Abstract:
An apparatus, system, and method are disclosed for aberration compensation. In one embodiment, a first compensation lens used in conjunction with a second compensation lens to produce a conical beam used to read from, or write to, an optical medium. An N th  order compensation equation is used to optimize aberration errors associated with accessing the optical medium. The present invention may include a displaceable focus lens positioned relative to an optical medium. The focus lens may be displaced when the conical beam&#39;s focal length is adjusted. A displacement equation is presented to determine the preferred placement of the focus lens. By compensating for aberration, read/write errors may be reduced while accessing optical media thus increasing system robustness and facilitating the use of additional layers on optical media.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to aberration compensation and more particularly relates to apparatus, methods, and systems for compensating for aberration when accessing an optical medium.  
         [0003]     2. Description of the Related Art  
         [0004]     Optical media have become increasing pervasive in digital systems. Movies, pictures, and other familiar content, as well as software programs, drivers, and data can all be stored in a non-volatile manner on optical media. Because the use of optical media is so flexible and pervasive, the motivation to increase storage density has increased. One approach to increasing the storage density of optical media involves the use of multiple data layers.  
         [0005]     Accessing an optical media with multiple layers is typically accomplished by using a single laser and selecting a unique intensity for each data layer. For example, one intensity is set for the data layer closest to the laser, another intensity is set for the data layer farthest from the laser, and likewise for each data layer therebetween. Although the laser has multiple intensities to accommodate each data layer, the laser is typically set at a fixed focal length resulting in increased data error rates over single layer optical media.  
         [0006]     Additional challenges result from the variability in optical characteristics from disk to disk and layer to layer. For example, variation in material composition or thickness may result in changes in the refractive index of the media and the desired focal point of the laser. Thus, the variability in optical characteristics and variation in layer depth as well as other factors result in increased aberration errors for multi-layered media.  
         [0007]     From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that compensates for aberration on a layer by layer basis. Beneficially, such an apparatus, system, and method would reduce errors when accessing optical media and increase system robustness and performance.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available aberration compensation systems used in conjunction with an optical data storage system. Accordingly, the present invention has been developed to provide an apparatus, system, and method for aberration compensation that overcome many or all of the above-discussed shortcomings in the art.  
         [0009]     The apparatus to compensate for aberration, in one embodiment, includes a compensation lens comprising a plurality of compensation planes interleaved with a plurality of electrodes. Each compensation plane has a refractive index which varies according to a voltage applied across electrodes adjacent to it. A voltage source provides a plurality of compensation voltages to the electrodes. Each compensation voltage may be a function of a plane index. In one embodiment, the compensation voltages substantially conform to an N th  order compensation equation wherein all odd terms of the compensation equation are substantially equal to zero.  
         [0010]     The apparatus, in one embodiment, has a second compensation lens that is substantially identical to the first and axially rotated to provide additional compensation. The compensation lenses may be used in combination to provide both vertical and horizontal compensation (or the like) of any aberrations within the media or optical elements associated therewith.  
         [0011]     In one embodiment, the compensation voltages conform to discrete compensation equation such as a quadratic, fourth order, sixth order, or eighth order equation. In one embodiment, the compensation equation is the following discrete eighth order equation, namely Vi=C 1 *i 2 +C 2 *i 4 +C 3 *i 6 +C 4 *i 8 . An optimization module may optimize the compensation equation (for example in response to data errors) by perturbing the coefficients of the compensation equation. In one embodiment, perturbing the coefficients of the compensation equation proceeds from the lowest order coefficients to the highest order coefficients (as needed) in order to eliminate data errors.  
         [0012]     A focus lens may also be included in the apparatus. In certain embodiments, the focus lens is adjustable in position and may be adjusted in conjunction with a change in focal length of the first and second compensation lenses. In one embodiment, the displacement of the focus lens is determined by several factors such as, a distance between an axial midpoint of the first and second compensation lens and the focus lens (‘Dc’), a distance between the axial midpoint of the first and second compensation lens and a laser (‘Ds’), a focal length of the first and second compensation lenses in combination (‘F1’) and a focal length of the focus lens (‘F2’). In one embodiment, a distance Dm from the focus lens to a data layer m in an optical media is adjusted to substantially conform to the equation Dm=[F 2 *Dc−F 2 *Ds*F 1 /(Ds−F 1 )]/[Dc−F 2 −Ds*F 1 /(Ds−F 1 )].  
         [0013]     A system of the present invention is also presented to compensate for aberration. The system may be embodied in a device used to access an optical medium. In particular, the system, in one embodiment, includes an optical head which is able to emit a beam of light, a focus lens, an optical medium, a compensation lens encompassing a plurality of electrodes, a voltage source configured to provide a plurality of compensation voltages. In one embodiment, the compensation voltages substantially conform to an N th  order compensation equation wherein all odd terms of the compensation equation are substantially equal to zero.  
         [0014]     The system may further include a second compensation lens that is identical in composition to the first. The two lenses may be utilized in combination by aligning them coaxial with a transparent spacer configured to separate them in a substantially parallel manner. A focus lens may also be included in the apparatus. In one embodiment, the focus lens is adjustable in position responsive to a change in focal length of the first and second compensation lenses.  
         [0015]     The displacement of the focus lens may determined by several factors such as, a distance between an axial midpoint of the first and second compensation lens and the focus lens (‘Dc’), a distance between the axial midpoint of the first and second compensation lens and a laser (‘Ds’), a focal length of the first and second compensation lenses in combination (‘F1’) and a focal length of the focus lens (‘F2’). In one embodiment, the displacement conforms to the equation Dm=[F 2 *Dc−F 2 *Ds*F 1 /(Ds−F 1 )]/[Dc−F 2 −Ds*F 1 /(Ds−F 1 )] where Dm is the distance from the focus lens to a data layer m in an optical media.  
         [0016]     A method of the present invention is also presented to compensate for aberration. The method in the disclosed embodiments substantially includes the steps necessary to carry out the functions presented above with respect to the operation of the described apparatus and system. In one embodiment, the method includes interleaving a plurality of electrodes with a plurality of compensation planes and providing a compensation voltage across each compensation plane. The method also may include adjusting a refractive index corresponding to each compensation plane by providing compensation voltages that conform to an N th  order compensation equation wherein all odd terms of the compensation equation are substantially equal to zero. The method may further include optimizing the compensation equation to reduce aberration error.  
         [0017]     The various embodiments of the present invention may provide particular advantages over the prior art. Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.  
         [0018]     Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognized that the invention may be practiced without one or more of the specific features of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.  
         [0019]     These features and advantages of the present invention will become more fully apparent from the following descriptions and appended claims, or may be learned by the practiced of the invention as set fourth hereinafter.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:  
         [0021]      FIG. 1  is a side view of one embodiment of an optical head with an optical medium in accordance with the present invention;  
         [0022]      FIG. 2  is a perspective view of one embodiment of an aberration compensator in accordance with the present invention;  
         [0023]      FIG. 3  is a chart of one embodiment of search matrices in accordance with the present invention;  
         [0024]      FIG. 4  is a schematic diagram of a consolidated optical head with an optical medium in accordance with the present invention;  
         [0025]      FIG. 5  is a schematic flow chart diagram illustrating one embodiment of a aberration compensation method of the present invention; and  
         [0026]      FIG. 6  is a schematic flow chart diagram illustrating one embodiment of a compensation adjustment method of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Many of the functional units described in this specification have been explicitly labeled as modules, in order to more particularly emphasize their implementation independence. For example, a module may be implemented as a hardware circuit comprising custom VLSI circuits or gate arrays, off-the-shelf semiconductors such as logic chips, transistors, or other discrete components. A module may also be implemented in programmable hardware devices such as field programmable gate arrays, programmable array logic, programmable logic devices or the like.  
         [0028]     Modules may also be implemented in software for execution by various types of processors. An identified module of executable code may, for instance, comprise one or more physical or logical blocks of computer instructions which may, for instance, be organized as an object, procedure, or function. Nevertheless, the executables of an identified module need not be physically located together, but may comprise disparate instructions stored in different locations which, when joined logically together, comprise the module and achieve the stated purpose for the module.  
         [0029]     Indeed, a module of executable code may be a single instruction, or many instructions, and may even be distributed over several different code segments, among different programs, and across several memory devices. Similarly, operational data may be identified and illustrated herein within modules, and may be embodied in any suitable form and organized within any suitable type of data structure. The operational data may be collected as a single data set, or may be distributed over different locations including over different storage devices, and may exist, at least partially, merely as electronic signals on a system or network.  
         [0030]     Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.  
         [0031]     Furthermore, the described features, structures, or characteristics of the invention may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of programming, software modules, user selections, network transactions, database queries, database structures, hardware modules, hardware circuits, hardware chips, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0032]      FIG. 1  is a schematic diagram of one particular embodiment of an optical head  100  and optical medium  160 . To communicate the distinctive features of the present invention, the elements cited in the claims attached hereto are depicted in particular embodiments within the attached FIGS. (including  FIGS. 1 and 2 ) in a manner that facilitates comprehension and discussion. Specific dimensions included for discussion purposes are not intended to limit the scope of the claims. One of skill in the art will also appreciate that various configurations and implementations may be derived that conform to the inventive concepts communicated herein.  
         [0033]     As depicted, optical head  100  includes a laser  101 . Laser  101  may be a gallium-aluminum-arsenide or similar diode laser which produces a primary beam of light  102 . In one particular embodiment, beam  102  is a blue-laser light source with a wavelength 405 nm. In another particular embodiment, beam  102  is a red-laser light with a wavelength of 650 nm. In yet another particular embodiment, beam  102  is an infra-red laser light with a wavelength of 780 nm. Beam  102  may be any wavelength of light or combination of wavelengths capable of accessing optical medium. Beam  102  is collimated by lens  103  and is circularized by a circularizer  104  which may be a circularizing prism. Beam  102  passes to a beamsplitter  105 .  
         [0034]     In the depicted embodiment, a portion of beam  102  is reflected by beamsplitter  105  to an aberration compensator  112  and an optical detector  107 . Laser light focused by lens  106  onto detector  107  may be used to monitor the intensity of beam  102 . An optimization module  109  may monitor aberration error directly or indirectly to make various adjustments to reduce aberration errors via the aberration compensator  112 . The rest of beam  102  passes to and is reflected by mirror  108  through focus lens  110  onto optical medium  160 .  
         [0035]     Optical medium  160  may include a plurality of data surfaces. In the depicted embodiment, beam  102  passes through a focus lens  110  and is focused onto one of eight data surfaces  161  thru  168  of optical medium  160 . The target layer is surface  165 , in  FIG. 1 . Focus lens  110  is mounted in a holder  114 . The position of holder  114  may be adjusted relative to optical medium  160  by a focus actuator motor  116 . In one embodiment, focus actuator motor  116 , is a voice coil motor.  
         [0036]     In the depicted embodiment, a portion of beam  102  may be reflected by targeted data surface  165  as reflected beam  120 . Beam  120  subsequently returns through lens  110 , is reflected by mirror  108 , and passes through compensator  112 . At beamsplitter  105 , beam  120  is reflected to a multiple data surface filter  122 . The beam  120  also passes through filter  122  and passes to a beamsplitter  124 . At beamsplitter  124  a first portion  130  of beam  120  is directed to an astigmatic lens  132  and a quad optical detector  134 . At beamsplitter  124 , a second portion  136  of beam  120  is directed through a half-wave plate  138  to a polarizing beamsplitter  140 . Beamsplitter  140  separates light beam  136  into a first orthogonal polarized light component  142  and a second orthogonal polarized light component  144 . A lens  146  focuses light  142  to an optical detector  148  and a lens  150  focuses light  144  to an optical detector  152 .  
         [0037]      FIG. 2  depicts one embodiment of a lens assembly  200 . Lens assembly  200  comprises a front compensation lens  213 , a rear compensation lens  215 , and a transparent spacer plate  217  positioned therebetween. Lens assembly  200  is one example of the aberration compensator  112  depicted in  FIG. 1 .  
         [0038]     In the depicted embodiment, a beam of light  219  is projected into rear lens  215  from right to left. Beam  219  may be independently focused by compensation lenses  213 ,  215  into conical beams  223 ,  225 , respectively, and focused to a common point  221  to the left of front lens  213 .  
         [0039]     In one embodiment, the thickness of spacer plate  217  is selected to be one half of one wavelength of beam  219 . Beam  219  may be any wavelength of laser light or combination of wavelengths capable useful for accessing the optical medium. The depicted compensation lenses  213 ,  215  are substantially identical in construction and rotated 90 degrees along the optical axis. For simplicity, only lens  215  will be discussed even though the following description applies equally to lens  213 .  
         [0040]     The depicted lens  215  includes a pair of parallel, glass plates,  231 ,  233 , and a plurality of thin, rectangular, insulative, polymer films  235  therebetween. In one particular embodiment, plates  231 ,  233  are one mm squares, the inner surfaces of plates  231 ,  233  are spaced apart by seventy microns which is also the width of the films  235 , and each film  235  has a length of one mm and a thickness of two microns.  
         [0041]     In the depicted embodiment, a pair of glass substrates  241 ,  243  are located at the upper and lower ends, respectively, of lens  215  with films  235  layered therebetween. Films  235  and substrates  241 ,  242  are substantially parallel to one another, and substantially perpendicular to plates  231 ,  233 . In one embodiment, films  235  and substrates  241 ,  243  are evenly spaced at approximately fifty micron intervals.  
         [0042]     To facilitate comprehension, the depicted lens assembly  200  includes five films  235  interleaved with six layers of a variable refractive index material  247  (such as a liquid crystal) and positioned between substrates  241 ,  243 . Each layer of variable refractive index material  247  is referred to herein as a compensation plane. In some embodiments, there are an equal number of layers of the refractive material above and below the centerline of the lens  213 . In the depicted illustration each compensation plane is labeled with a layer index i (namely −3, −2, −1, 1, 2, 3) to denote the six layers shown. With a 50 micron layer-to-layer interval and a one mm square compensation lens, the actual number of layers would be approximately twenty, (1000/50).  
         [0043]     The upper and lower surfaces of each film  235  and the inner surfaces of substrates  241 ,  243  have an electrode  245  formed on them. In the embodiment shown, electrodes  245  are semi-circular in shape with a radius of two mm. In another embodiment, electrodes  245  may be formed in any other shape that also would produce a positive focal length. Electrodes  245  may have a thickness that is less than ten nm, but they are shown much thicker for illustration purposes. Electrodes  245  may be sputtered to the desired shape with a patterned mask, or sputtered over the entire rectangular surface of films  235  and substrates  241 ,  243 , and then chemically etched with a patterned photo-resist to obtain the desired shape. Other processes, such as photolithography, may also be used to obtain the desired pattern for electrodes  245 .  
         [0044]     After electrodes  245  are formed, an alignment material (not shown) such as polyimide may be spin-coated or printed on top of the electrodes and the remaining surface area of the underlying substrate. However, any material for homogenous parallel alignment, such as polyvinyl alcohol, may be used. The alignment material has a thickness of thirty nm or less. After the alignment material has coated the electrodes and their substrates, an alignment process, such as rubbing, is performed on the alignment material to set the desired alignment direction for the liquid crystals. Other alignment processes, such as photoalignment, which establish parallel homogenous alignment, may also be used.  
         [0045]     The films  235  may be mounted in spacers (not shown) to maintain their spacing which in the depicted embodiment is approximately 50 microns. The spacing between films  235  could be larger or smaller, as long as the alignment effect is maintained. The films  235 , substrates  241 ,  243 , and plates  231 ,  233  are then assembled together to form lens  213  before the refractive material  247  is injected into the spacers or cells. In one embodiment, the refractive material  247  is a liquid crystal.  
         [0046]     In one embodiment, a voltage  255  is applied to produce a selected refractive index for the refractive material  247  for each compensation plane in lenses  213 ,  215 . Each compensation plane may have a different voltage  255  to produce a refractive index for that layer, so that a common focus point  221  is achieved. The voltage across each compensation plane may be constant, resulting in a stepped voltage profile  280 .  
         [0047]     In one embodiment, the total thickness of each film  235 , including an electrode  245  and outer layer of alignment material on each surface (which are substantially negligible at twenty nm and sixty nm total) is approximately two microns. Assuming one film  235  for every fifty microns of transmission width, the amount of light transmitted by lens assembly  200  may be diminished by only four percent (4%) per lens  213 ,  215 , or eight percent (8%) total. In one embodiment, the value of δz is fifty-two microns distinguished as fifty microns for the liquid crystal plus two microns for the film.  
         [0048]      FIG. 3  illustrates a set of search matrices  300  to perturb coefficients for aberration compensation in accordance with the present invention. The search matrices include a matrix of quadratic coefficients  310 , a matrix of fourth-order coefficients  320 , a matrix of sixth-order coefficients  330 , and a matrix of eighth-order coefficients  340 . The search matrices  300  may be used by the optimization module  109  to optimize an Nth order compensation equation used to provide the compensation voltages to the aberration compensator  112 . In the depicted embodiment, the search matrices  300  correspond to an 8 th  order compensation equation. While only a 3×3 submatrix is shown for purposes of illustration, search matrices  300  may be of any appropriate size.  
         [0049]     Matrix  310  may have coefficient values  301  shown as C 1  and C 5 . In the depicted embodiment, coefficient values  301  represents the current quadratic coefficients used to generate the compensation voltages for compensation lenses  213  and  215  respectively. Index  301  may have neighboring cells  302 - 309 . Cells  302 - 309  represent possible offsets from the current coefficients in index  301 .  
         [0050]     Similarly, search matrices  320 ,  330 ,  340  may have indices  311 ,  321 ,  331  respectively. In the depicted embodiment, indices  311 ,  321 ,  331  represent the current fourth-order, sixth-order, and eight-order coefficients, respectively, used to generate the compensation voltages and compensate for aberration errors in the optical head  100  and optical medium  160 . Indices  311 ,  321 ,  331  may have neighboring cells  312 - 319 ,  322 - 329 ,  332 - 339  respectively. In the depicted embodiment, cells  312 - 319 ,  322 - 329 ,  332 - 339  are possible offsets from the current coefficients in indices  311 ,  321 ,  331 .  
         [0051]     The coefficients of indices  301 ,  311 ,  321 ,  331  may be offset by a value, δC. δC may be different for each search matrix. In one embodiment, δC corresponds to the finest resolution achievable for the compensation voltages. In another embodiment, δC is initially set at a course resolution and subsequently set to a finer resolution to increase the optimization resolution. As depicted, each cell of the search matrices  300  represents two coefficients, one coefficient is for aberration compensation of the first liquid crystal lens and one is for aberration compensation of the second liquid crystal lens. When selecting new coefficients for aberration compensation, at least one of the coefficients of the neighboring cells may be perturbed by a value, δC.  
         [0052]     If each lens of aberration compensator  200  is substantially identical in construction, an aberration compensation equation may be derived for independently focusing light into a canonical beam which has a common algebraic form for both lenses. For simplicity, two equations will be presented using distinguishing variables for the coefficients. For lens  215 , the discrete equation for aberration compensation of light into a canonical beam is represented herein as C 1 *i 2 +C 2 *i 4 +C 3 *i 6   +C 4 *i   8 . Likewise, for lens  213 , the discrete equation for aberration compensation of light into a canonical beam is represented herein as C 5 *i 2 +C 6 *i 4 +C 7 *i 6 +C 8 *i 8 . In the discussed embodiments, odd powers of layer index i are excluded to ensure the step-voltage profile  280  is symmetric about the centerline of lens  213 ,  215 .  
         [0053]     In one embodiment, the optimization module  109  first perturbs the quadratic coefficients C 1  and C 5  via search matrix  310 . The coefficients C 1  and C 5  maybe offset by adding or subtracting a value, δC, from at least one of the coefficients according to the search matrix  310  until aberration is minimized. In one embodiment, when additional compensation is needed, coefficients of the next higher ordered term are perturbed in accordance with the corresponding matrix  320 ,  330 ,  340 .  
         [0054]      FIG. 4  is a consolidated depiction of optical head  100  with optical medium  160 . Some components of optical head  100  have been omitted to ease illustration. The consolidated optical head  400  includes a laser diode  402 , a laser beam  404 , a variable aberration compensator  406 , and a focus lens  408 . The depicted spacings include a distance  410  (‘Ds’) between laser diode  402  and variable aberration compensator  404 , a distance  412  (‘Dc’) between variable aberration compensator  406  and focus lens  408 , a distance  414  (‘Dm’) between focus lens  408  and a data layer on optical medium  160 , and a direction  420 .  
         [0055]     As depicted, laser diode  402  emits a beam of light  404  in direction  420 . Beam  404  passes through variable aberration compensator  406 , then focus lens  408 , and is focused onto a data layer on optical medium  160 , where data layer  165  as depicted as the target layer. In one embodiment, the focus lens  408  is moveable by a voice coil motor so as to be closer to or further from the optical medium. Focus lens  408  has a focal length, F 1 , and variable aberration compensator  406  has a focal length, F 2 . When the focal length of the aberration compensator is altered, the distance  414  may require adjustment. The adjustment to distance  414  may be made according to the equation Dm=[F 2 *Dc−F 2 *Ds*F 1 /(Ds−F 1 )]/[Dc−F 2 −Ds*F 1 /(Ds−F 1 )]. Adjusting according to the particular equation facilitates focusing the head  100  to a different layer  165  of the optical medium  160  independent of the current aberration compensation coefficients.  
         [0056]     The schematic flow chart diagrams that follow are generally set forth as logical flow chart diagrams. As such, the depicted order and labeled steps are indicative of one embodiment of the presented method. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the illustrated method. Additionally, the format and symbols employed are provided to explain the logical steps of the method and are understood not to limit the scope of the method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown.  
         [0057]      FIG. 5  depicts one embodiment of a compensation method  500  of the present invention. The depicted method  500  includes selecting  510  coefficients to compensate for aberration, determining  520  the media focus, and displacing  530  the focus lens to the new media focus. The method  500  facilitates improved error rates particularly when accessing multi-layer media.  
         [0058]     Selecting  510  coefficients to compensate for aberration includes perturbing coefficients C 1 -C 8  to receive a minimal amount of aberration errors. In a depicted embodiment, selecting  510  coefficients C 1 -C 8  to compensate for aberration uses one of the various optimization processes discussed in conjunction with the search matrices  300 . For example, selecting may begin with the second order coefficients and proceed to higher order coefficients as needed.  
         [0059]     Subsequently the method proceeds by testing  515  for a change in the aberration compensator focal length. If the focal length has not changed, the method ends  540 . If the focal length has changed, the method continues by determining  520  the media focus distance Dm. As disclosed herein the media focus distance is the distance from the focus lens to a data layer m on an optical medium. In the depicted embodiment, the focus lens is adjusted to conform to the equation Dm=[F 2 *Dc 31  F 2 *Ds*F 1 /(Ds−F 1 )]/[Dc−F 2 −Ds*F 1 /(Ds−F 1 )] where Dc is a separation distance between an axial midpoint of the first and second compensation lens and the focus lens, Ds is a separation distance between the axial midpoint of the first and second compensation lens and a laser, F 1  is a focal length of first and second compensation lens in combination, and F 2  is a focal length of the focus lens.  
         [0060]     Displacing  530  the focus lens to the new media focus distance may include repositioning a frame which secures the focus lens. In one embodiment, the frame is repositioned relative to the optical medium by an actuator.  
         [0061]      FIG. 6  depicts of a compensation adjustment method  600 . The depicted compensation adjustment method  600  includes initializing  610  the coefficient index, executing  620  the current coefficients, testing  625  for data errors, perturbing  630  the coefficient index, identifying  640  the optimum coefficients, redefining  650  the index of coefficients, and advancing  660  to the next search matrix. The compensation adjustment method  600  may be conducted in response to a read error, a calibration error, or the like. In one embodiment, the compensation adjustment method  600  is conducted in conjunction with the search matrices  300 .  
         [0062]     Initializing  610  the index of coefficients may include supplanting the current coefficients for the compensation equation with a temporary or default set of coefficients. In certain embodiments, the coefficients are not supplanted and operation  610  may be omitted. Executing  620  the current coefficients includes subjecting the currently selected coefficients to a read test. In one embodiment, the coefficients are used to generate and apply the compensation voltages corresponding to the current coefficients to their respective compensation planes.  
         [0063]     In conjunction with the read test, the optical head may be positioned to read data from the point of error that necessitated invoking the compensation adjustment method  600 . The method may continue by testing  625  for data errors in the read test. If the data can be read without errors, then further searching for coefficients may not be required and the method may advance to saving  650  the coefficients for continued usage. However, if the data cannot be read, the method continues to search for better coefficients.  
         [0064]     Perturbing  630  the coefficients may include incrementing or decrementing one or more indices into a search matrix in order to test different coefficients. For example, if the method were using the compensation coefficients from cell  301 , then perturbing  630  the coefficients would move the center cell  301  to an adjacent cell such as cell  302  or  303  (See  FIG. 3 ). In one embodiment, the index values into a search matrix are randomly perturbed. In another embodiment, the perturbation continues in a selected direction until the error rate increases whereupon a different search direction is selected.  
         [0065]     The perturb again test  635  ascertains whether additional perturbations maybe useful. If so, the method loops to the execute constants operation  620 . If not, the method advances to identifying  640  the optimum coefficients. Identifying  640  the optimum coefficients includes ascertaining which perturbed coefficients produced the best. If the results are satisfactory, a flag may be set indicating that additional adjustment is not needed.  
         [0066]     The more adjustment needed test  645  ascertains if additional adjustment to the compensation coefficients is needed. If additional compensation is needed, the method advances  660  to the next (higher ordered) search matrix. If additional compensation is not needed the method continues by saving  650  the optimized coefficients. Subsequently, the method ends  670 .  
         [0067]     The present invention provides improved aberration compensation particularly with multi-layered optical media. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.