  26 ′ is to adjust the magnification of the optical system, thereby changing the spacing between the laser beams spots. FIG. 5 depicts an illustrative arrangement of optical components for providing a variable power optical system. 
     Optical pickup  50  of FIG. 5 is similar to optical pickup  10  of FIG.  1  and includes the same optical components described with respect to FIG.  1 . However, optical pickup  50  includes an additional group of optical components  52  that provides a variable power, or magnification, system. Many different optical components may be used to provide a variable power optical system, several of which are described below in connection with FIGS. 6-9. 
     Optical components  52  of FIG. 5 may include a group of three lenses as shown in FIG.  6 . Lens  60  is stationary, while lens  62  is moved axially to vary the power of the optical system. Because altering the spacing between lenses  60  and  62  also causes a shift in the image plane of the optical system, lens  64  is moved to counteract the image plane shift. As indicated by the dotted lines in FIG. 6, the correct position of lens  64  is a nonlinear function of the position of lens  62 , which may be empirically determined. 
     Two alternative variable power optical systems are described with respect to FIGS. 7A and 7B, wherein optical components  52  of FIG. 5 include a number of fixed lenses  72  and movable lenses  74 . Movable lenses  74  are linked together so that they move in unison to vary the power of the optical system. Some shifting of the image plane may occur as the power is varied, but over small ranges of power change, the image plane shift is small. Additional groups of lenses may be added to further reduce image plane shift. 
     Yet another embodiment of a rotational error correction mechanism of the present invention is shown in FIGS. 8A and 8B. As shown in FIG. 8A, prism  80  may be used as an anamorphic lens. When inserted into the optical path between collimator  15  and prism mirror  17  of FIG. 5, prism  80  magnifies the laser beam spot pattern in a single dimension. For example, reducing the width of the laser beam spot pattern from w 1  to w 2 . The power of prism  80  is determined by the angle of its faces with respect to the optical path, so optical power may be varied by rotating prism  80  about a line parallel to its axis, thereby compensating for the rotational errors. 
     Prism  80  also causes an angular deviation of the incident laser beams. The amount of the deviation is a function of the angle of the prism faces with respect to the light beam. As illustrated in FIG. 8B, second prism  82  may be used to eliminate or reduce the angular deviation. Thus, by suitably rotating prism  82  the angular deviation introduced by prism  80  may be counteracted. 
     It should be noted that the laser beams exiting prism  82  have a lateral offset relative to the light beams entering prism  80 . Small offsets may appear as a tracking error which will be compensated for by the optical disk reader&#39;s tracking subsystem. Alternatively, a glass plate may be used to remove the lateral offset. Further details and designs of variable power systems using lenses and/or prisms may be found in Chapter 9 of  Modern Optical Engineering,  Warren J. Smith, McGraw-Hill Book Company, New York, 1966, which is incorporated herein by this reference. 
     In a further exemplary embodiment of a variable power anamorphic optical system, cylindrical lens  90  may be used wherein the radius of curvature of the lens varies along a length of the lens as shown in FIG.  9 . Cylindrical lens  90  is positioned in the optical path such that its flat face is orthogonal to the optical path and its axis is perpendicular to the plane formed by the multiple laser beams. As with the prisms of FIGS. 8A and 8B, a cylindrical lens provides magnification in only a single dimension, wherein the degree of magnification in that dimension is determined by the radius of the curved surface of the lens. By using a lens in which the curvature varies along its length, the horizontal magnification of the images may be controlled through vertical movement of variable radius cylindrical lens  90 . As in the case of the prisms of FIGS. 8A and 8B, cylindrical lens  90  may cause a small angular deviation of the laser beams. 
     The systems for correcting rotational errors discussed in connection with FIGS. 5-9 work by changing the optical power of the optical pickup to adjust the spacing between the multiple laser beams. In yet another embodiment of the present invention, an optical disk reader may compensate for the magnification error by changing the position of one or more of the optical components in the optical pickup. 
     For example, referring back to FIG. 1, the beam from light source  11  is split into multiple diverging beams by grating  13 . Collimator lens  15  refracts the multiple laser beams so that they are approximately parallel. Because the beams diverge linearly, the amount of divergence may be changed by changing the separation between diffraction grating  13  and collimator lens  15 . Increasing the spacing increases the divergence, and therefore, the spacing between the laser beams. Conversely, reducing the spacing between diffraction grating  13  and collimator lens  15  reduces the spacing between the laser beams, as shown in FIG.  10 . 
     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. 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.