Abstract:
A method and a system for formatting a magnetic tape with identifiable optical servo tracks are disclosed. Multiple optically detectable servo tracks extending lengthwise parallel to the tape transport direction are written simultaneously by directing a spatial beam dot pattern unto the magnetic tape. The beam dot pattern is capable of imparting not only a unique identification, but also other information such as tape longitudinal address, and manufacturer&#39;s data to each servo track. The spatial beam dot pattern may also include two arrays of beam dots created via the use of two one-dimensional diffractive optical elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/361,669 to Hong Chen et al., filed Jul. 27, 1999, and assigned to the same assignee as the present application. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to optical servo track identification on tape storage media.  
         BACKGROUND  
         [0003]    Digital magnetic tape recording provides an important function of data backup in a hierarchy of data storage platforms. Conventionally, at least two approaches are employed for recording digital information onto magnetic recording tape. One approach calls for moving the tape past a rotating head structure that records and plays back user information from discontinuous transverse tracks. Interactive servo systems are employed to synchronize rotation of the head structure with travel of the tape. Another approach is to draw the tape across a non-rotating head at a considerable linear velocity. This approach is sometimes referred to as digital linear tape (DLT) recording and playback.  
           [0004]    Increased data storage capacity and retrieval performance is being required of all commercially viable mass storage devices and media. In the case of digital linear tape recording, a trend is toward multi-head, multi-channel fixed head structures with narrowed recording gaps and track widths. Such narrow recording heads allow many linear tracks to be formed on a magnetic tape of predetermined width, such as one-half inch width tape. Tape substrates are also being made thinner, with increased tape lengths being made possible in small diameter reel packages.  
         SUMMARY  
         [0005]    In an aspect, the invention features an optical servo system including a laser source for generating a single beam of light, a diffractive optical element (DOE) positioned to receive and split the single beam into a linear array of beams, a lens to focus the linear array of beams to form a spatially separated array of beam dots, and a spatial light modulator (SLM) positioned to receive and selectively filter the linear array of beam dots to form a time dependent spatial pattern.  
           [0006]    One or more of the following features may also be included. The system may also include lenses positioned to image the filtered beam dots onto a moving magnetic tape to cause a time dependent etched pattern on the tape, each one of the dots corresponding to one of a plurality of servo tracks. A second DOE may be positioned prior to a final focusing lens so that the tracks are simultaneously duplicated into multiple copies across a width of the moving tape. The filtered beam dots may form and etch a pattern on a non-magnetic surface of the moving magnetic tape or on a magnetic surface of the moving magnetic tape. The pattern may indicate track identification, a position along the tape, manufacturer&#39;s data, and control information for each of the plurality of servo tracks on the magnetic tape.  
           [0007]    In another aspect, the invention features a method of generating a pattern on a magnetic tape including generating a single beam of coherent light, splitting the single beam in a diffractive optical element (DOE) into a linear array of beams, focusing the beams to form an array of spatially separated beam dots, and filtering the plurality of beam dots in a spatial light modulator (SLM).  
           [0008]    One or more of the following features may also be included. The method may also include positioning lenses to image the filtered beam dots on a moving magnetic recording tape with lenses, the dots forming a time dependent etched pattern on the tape. The method may also include etching the filtered beam dots on a non-magnetic surface of the moving magnetic recording tape or etching the filtered beam dots on a magnetic surface of the moving magnetic recording tape. The pattern may indicate track identification, position along the tape, control information, and manufacturer&#39;s data for each of a plurality of servo tracks on the magnetic recording tape.  
           [0009]    In another aspect, the invention features an optical servo writer system including a laser source for generating a single beam of light, a diffractive optical element (DOE) positioned to receive and split the single beam into a two dimensional array of beams, a lens to focus the two dimensional array of beams to form a two dimensional array of spatially separated beam dots, and a spatial light modulator (SLM) positioned to receive and selectively filter the two dimensional array of beam dots.  
           [0010]    One or more of the following features may be included. The system may include lenses positioned to image the selectively filtered beam dots onto a moving magnetic tape, the dots forming a time dependent etched pattern on the tape. The system may include a second DOE positioned prior to a final focusing lens such that tracks are duplicated into multiple copies across a width of the moving magnetic tape. The SLM may be an active spatial modulator in which each individual cell of the SLM can be selectively turned on and off to allow passage or blockage of light or a passive spatial filter in which a filtered pattern is a pre-determined fixed spatial pattern. Selectively filtered beam dots may form and etch the pattern on a non-magnetic surface of the magnetic tape or on a magnetic surface of the magnetic tape. The pattern may indicate track identification, position along the tape, control information, and manufacturer&#39;s data for each of a plurality of data tracks on a recording surface of the magnetic tape.  
           [0011]    In another aspect, the invention features a method of generating a pattern on a magnetic tape including generating a single beam of coherent light, splitting the single beam in a diffractive optical element (DOE) into a two dimensional array of beams, focusing the two dimensional array of beams by a lens to form a two dimensional array of spatially separated beam dots, and spatially filtering the plurality of beams dots with a spatial light modulator (SLM).  
           [0012]    One or more of the following features may be included. The method may include imaging the spatially filtered beam dots on a moving magnetic tape with lenses. A second DOE may be positioned prior to a final focusing lens such that tracks are duplicated into multiple copies across a width of the moving tape. The spatially filtered beam dots may form and etch a pattern on a non-magnetic surface of the magnetic tape or on a non-magnetic surface of the magnetic tape.  
           [0013]    In another aspect, the invention features a system including a laser source for generating a single beam of light, a passive optical element to split the beam into two beams with an angle separation, and a bi-DOE having two DOEs etched into a single glass substrate, each DOE having different angle orientations for a diffracted array of beams.  
           [0014]    One or more of the following features may also be included. The system may also include a lens positioned to receive and focus the two beams to generate a pattern having two linear arrays of dots forming a small angle with respect to each other on a moving magnetic tape. The passive optical element may be a Wollaston prism or an optical wedge. The two DOEs may be one-dimensional, each producing a one-dimensional array of beams. A third DOE may be positioned prior to a final focusing lens such that tracks are duplicated into multiple copies across a width of the moving tape. The two beams may form and etch a pattern on a non-magnetic surface of the magnetic tape or on a magnetic surface of the magnetic tape. The pattern may indicate track identification for each of a plurality of servo tracks on the magnetic tape.  
           [0015]    In another aspect, the invention features a system including a laser source for generating a single beam of light, a half-wave-plate in combination with a polarizing beam splitter positioned to receive and split the single beam into two beams, an optical wedge positioned in a first beam to impart a small angle deviation to one of the tow beams, a set of mirrors to direct a second beam such that the first beam and second beam can be recombined, a first DOE positioned to split the first beam into a first linear array of beams, a second DOE positioned to split the second beam into a second linear array of beams, and a second polarizing beam splitter positioned to receive and recombine the first and second array of beams.  
           [0016]    One or more of the following features may also be included. The system may include a lens positioned to receive and focus the recombined array of beams to generate a repeated pattern having two linear arrays of dots forming an angle with respect to each other on to a moving magnetic tape. The system may also include a third DOE positioned prior to a final focusing lens such that tracks are duplicated into multiple copies across the moving tape. The first beams and the second beams may form and etch a pattern on a non-magnetic surface of the magnetic tape or on a magnetic surface of the magnetic tape. The pattern may indicate track identification for each of a plurality of servo tracks on the magnetic tape.  
           [0017]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.  
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0018]    [0018]FIG. 1 indicates several bands of servo tracks recorded on a magnetic tape.  
         [0019]    [0019]FIG. 2 is a first example of a V-like servo track pattern.  
         [0020]    [0020]FIG. 3 is a second example of a cross pattern servo track pattern.  
         [0021]    [0021]FIG. 4 is a third example of a diamond with a diamond missing servo track pattern.  
         [0022]    [0022]FIG. 5 is a fourth example of a servo track pattern.  
         [0023]    [0023]FIG. 6 is an example of a general servo pattern where each track can be individually coded.  
         [0024]    [0024]FIG. 7 is an addressable fiber-coupled laser system for writing a servo track pattern.  
         [0025]    [0025]FIG. 8 is a system for writing a servo track pattern using an acousto-optic modulator.  
         [0026]    [0026]FIG. 9 is a system for writing a servo track pattern using a combination of one-dimensional DOE and one-dimensional spatial light modulator (SLM).  
         [0027]    [0027]FIG. 10 is a system for writing a servo track pattern using a combination of a two-dimensional DOE and a two-dimensional spatial light modulator (SLM).  
         [0028]    [0028]FIG. 11 is a system for writing servo track patterns using a spatial light modulator and an imaging lens.  
         [0029]    [0029]FIG. 12A is a simple system for generating passive servo patterns using a bi-DOE.  
         [0030]    [0030]FIG. 12B shows a composition of the bi-DOE of FIG. 12A.  
         [0031]    [0031]FIG. 13 is a system whereby the bi-DOE of FIG. 12 is expanded to allow the two dot patterns to be separated along the tape motion direction.  
         [0032]    [0032]FIG. 14 is a further extension of the system of FIG. 13, whereby the bi-DOE is replaced with two separate DOEs in two separated optical paths.  
         [0033]    [0033]FIG. 15 is a block diagram of an exemplary system for manufacturing a tape that has servo tracks thereon.  
     
    
     DETAILED DESCRIPTION  
       [0034]    [0034]FIG. 1 shows an example of a servo track. A magnetic tape  10  is provided with several servo bands (labeled Band  1 , Band  2 , Band  3 , and Band  4 ), wherein each servo band may include a servo track pattern composed of a plurality of servo tracks  12 ,  13 ,  14 , each of the servo tracks  12 ,  13 ,  14  can be engraved onto the magnetic tape  10  by a marking mechanism. The servo marks are typically spaced apart a uniform distance. The servo tracks can also be continuous lines instead of being discrete marks. The servo tracks can be read with an optical pickup device. A servo band may include servo tracks on a non-magnetic surface of the tape  10 , and each servo track may correspond to multiple data tracks on the magnetic side of the magnetic tape  10 . The many-to-one correspondence relies on the use of a multiple channel magnetic recording head (not shown).  
         [0035]    The servo tracks  12 ,  13 ,  14  extend the full length of the magnetic tape  10 . These servo tracks  12 ,  13 ,  14  are employed for aligning recording heads (not shown) with a data track recorded on to an opposite (i.e., magnetic) side of the magnetic tape  10 . The servo tracks  12 ,  13 ,  14 , provide for precise alignment of a recording head with a data track regardless of LTM during the recording of data as the tape  10  moves across the recording head. Thus, more data tracks can be packed on the magnetic side of the tape  10 , thereby increasing total data storage capacity for the tape  10 . The number of servo tracks  12 ,  13 ,  14 , that are marked onto a tape  10  depends, in part, on the size of the servo mark, the pitch between servo marks, and the pitch between servo tracks.  
         [0036]    When a servo feedback loop (not shown) is employed, an actuator by which a combined optical pick up device and magnetic recording head are driven will move up and down according to the tape lateral motion. However, due to the existence of tape defects, and unexpected shocks and vibrations of a tape drive, the servo feedback loop may become disrupted, and opened. Under such a circumstance the servo feedback loop can be closed again when the upset event is passed. But the servo feedback loop will not know which track is acquired since there is no distinction among the various tracks. One has to make the actuator mechanism go to the bottom or the top of the tape  10  and seek by counting the tracks until the desired track is reached. This compromises the drive performance and runs the risk of incorrect counting unless numerous protective algorithms are provided.  
         [0037]    Although marking mechanisms can adjust the spatial characteristics of the generated servo patterns, the servo tracks themselves do not carry track information for determining their lateral position on the tape. The systems and methods described herein provide track information, such as track identification, manufacturer ID, manufacturer&#39;s data, longitudinal addresses, and/or control information.  
         [0038]    Referring to FIG. 2, one servo band is shown having an exemplary V-shaped track pattern repeated along the tape  10 . The track pattern is formed by alternatingly and/or sequentially engraving the tape  10  in a manner to be described below. The spacing and orientation of track marks  17   a ,  17   b ,  18   a ,  18   b  can be adjusted. With the tape  10  moving in a transport direction  15 , an optical servo head (not shown) reading the track marks  17   a ,  17   b  along servo track  17  will encounter two closely spaced track marks  17   a -  17   b , followed by a larger spacing,  17   b -  17   a . The optical servo head, when reading the track marks  18   a ,  18   b  along servo track  18  will also encounter two closely spaced track marks  18   a - 18   b  followed by a larger spacing  18   b - 18   a . However, the spacing between the track marks  18   a - 18   b  is greater than the spacing between the track marks  17   a - 17   b , while the spacing between the track marks  18   b - 18   a  is smaller than the spacing  17   b - 17   a . The track marks can be written on the tape  10  so that each track has a unique combination of a small and a large spacing. In so doing, the spacing is converted by the moving tape  10  into timing signals that can be detected by the optical servo head, thereby uniquely identifying the respective track.  
         [0039]    In the case of identification ambiguity for the upper tracks versus the lower tracks in a given servo band due to inversion symmetry with respect to the middle of the servo band, the optical servo head can simultaneously read signals from servo marks located on the adjacent tracks to remove the ambiguity.  
         [0040]    Other exemplary servo track patterns are illustrated in FIGS.  3  to  5 . For example, FIG. 3 shows a repetitive X-shaped track pattern with markings  34   a ,  34   b ,  35   a ,  35   b  and so forth. FIG. 4 shows a repetitive rectangular array with a missing diagonal, and with two missing columns to separate neighboring rectangles. It shows distinct tracks  42 ,  43 ,  44  and so forth. The rectangle may well be diamond shaped or other appropriate shape. FIG. 5 shows yet another repetitive pattern that allows track identification. The pattern is a missing sub-rectangle or diamond array with one or more surviving slanted lines; it shows servo tracks  52 ,  53 ,  54  and so forth.  
         [0041]    The repetitive patterns shown above can be generated with passive optical means, that is, no real-time pattern generator. Examples are shown in FIGS.  10 - 14 . Such repetitive patterns provide track identification, but they do not permit additional features such as longitudinal addresses, manufacturer&#39;s data and so forth. A preferred servo pattern is generated with an active pattern generator that allows each track to be individually coded. An example is shown in FIG. 6, where each track can be coded with a track address that is constant for the given track, followed by, for example, a longitudinal address that is a variable, and then by other information of value to the user, which could also be a variable. The process can be repeated along the track with a pattern generator. Example tracks are  61 ,  62 , and  63 .  
         [0042]    Referring to FIG. 7, a radiation source  82  having a plurality of individually addressable emitters, such as a semiconductor diode laser bar with ten or more emitters, provides tracks with individual coding. The emitters can be individually turned on or off at high speed. Such laser bars can emit light in the visible or IR spectral range. Each individually addressable emitter of the radiation source  82  is coupled to an optical fiber  86 , for example, through an arrangement of input lenses  84 . The light from each emitter is guided in a separate fiber and imaged, optionally through an arrangement of output lenses  88 , onto the tape  10 , to form track marks  87   a ,  87   b ,  87   c ,  87 D, and  87   e . In this example, the emitters of radiation source  82  may be arranged so as to be oriented either perpendicular, or at a slanted angle, with respect to the transport direction. This arrangement generates servo patterns illustrated in FIGS. 2 through 6.  
         [0043]    [0043]FIG. 8 shows another arrangement to form individual servo tracks with the use of an acousto-optic device, or a Bragg cell,  104 . Different spots  107 ,  109   a ,  109   b  on the tape  10  may be addressed and engraved with track marks using a single laser  100  and the optics system shown. The optical system may include a beam expander  102  that expands the laser beam to a suitable diameter to produce a spot with a desired spot size using a scanning lens  108 . The Bragg cell  104  is actuated by a RF generator  106  that induces periodic changes in the refractive index of the Bragg cell  104  that in turn causes the laser beam to be diffracted. Changing the RF frequency can control the angle of diffraction. The scanning lens  108  then focuses the scanning beam onto the tape  10 . A zero order spot is formed at the line of sight location  107 , whereas a first order spot is formed at  109   a  and  109   b , respectively on the tape  10 .  
         [0044]    The optical system of FIG. 8 can be operated to write a general servo pattern as follows. A signal generated by RF signal generator  106  having a first (higher) frequency is applied to the Bragg cell  104  to deflect the laser beam to the position  109   b  of FIG. 10 and write spot  29 . After the tape  10  has moved in the transport direction a distance corresponding to the lengthwise separation between track marks, a second spot is written by applying to the Bragg cell  104  a signal with a second frequency which deflects the laser beam, for example, to the position  109   a . Repeating the process of moving the tape  10  and changing the RF frequency of the signal write additional track marks. Due to the limited number of resolution spots, and limited scanning speed, this approach is suitable for tape drives using relatively narrow tape widths, and servo writers running at relatively low speed.  
         [0045]    Referring to FIG. 9, a beam expander  122  expands a beam from the laser  120 , with the expanded beam striking a diffractive optical element (DOE)  123 . The DOE is designed to split an incoming beam into a linear array of multiple beams with equal angular spacing, and intensity. The DOE  123  can be positioned at a front focal plane of Fourier transform (FT) lens  124  so that the multiple diffracted light beams are Fourier-transformed at the back focal plane of the lens  124 , thereby becoming equally spaced dots representing the tracks. The same pattern of equally spaced dots representing tracks enters a one-dimensional spatial light modulator (SLM)  125 . The pattern that is applied to the SLM  125  is time dependent. A second FT lens  126  Fourier transforms the dot pattern into a second DOE  127  to split the multiple beam patterns further into several identical bands that are written simultaneously onto the moving tape  10 . In this way, track marks having a unique track ID within one band are copied across many bands simultaneously.  
         [0046]    The DOE is a complex optical phase pattern on an optical substrate, provided with modern micro-lithography technique. A typical substrate is the optical glass. Upon illumination with a laser beam, a desired spatial pattern can be observed in the far field or near field, depending on the design. Herein, far field DOEs are used. DOEs are sometimes called Fourier generators. An earlier name for DOE is the kinoform, and that name is still used sometimes.  
         [0047]    The complex phase pattern is generated by mathematical computation using the laws of propagation of a coherent light beam in free space. Numerous algorithms have been developed for achieving good convergence for the mathematical computation. A mask can then be made for the complex phase pattern, from which the glass can be patterned and etched resulting in the DOE.  
         [0048]    The DOE is distinctly different from a hologram, even though both technologies depend on the use of the laser beam. The hologram is recorded on a special recording medium by the interference of the complex phase and amplitude pattern with a reference beam. One viewpoint borrowed from communication theory is that the reference beam provides a spatial carrier and the complex pattern is the base-band modulation signal. The DOE does not use a spatial carrier, and records the base-band pattern directly. It results in the following major advantages:  
         [0049]    1. The diffraction efficiency defined as the output beam intensity going into the desired output pattern divided by the incident laser beam approaches 90% to 100% for the DOE.  
         [0050]    2. The hologram is an off-axis construction, while the DOE is an on-axis device. That is, the illumination beam and the desired pattern have an offset angle to each other in the case of the hologram. This causes cumbersome optical alignment compared to the DOE.  
         [0051]    3. Since the hologram requires a carrier, the spatial resolution is very demanding. The DOE requires far less spatial resolution since only the complex signal itself has to be resolved.  
         [0052]    4. A batch process based on micro-lithography, using a mask set and the lithography and etching tools commonly used in the semiconductor industry, fabricates the DOEs. That is, one can produce numerous parts with one batch run. The hologram has to be made one at a time, and is quite time consuming when a large number of parts are needed.  
         [0053]    The DOE design is flexible, since the design is a mathematical construct; the hologram is not flexible since it is made from the interference of two beams onto a recording media.  
         [0054]    The spatial light modulator  125  can be made of several candidate devices. One candidate device is made of the electro-optical crystal such as LiNbO 3 , LiTaO 3 , KDP, and so forth. Another candidate device is the acousto-optic modulator array made of crystals such as Tellurium Oxide (TeO 2 ), Gallium Phosphide (GaP), Gallium Arsenide (GaAs), and so forth. Yet another candidate is the liquid crystal spatial light modulator, especially high-speed ferro-electric liquid crystal varieties. In each case, a linear spatial modulator can be fabricated and electroded to turn on or turn off each element in the beam dot array.  
         [0055]    Referring to FIG. 10, an optical system is used to produce a track ID by means of a two dimensional (2D) DOE  123  in conjunction with a 2D SLM  125 . The 2D SLM  125  can be an active device, or a passive spatial filter as shown in FIG. 10. An active device allows one to place any track information on each track, including track identification, manufacturer ID, manufacturer&#39;s data, longitudinal addresses, control information, and so forth. A passive device will allow only track ID to be coded in a repetitive fashion. In the example of FIG. 10, the first (2D) DOE  123  splits the expanded laser beam into a 2-dimensional array of laser beams having approximately equal intensity and a specific shape, such as the shapes  164 . The spatial filter  162  blocks certain spots, such as those along a diagonal of shape  164 , resulting in the pattern of  166 . Each track of the pattern  166  has its own distinct track ID. A second FT lens  126  Fourier transforms the pattern  166  unto a second linear DOE  127  that duplicates or multiplies the Fourier transform of pattern  166  perpendicular to the direction of the tape motion, thereby generating multiple bands of similar tracks. A FΘ scan lens  128  images the multiple bands and the multiple tracks  168   a ,  168   b , on the tape  10 .  
         [0056]    Referring to FIG. 11, a servo pattern can be produced by first generating an image of the servo pattern on a spatial light modulator (SLM)  114 . A laser beam, after being expanded by the beam expander  112 , illuminates the SLM  114 . An imaging lens  118  images the pattern on to the tape  10 . The SLM  114  can be either one-dimensional or two-dimensional. In the case that the SLM  114  is of the reflective type rather than the transmission type as shown in FIG. 11, the system functions well if the optics is modified. Since any SLM has a fill factor for each cell in the array, it is clear that the system shown in FIG. 11 requires more laser power than those systems shown in FIG. 9 and FIG. 10.  
         [0057]    [0057]FIGS. 12 through 14 illustrate a passive means of generating track identification marks.  
         [0058]    Referring to FIG. 12A, a bi-DOE  134  is used to generate a repetitive cross pattern. A beam expander  132  expands a laser beam  130 , and the expanded beam illuminates the bi-DOE  134 . Each of the two DOEs in the bi-DOE  134  has a specific fringe orientation. One DOE generates a linear array of beams tilted in one direction whereas the other DOE tilts a second linear array of beams in the other direction. When the two diffracted beams are focused by a scanning lens  136  onto the tape  10 , the focused spots form the cross pattern  144 . Here  144   a  are beam dots generated by one DOE whereas  144   b  are beam dots generated by the other DOE in the bi-DOE  134 . A cross sectional view of the bi-DOE  134  is shown in FIG. 12B. Slanted lines in  134 A and  134 B are representative of the orientation of the fringes inside the two DOEs.  
         [0059]    Referring to FIG. 13, a V-like pattern is illustrated. The V pattern can be made to have no ambiguity for the identification of each track compared to the cross pattern  144  (of FIG. 12A). The expanded laser beam is split into two beams with an angle to each other,  153   a  and  153   b , using a Wollaston prism  152 , or an optical wedge. Each beam is incident on a corresponding portion of a bi-DOE  156 . The fringe orientations of the two DOEs in bi-DOE  156  are similar to the ones shown in bi-DOE  134  in FIG. 12. The bi-DOE  156  generates two slanted beam arrays,  155   a  and  155   b . The FΘ lens  136  focuses the two beam arrays into two slanted beam dot arrays separated along the tape path direction by an amount determined by the angle between the beams  153   a  and  153   b . The result is the V shape, shown as  159   a  and  159   b.    
         [0060]    Referring to FIG. 14, a collimated laser beam is split into two beams. Each of the two beams goes through an individual DOE  203 ,  205  with the proper fringe orientation, and the two beams are combined and focused by the FΘ lens  208  into two linear arrays.  
         [0061]    Beam  62  passes through an adjustable half wave plate  200 , followed by a polarizing cube  201  to split the single beam  62  into two beams with orthogonal polarization directions, and equal power. The half wave plate  200  receives beam  62  with a given polarization state. The polarization is rotated after the beam emerges from half wave plate  200 , which can be decomposed into a p-component and s-component. The polarization beam splitter  201  passes a first light beam having a first polarization direction, while reflecting the second light beam having a different orthogonal polarization direction. The first light beam passes through an optical wedge  202 , a first DOE  203  and a second polarization beam splitter  207 . The purpose of the optical wedge  202  to impart an angle to the beam, just like what the Wollaston prism  152  does in FIG. 13. The second light beam is reflected by two mirrors  204  and  205  before passing through the second DOE  206  and then reflected by the second polarization beam splitter  207 . The two light beams exiting the second polarization beam splitter  207  are offset with respect to one another in the direction of the tape motion indicated by arrow  28 . The offset can be adjusted by selecting an appropriate wedge angle of the wedge  202 . The relative angle between the two linear array dots in the track pattern is designed into the two DOE&#39;s  201  and  206  via respective fringe orientation and fringe spacing control. The two beam patterns are focused by the FΘ lens  208  to form a track pattern on the moving tape  10 .  
         [0062]    [0062]FIG. 15 shows a block diagram of an exemplary system  240  for forming servos tracks and servo marks on the magnetic tape  10 . The system  240  includes an optical marking system  241  to mark a servo track onto a surface of the magnetic tape  10 . The marking unit  241  includes an optical marking engine (OME)  242  with a light source, such as a laser, marking optics  244 , as well as a tape transport path  246 . The tape transport path  246  is defined by two spools,  248  and  250 , and tape stabilizer  252 , which minimizes tape movement, in particular lateral tape motion (LTM), at the beam marking area  244  defined by a multiplicity of beams formed by the marking optics  244 . The beam marking area is depicted schematically as a triangle  254 . The optical marking engine  242  and marking optics  244  can incorporate any of the examples for writing a servo pattern described with reference to FIGS.  7 - 15  above.  
         [0063]    A verifier  256  is located downstream from the beam marking area  244 , and carries out the function of direct read-after-write, thus verifying that marks engraved onto the tape surface are appropriately written. Controller  260  may be a computer that may include software for controlling the tape transport, the optical marking engine  242 , and the verifier  256 , some or all of which can be mediated via a control interface  262 . One feature of the depicted verifier  256  is that the direct read after write signals can be conditioned and processed via a digital signal processor (not shown) that can be incorporated in the controller  260 , to establish pass/fail criteria for various features of interest. Thus the optical marking system  241  either can be de-activated under the most severe offense for which correction while operation is not feasible or, for a milder offense that can be corrected while in operation, a failed portion of the tape  10  can be identified and its location recorded in the controller  260 . The failed portion of the tape  10  can then be discarded in a subsequent process.  
         [0064]    A number of embodiments of the invention have been described. Other embodiments are within the scope of the following claims.