Abstract:
A method for preformatting an optical recording medium with a plurality of parallel guide tracks, each guide track controlling the recording of a band of data tracks; said method includes providing a source laser beam along a predetermined path at a wavelength which is selected to be within the sensitivity range of the optical recording medium and providing a beam splitter in the predetermined path which divides the source laser beam into a multiplicity of preformatted recorded beams directed to impinge upon the optical recording medium. The spacing or pitch between the beams at the optical recording medium is uniform.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the fabrication of optical media with preformatted guide tracks for multi-track optical recording. 
     BACKGROUND OF THE INVENTION 
     The high capacity of optical recording media is achieved by encoding data in very narrow data tracks, typically about 1 μm wide. A tracking servo system is required to position the tracks so close together without overlap. The tracking servo system functions by sensing guide tracks that are preformatted on the optical recording medium. 
     The recording data rate of an optical recording system can be greatly increased by multi-track recording, whereby a band of data tracks are recorded simultaneously by a single optical head. The relative positions and parallelism of the tracks within the band are held fixed by optical alignment of recording sources within the head. However, a guide track is still required for each band of data tracks so that the tracking servo system can position data bands close together without overlap. 
     Existing methods for preformatting optical media are suitable to format optical disk media for single-track recording. However, these preformatting methods are slow and expensive, sometimes dominating the total cost of media manufacturing. Furthermore, these methods are not readily adaptable to create guide track formats for multi-track optical recording. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for flexible, high speed preformatting of optical media for multi-track optical recording and to reduce the cost of optical media fabrication by reducing the complexity of the media preformatting process. 
     This object is achieved by a method for preformatting an optical recording medium with a plurality of parallel guide tracks, each guide track controlling the recording of a band of data tracks, said method comprising: 
     a) providing a source laser beam along a predetermined path at a wavelength which is selected to be within the sensitivity range of the optical recording medium; 
     b) providing a beam splitter in the predetermined path which divides the source laser beam into a multiplicity of preformatted recorded beams directed to impinge upon the optical recording medium, the spacing or pitch between the beams at the optical recording medium being uniform as viewed from a scan direction, such pitch being greater than at least three times the spacing between the data tracks after they are recorded; 
     c) simultaneously focusing the preformatted recording beams at a focal surface to form an array of focused spots; 
     d) positioning the optical recording medium with its recording surface at the focal surface; and 
     e) providing relative motion between the optical recording medium and the focused spots in the scan direction to form visible guide tracks in a recording zone of the optical medium that trace out the path of each focused spot. 
     ADVANTAGES 
     The present invention has as an advantage that a precise guide track format for multi-track optical recording is formed at a high rate and at low cost. The invention also improves system performance in that reduced guide track runout leads to improved tracking accuracy and increased media capacity. 
     It is a further advantage of the present invention that it simplifies media manufacturing by allowing high-speed servowriting of the preformat pattern, thereby eliminating the need for specialized surface replication equipment and complex media structures. Servowriting imposes no additional constraints on the media design because it is a laser-marking process similar to data recording. Simplification of the servowriter system is achieved according to the present invention by writing a plurality of guide tracks with a single modulated laser beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the use of guide tracks to position data bands for multi-track optical recording according to the prior art; 
     FIG. 2 illustrates a prior art method of creating a guide track using a servowriter; 
     FIG. 3 describes a multi-track servowriter according to the prior art that projects a plurality of record beams through one objective lens; 
     FIG. 4 illustrates multi-track preformatting using a single modulated laser beam according the present invention; 
     FIG. 5 illustrates another implementation of guide track preformatting according to the present invention; 
     FIG. 6 shows a cross-section of the interface between the servowriter head and a flexible optical recording medium according to the present invention; 
     FIG. 7 shows how high frequency guide track dither can be accomplished electro-optically according to the present invention; and 
     FIG. 8 illustrates how a laser beam shaping can improve guide track uniformity according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Optical recording media store information in the form of very narrow tracks of data marks. High data density is realized by disposing these data tracks very close together, typically at a track pitch of 1-2 μm. Accurate positioning of the data tracks is accomplished by a closed-loop tracking servo. The tracking servo develops its control signals by optically sensing the positions of preformatted guide tracks on the medium surface. A separate guide track is provided adjacent to each data track, or each band of data tracks for a multi-track recording system. During recording of a data track or a band of data tracks, the tracking servo keeps the optical head registered to the associated guide track. 
     FIG. 1 shows the use of guide tracks to position data bands for multi-track optical recording according to the prior art. An optical recording medium  10  includes preformatted guide tracks  12   a-c . An illuminated spot  14  of guide track  12   b  is illuminated by an optical head  16  through objective lens  18  while the medium is moved relative to the optical head in a scan direction  20  parallel to the guide tracks. A track position sensor  22  in the optical head detects the illuminated guide track and sends a tracking error signal  24  to a tracking servo circuit  26 . The tracking servo circuit drives a tracking actuator  28  to move the objective lens so that its cross-track position is held fixed relative to guide track  12   b . With the optical head thus stabilized, the optical head focuses a plurality of recording laser beams  30  onto the medium. As the medium continues its scanning motion, the recording laser beams mark the surface, forming a plurality of data tracks  32  all of which run parallel to guide track  12   b . In the figure, another band of data tracks,  32 ′, was recorded previously and referenced to guide track  12   a.    
     The guide track pitch must be at least three times the data track pitch so that a plurality of data tracks can be recorded in the intervening unformatted gaps. The guide track pitch must also accommodate any residual cross-track runout of the optical head relative to the preformat pattern so that the data band never runs into an adjacent guide track. By contrast, in the absence of preformatted guide tracks, the pitch between data tracks would have to be much larger to accommodate the open-loop cross-track runout of the optical head relative to the medium. Thus guide tracks increase storage capacity even as they restrict the recordable areas of the medium. 
     FIG. 2 illustrates a prior art method of creating guide tracks using a servowriter. A recording beam  34  from a servowriter head  36  is focused by an objective lens  18  to form a focused spot  38  on the featureless surface of an optical recording medium  10 . The recording medium is scanned in a direction  20  relative to the focused spot which then marks the surfaces, forming a visible guide track  12 . Successive scans of the medium are interspersed with cross-track stepwise motions of the servowriter head in a direction  40  to form a set of parallel guide tracks  12 ′ that cover the recording zone on the surface of the recording medium. The servowriter system can include a modulator  42  that modulates the power in the focused spot, thus creating patterned guide tracks. The servowriter also includes registration means  44  for accurately controlling the cross-track position of the focused spot on the medium and for repositioning the focused spot for formatting of each guide track. 
     The optical recording medium in FIG. 2 is pictured as an optical card. However, the servowriting method is suitable for preformatting guide tracks in other forms of optical recording media, including optical disk and optical tape. 
     An important limitation of prior art servowriter systems is that they are slow. It is known in the art that an optical head can project a plurality of record beams through one objective lens to simultaneously record a plurality of tracks. FIG. 3 illustrates a multi-track servowriter that incorporates this principle. Lasers  46   a  and  46   b  in the servowriter head form two recording beams,  34   a  and  34   b . The recording beams are directed by mirror  48  to pass through objective lens  18 , forming focused spots  38   a  and  38   b  on the featureless recording surface of an optical recording medium  10 . The recording medium is scanned in a direction  20  relative to the focused spots, simultaneously writing guide tracks  12   a  and  12   b . The recording beams are modulated by modulators  50   a  and  50   b  to form patterned guide tracks. Successive scans of the medium are interspersed with cross-track stepwise motions of the servowriter head in a direction  40  with a step size equal to twice the nominal guide track spacing to form a set of parallel guide tracks  12 ′ that cover the surface of the recording medium. Such a servowriter system can preformat the medium faster than a single-spot servowriter. However, it is still limited to formatting a region no wider than the field of view of the objective lens, usually less than 100 μm. Therefore, writing all the necessary guide tracks on a media unit requires multiple servowriter scans. 
     Established optical disk technology includes methods for embossing or replicating an entire media surface to create a preformat pattern guide tracks and preformat patterns on an entire media surface. For example, substrates for rewritable CD disks area commonly injection molded. However, although precision injection molding or embossing processes are compatible the discrete manufacturing flow for optical disks substrates, these processes are not compatible with continuous production of optical tape or optical card substrates, and their cost per unit area is high. 
     Photographic exposure has been suggested as a method for preformatting optical tape (see U.S. Pat. No. 4,884,260). Photographic replication can be rapid and accurate. However, this approach requires that expensive photosensitive layers be added to the media. Furthermore, small guide track features, i.e. much smaller than 10 μm, cannot be replicated in a high-speed photographic process. 
     FIG. 4 illustrates multi-track preformatting using a single modulated laser beam according the present invention. The single beam from laser  46  is modulated by modulator  50  and divided by beam splitters  52  to form a plurality of recording beams  34   a-d . The recording beams are focused through a plurality of objective lenses  18   a-d , forming focused spots  38   a-d  that are coincident with the featureless recording surface of an optical recording medium. The focused spots simultaneously record a plurality of identical, parallel guide tracks  12   a-d  across the recording zone of an optical recording medium  10 . The width of the preformatted region of the optical recording medium is not limited to the field of view of the objective lenses. 
     FIG. 5 illustrates a another implementation of guide track preformatting according to the present invention. In a servowriter head  36 , the recording beam  34  from laser  46  is modulated by modulator  50 . The recording beam is expanded by an anamorphic beam expander  54  and deflected by mirror  48  to fill a plurality of lenslet elements on a microlens array  56 . Each lenslet functions both as a beam splitter, dividing off a part of the recording beam, and as an objective lens, forming a recording spot at the focal plane of the microlens array. An optical recording medium is positioned at the focal plane of the microlens array. As the medium is moved in a scanning direction  20 , the servowriter head writes a plurality of identical, parallel guide tracks  12 ′, one for each illuminated element of the microlens array. In this way, an optical medium such as an optical tape can be preformatted in a single pass under the servowriter head forming guide tracks across the entire recording zone if the microlens array is as wide as the medium and includes a lenslet for each of the required guide tracks. FIG. 5 illustrates a microlens array with a linear array of lenslets. It will be appreciated, however, that the microlens elements can be disposed in a two-dimensional array. 
     FIG. 6 shows a cross-section of the interface between the servowriter head and a flexible optical recording medium according to the present invention. A recording beam  34  passes through a beam splitter  58  to illuminate a microlens array  56  with lenslet elements that divide the beam and focus it to form focused spots  38   a-d  at a focal surface. The recording surface of a flexible optical recording medium  10  is maintained coincident with the focal surface by a stabilizer  60 . The stabilizer can include, for example, a roller, a dynamic air bearing, or a porous pumped air bearing. The stabilizer can also include edge guide features or mechanisms that provide transverse registration of the recording medium. 
     Light reflected at the surface of the recording medium returns through the microlens elements to form a sensor beam  62  that is deflected by the beam splitter, through a focus sensor lens  64 , and onto focus sensors  66   a  and  66   b . A Focus sensor  66   a  measures the defocus of spots  38   a  and  38   b . Focus sensor  66   b  measures the defocus of spots  38   c  and  38   d . The difference between error signals from the two offset focus sensors is an indication that the media is tilted relative to the microlens array. Positioning screws  68   a  and  68   b  that control the orientation of the microlens array relative to the stabilizer are adjusted to minimize the error signals from focus sensors  66   a  and  66   b , respectively, thus eliminating focus and tilt error at the interface between the medium and the focused spots. If the dynamic runout of the stabilized medium in the vertical direction perpendicular to the recording surface is less than the microlens depth of focus, typically several μm, then adequate focusing can be accomplished by static adjustments without a closed-loop focus system. 
     The transverse position of guide tracks preformatted on the recording medium depends on the position of the microlens array relative to the medium. An actuator  70  can be provided to dynamically control that relative position by moving the microlens in the transverse direction. Such an actuator can compensate for tape runout, for example if the edge guiding functionality of the stabilizer  60  is imperfect. The actuator can also be used to create guide tracks with a periodic cross-track dither or wobble. In this case, the guide tracks are not straight lines, however the uniform spacing and parallelism of the guide tracks is maintained. 
     It can be desirable to preformat guide tracks with cross-track features of very high spatial frequency. If the objective lenses or media cannot be moved quickly enough, high frequency dither can be effected by deflecting the recording beam. FIG. 7 shows how high frequency guide track dither can be accomplished electro-optically according to the present invention. A polarization modulator  72  switched the recording beam  34  to an orthogonal polarization state. In the switched state, the recording beam is refracted by a birefringent prism  74  to form a deflected recording beam  34 ′ that it illuminates a microlens array  56  at a different incidence angle relative to beam  34 . The deflected beam is focused by the elements of the microlens array to form a plurality of focused spots  78   a-d  that are offset from the undeflected focused spots  38   a-d  in the transverse direction. The magnitude of the offset is equal to the differential angle of refraction through the birefringent prism multiplied by the focal length of the microlens elements. 
     FIG. 8 illustrates how a laser beam shaping can improve guide track uniformity according to the present invention. The record beam  34  is converged by a lens  54 ′ to form an illumination pattern  80  on a microlens array  56  which divides the beam and forms a plurality of focused spots  38   a-d . The guide tracks written by these focused spots will be identical if the spots have substantially uniform optical power and size. In order for the focused spots to be uniform in power, the total illumination of each of the microlens elements must be the same. And in order for the focused spots to be uniform in size, the illumination pattern should be similar at each microlens element. The recording beam as emitted by a laser usually has a circular Gaussian intensity that would deliver more illumination to lenslets near the center of the optical axis than to those at the edges. The recording beam should be filtered or shaped, for example by an aperture  82 , to provide uniform lenslet illumination. Uniform illumination can also be achieved using a variable density filter that transmits more light at the edges than at the center. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  optical recording medium 
       12   a-d  guide tracks 
       12 ′ guide tracks 
       14  illuminated spot 
       16  optical head 
       18  objective lens 
       18   a-d  objective lenses 
       20  scan direction 
       22  track position sensor 
       24  tracking error signal 
       26  tracking servo circuit 
       28  tracking actuator 
       30  recording laser beams 
       32  data tracks 
       32 ′ data tracks 
       34  recording beam 
       34 ′ recording beam 
       34   a-d  recording beams 
       36  servowriter head 
       38  focused spot 
       38   a-d  focused spot 
       40  motion of the servowriter head 
       42  modulator 
       44  registration means 
       46  laser 
       46   a  laser 
     PARTS LIST (con&#39;t) 
       46   b  laser 
       48  mirror 
       50  modulator 
       50   a  modulator 
       50   b  modulator 
       52  beam splitters 
       54  anamorphic beam expander 
       54 ′ lens 
       56  microlens array 
       58  beam splitter 
       60  stabilizer 
       62  sensor beam 
       64  focus sensor lens 
       66   a  focus sensor 
       66   b  focus sensor 
       68   a  positioning screw 
       68   b  positioning screw 
       70  actuator 
       72  polarization modulator 
       74  birefringent prism 
       78   a-d  focused spots 
       80  illumination pattern 
       82  aperture