Abstract:
A method and apparatus for retrieving information from a three dimensional storage medium uses a three dimensional storage medium having an active medium capable of exhibiting first and second states, a data unit being represented by the ratio between the concentration of the first and second states in a given volume portion of the medium and a data sequence is represented by a sequence of data units. The active medium is irradiated with light as to concentrate light flux through a volume portion of the storage medium so as to generate in the volume portion a detectable non-linear optical response characteristic of the concentration ratio, which is detected and used for tracking.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to methods and apparatus for reading information from a 3D optical storage medium.  
       BACKGROUND OF THE INVENTION  
       [0002]     It has been suggested in the art, to store information in three dimensional optical storage apparatuses. One of the problems to be solved in such systems is how to read information from a particular point without letting the reading light beam being distracted by the storage medium positioned between the reading light source and the said particular point.  
         [0003]     WO 01/73779 co-owned by the owner of the present invention, suggests reading the information by two-photon absorption. In this method, the information stored in a particular point is characterized by the absorption coefficient in a certain frequency ν, and the reading is carried out with two light beams having frequencies ν 1  and ν 2 , so that ν 1 +ν 2 =v (ν 1 −ν 2 =v is also possible). Only when the two light beams intersect, the light may be absorbed and reading takes place. In all the points where the two beams do not intersect, there is no light of frequency ν, and therefore no reading. The storage medium should be transparent to light having a frequency ν 1 , and also to light having the frequency ν 2 .  
         [0004]     Regarding the storage means, it is suggested in WO 01/73779 to use a matrix carrying stilbene derivatives, having one characteristic absorption in a given frequency when in the cis isomer and another, when in the trans.  
         [0005]     In optical storage media such as optical disks in general and DVDs in particular, data is stored along tracks formed in the bulk of the optical disk and is read by focusing a laser beam produced by semiconductor diodes on to the tracks, while spinning the disk on its axis. The tracks generally comprise spiral tracks on which data is written and from which the data is read.  
         [0006]     Obviously in order to retrieve data correctly it is essential that the reading head can locate and follow a desired track. In practice this leads to two different kinds of tracking problem: skipping from one track to another and faithfully following a single track. For the purpose of the present discussion, it suffices to observe that these two different tracking problems require different solutions and to the extent that the method of tracking is relevant to the present invention, the present invention is concerned only with the second of the two problems.  
         [0007]     U.S. Pat. No. 5,592,462 (Beldock) issued Jan. 7, 1997 entitled “Three-dimensional optical data storage and retrieval” discloses a three dimensional optical data storage and retrieval system having a three dimensional optical data storage medium and an apparatus for providing access to data stored on the medium. In accordance with one aspect, the data storage medium includes a number of concentric shells each of which has a curvilinear data storage surface for storing data in a number of substantially parallel data tracks. According to another aspect, the data storage medium includes a number of data storage surfaces, which are rotatable about a corm non axis, each data storage surface for storing data in a number of substantially circular data tracks and having an optically transparent window, which transects each of the data tracks. In use, each shell or data storage surface is rotated about a common axis and tracking is achieved by directing the reading beam through the optically transparent windows on to a data track of interest. Thus, this reference is not applicable to retrieving data from a solid optical storage medium wherein the data is stored in multiple layers.  
         [0008]     The manner in which CD and DVD reading head track a destination track is based on focusing the reading spot on to the track and measuring the intensity of a reflected spot by position sensitive detectors. This allows calculation of the position of the reading spot and subsequent adjustment of the reading head&#39;s location based on the measured error.  
         [0009]     U.S. 20010040844 published Nov. 15, 2001 (Sato et al.) entitled “Tracking servo apparatus of optical information recording and reproducing apparatus” discloses a tracking servo apparatus using this technique. Thus, reflection light obtained when a laser beam is irradiated onto a recording surface of an optical disc is photoelectrically converted, thereby obtaining a photoelectric conversion signal. A tracking error signal showing an amount of deviation of an irradiating position of the laser beam for a track in a disc radial direction on the recording surface is generated by the photoelectric conversion signal. A spherical aberration occurring in an optical system is detected, a level of the tracking error signal is corrected on the basis of the detection result, and the irradiating position of the laser beam is moved in the disc radial direction in accordance with the level-corrected tracking error signal.  
         [0010]     Likewise, U.S. Pat. No. 6,233,210 published May 15, 2001 (Schell; David L.) entitled “Optical drive error tracking method and apparatus” discloses a method and apparatus for obtaining a tracking error signal for an optical disk player which is general across the various data formats found in CD audio disks and DVDs. A photodetector having at least four active areas is used to sense the reflected laser beam. A differential amplitude tracking error signal is generated by comparing the signal strength in the different active areas.  
         [0011]     These references are typical of known solutions for maintaining the read/write head in communication with a desired track using a photodetector having multiple sections that serves as a position-sensitive detector for detecting a component of the read/write laser beam reflected from the surface of the optical disk.  
         [0012]     For both CDs and DVDs, axial compensation translates to a focusing adjustment of the read/write beam.  
       SUMMARY OF THE INVENTION  
       [0013]     It is an object of the invention to provide a method and system for retrieving information from a three dimensional storage medium.  
         [0014]     It is a further object of the invention to correct tracking errors in such a method and system where the read spot may drift in two essentially orthogonal directions.  
         [0015]     In accordance with a broad aspect of the invention, there is provided a method for retrieving information from a three dimensional storage medium, the method comprising: 
        using a three dimensional storage medium comprising an active medium capable of being in two states, wherein a data unit is represented by the ratio between the concentration of the first and second of said two states in a given volume portion of said medium and a data sequence is represented by a sequence of such data units;     irradiating said active medium with light as to concentrate light flux through a volume portion of said storage medium so as to generate in said volume portion a detectable non-linear optical response characteristic of said concentration ratio;     detecting said non-linear optical response to retrieve information stored in said volume portion; and     tracking a data sequence for retrieving said data sequence in a reproducible manner.        
 
         [0020]     Within the context of the description and appended claims, the term “data unit” refers to a bit or symbol of a finite alphabet. Preferably, the data sequence is tracked via a tracking feedback signal for positioning the light at a predetermined volume portion of the storage medium.  
         [0021]     According to a further aspect of the invention there is provided a method for correcting tracking errors in an optical storage medium having multiple tracks arranged in different layers of the optical storage medium, the tracking comprising: 
        (a) directing a reading spot that is nominally focused on to a track in the optical storage medium,     (b) continually moving the reading spot in axial and radial directions,     (c) receiving a signal having an amplitude which varies according to respective offsets from the track in radial and axial directions,     (d) using the received signal to determine a direction of a respective offset from the track in radial and axial directions, and     (e) adjusting a location of the reading spot accordingly.        
 
         [0027]     Any active medium known in the art is suitable for use according to the present invention. Some non-limiting examples to active media are those described in WO 01/73779 and in U.S. Pat. No. 5,268,862, both of which are incorporated herein by reference, stillbene derivatives, and azobenzene derivatives.  
         [0028]     The active medium is preferably embedded in a supporting matrix, for instance, as a dopant or, when the supporting matrix is a polymer, as a monomer co-polymerized with the supporting matrix. The supportive matrix should be transparent for the light irradiated on it by the method of the invention and to the light generated by the non-linear optical process. Non-limiting examples of supportive matrices suitable for use according to the present invention are polyethylene, polypropylene, polycarbonate, and polymethylmetacrilate (PMMA).  
         [0029]     Typically, the data is stored in a binary mode, so that the concentration ratio representing one digit is 1:0 and the concentration ratio representing the other digit is 0:1. Here, 1 and 0 are not absolute values but rather should be interpreted as the highest and lowest concentrations that may be achieved during the writing process, which is not discussed herein.  
         [0030]     As an alternative to storing the data in binary mode, other schemes may be devised where the there are more than two states of the media (e.g. completely in isomerics states A or B, in the thermal equilibrium or close to it, in one of a multitude of photo stationary states and more). The size of the alphabet used in the encoding-decoding process depends on the encoding-decoding method used and on the signal separation and the signal resolution (signal to noise ratio) of the system. Many encoding-decoding methods are known in the art, DC free and run length limited encoding-decoding are non-limiting examples of such families of codes.  
         [0031]     The size of the volume portion from which a data unit is retrieved according to the invention is the size of the light spot wherein the flux is large enough to generate a detectable non-linear reaction. Generally, smaller spot sizes may compensate for weaker light intensities. Therefore, working with spots having a radius of less than 30 μm is advisable, and spots having a radius equal to or smaller than the wavelength of the irradiated light is preferable.  
         [0032]     Small spots allow the use of cheaper light sources. However, it may be beneficial to work at high intensity of a given light source, even if a non-linear response is detected at lower intensities. This is so because above some intensity, due to saturation effects, the response is no longer sensitive to the intensity, and the reading is also not sensitive thereto. This way noise may be reduced from the measurement.  
         [0033]     Detection of the signal requires its separation from other light signals that may exist in the environment. Such separation may be achieved by any method known for this purpose in the art, such as directing the non-linearly generated signal into a direction where it is the only source for light of its frequency by satisfying phase matching conditions; filtering the light through a filter, prism, grating, polarizers, etc.; using phase sensitive detection, lock-in amplifier, a box-cars, and/or gated averaging method. All the available methods may be applied whether the beams irradiating the active medium are collinear or not.  
         [0034]     Non-linear optical processes are very sensitive to the flux of light. As the flux varies as a function of volume element when a beam of light is focused through matter, a dramatic increment in the effective efficiency occurs when approaching the focal point. The effective result of this is that with the appropriate photon flux, the process occurs only at the locus of maximal power, that is the focal point. When using a single light source at a single wavelength, this focal point is straightforwardly defined as the location where the beam waist is minimal.  
         [0035]     When using more than one wavelength the most efficient way to achieve the same effect of high localization is by overlapping the focal point of all photon sources involved in the relevant process. When different foci are not exactly overlapping, the same process occurs, albeit less efficiently, in the volume generated by the crossing of the effective volume elements generated by the foci of all light sources. To gain additional effective efficiency in a one-wavelength process higher fluxes can be achieved through the use of more than one light source using the same methods as for the multi-wavelength case.  
         [0036]     The superposition principle allows one to consider a single beam as a plurality of beams whose respective foci are trivially located in the same location. Therefore within the context of the present invention and the appended claims where reference is made to two or more intersecting light beams, it is to be understood that a single light beam may give rise to a non-linear process analogous to the overlapping of two or more monochromatic beams, and this is encompassed by reference in the claims to the intersection of two or more light beams.  
         [0037]     One family of non-linear optical responses suitable for use according to the present invention is a multi-photon fluorescence, such as, but not limited to, two-photon fluorescence.  
         [0038]     Non-limiting examples of non-linear optical responses related to a X process, are four wave mixing processes such as Stimulated Raman Scattering, Coherent Anti-Stokes Raman Scattering (CARS), Raman induced Kerr effect, and degenerate four-wave mixing. Similar X (5)  processes and higher are also known in the art and may be used according to the present invention.  
         [0039]     According to another aspect of the present invention there is provided an apparatus for retrieving information from a three dimensional storage medium by generating a non-linear optical response of said storage medium, detecting said non-linear optical response and analyzing and processing it. Such an apparatus includes, in order to generate and detect a non-linear optical response, at least one light source, which in some cases (such as CARS) must be coherent; a detector for detecting light, which is different in at least one characteristic from the light provided by said light sources. In this context, examples of light characteristics are the light wavelength, polarization, and propagation direction. And means for tracking, i.e. process the signals received from the medium to get a tracking feedback signal and correct the location of the read spot accordingly. The apparatus may also include means, known per se in the art for analyzing and processing detected signals and retrieving information therefrom. These may comprise means for digitizing the detected signal, such as an A/D converter, and an algorithmic error detection means, such as error detector code running on a computer or on an electronic chip.  
         [0040]     A light source according to the present invention may be an active light source like a laser, or a passive light source like a mirror. A beam splitter, for example, may be considered as two (passive) light sources.  
         [0041]     According to one embodiment of the invention the data sequences are arranged as layers within the medium, each layer consisting of a spiral track of the respective data sequences, where the medium is shaped as a disk, and rotated around its axis by the apparatus. The purpose of the invention is to track the spiral track corresponding to a required data sequence in r and z coordinates when the disk rotates. It is assumed that the track suffers limited amount of run-out both in r (radial run-out) and z (axial ran-out) coordinates. Such distortions can occur in the event that the axis of rotation is slightly off the disk center and slightly non-parallel to the disk plane normal, such that the data spiral moves relative to the reading spot while the disk rotates. The invention enables tracking the data spirals by calculating a tracking error signal that is used as feedback for the servo-mechanisms that control the r and z position.  
         [0042]     The basic tracking principle is to perpetually move (modulate) the reading spot in a periodical path around its nominal current position (traveling the r-z plane by two orthogonal functions of time). This modulation causes a modulation in amplitude and phase of the read signal that depends on the position of the reading spot relative to the data spiral. This dependence is used to determine the tracking error.  
         [0043]     As a simplified example of the way the tracking algorithm calculates an error signal, consider a 2-D case. Assume the z coordinate is fixed such that the lasers spot is focused at the proper height and there is no axial run-out. As the reading spot propagates along the track, the spot&#39;s radial position is modulated in the radial direction so that the spot is half the time in (towards the center of the disk) and half the time out relative to the track (i.e. r&lt;r 0  half of the time and r&gt;r 0  half of the time). It should be noted that the offset relative to track center has to be small to ensure that signal is still detected with a signal to noise ratio that is high enough for other functions such as symbol detection or synchronization to be accomplished. If the signal has a fixed average and the tracking is perfect, than the average of the ‘in’ signal (signal detected when spot is ‘in’ relative to the track) is equal to the ‘out’ signal. If the spot&#39;s position begins to diverge from the track&#39;s position e.g. because of eccentricity of the disk, the expansion of the spiral or some other reason, then the difference between the ‘in’ and ‘out’ parts of the modulated signal, out-in, is negative if there were a small ‘run-out’ or positive if there was ‘run-in’.  
         [0044]     Two main factors determine the frequency of the modulation. It should be high enough to be able to respond to fast changes of the relative location of the track and the spot, but low enough to average enough data units so that the signal will be independent of the stored data. The averaging of the data can be accomplished by window integration or other appropriate low pass techniques. To ensure that in each integration window the signal is data-independent, DC free encoding techniques are used.  
         [0045]     In another embodiment of the tracking mechanism the error signal calculation is accomplished by multiplying (inner product) the time variation of the data envelope (the read signal) with the reading spot modulation function. In this scheme the error signal is weighted by the strength of the modulation, i.e. signal measured when the amplitude of the modulation is high contribute more to the error signal. Further refinement of the invention is to include delay compensation before the multiplication between the signal and the modulation.  
         [0046]     The tracking errors are used as feedback signals for the servo machine controlling the nominal spot position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]     In order to understand the invention and to see how it may be carried out in practice, specific embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:  
         [0048]      FIG. 1  is a graph showing CARS spectra of solid solutions having two different concentration ratios between cis and trans isomers of a given compound;  
         [0049]      FIG. 2  is a schematic illustration of an apparatus according to the present invention;  
         [0050]      FIG. 3  is a block diagram showing functionally a read/write system for use with the invention;  
         [0051]      FIGS. 4   a  and  4   b  are pictorial representations showing the effect of sinusoidally modulating the position of the reading head in the system of  FIG. 2 ; and  
         [0052]      FIGS. 5 and 6  are block diagrams showing details of a tracking system for use with the system shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF INVENTION  
       [0053]     A solid solution of 10% cis-4,4′-dimethoxy-α,α-diciano stillbene (hereinafter compound A) in PMMA was irradiated with collinear laser beams of 844 and 1037 nm that were focused through a lens to a spot smaller than 10 μm. CARS signal at a wavelength of 711 mn was detected. The spectra of the signal detected from a solid solution of the cis isomer and of a cis-trans mixture is given in  FIG. 1 .  
         [0054]      FIG. 2  is an illustration of an apparatus  100  according to the present invention for retrieving information from a three dimensional storage medium (hereinafter referred to as “disk”)  102  having an information carrying volume (not shown). The apparatus  100  includes two lasers  104  and  106 , each being a source for a beam of coherent light ( 110  and  112  respectively), and a detector  120  for detecting a beam of coherent light  116  which is of different wavelength to the light provided by the lasers  104  and  106 . The detector  120  transfers an electric signal, created therein due to the detection of an optical signal produced by the beam of coherent light  116 , to a low noise amplifier  123 , which can be a lock in amplifier to a tracking unit  125 , so that the data sequence may be faithfully followed, and to an A/D converter  126 , whose output is fed to a decoder and error detection and correction (ECC) unit  128  (constituting an algorithmic error detector), so that information encoded in the data sequence may be retrieved.  
         [0055]     In the embodiment shown in  FIG. 2  there is also a disk mount  202  for mounting thereon the disk  102 , so that the disk, when mounted, may be rotated around its center by a motor  205 . The two light sources  104  and  106  are connected to two optical fibers  104 ′ and  106 ′ arranged to direct the light signals  110  and  112  to a dichroic mirror,  220 . The signal  112  is transferred through the mirror and the signal  110  is reflected thereby. Thus, the light may irradiate the disk  102 , in such a manner that the beams common focus  222  is located within the information carrying volume of the disk  102 .  
         [0056]     The optical unit  101  is mounted on an arm  210 , which may rotate around an arm axis  230 . A lens  240 , of the kind used in CD players, is positioned between the dichroic mirror  220  and the disk  102 . Its position in the direction parallel to the disk&#39;s surface  102 ′ is controlled by the combination of the rotations of the arm  210  around the arm axis  230  and the disk  102  around the disk mount  202 . Its position in the direction perpendicular to the disk&#39;s surface  102 ′ is controlled by a magnetic coil  242 , which is also used to control small radial motions of the lens and thus the position of the common focus  222  within the disk  102  may be fully controlled. To achieve tracking, the location of the common focus is modulated by moving the lens  240  by applying a periodical electric signal to the magnetic coil  242 . The coherent light sources  110  and  112  in combination with the dichroic mirror  220 , the lens  240  and the magnetic coil  242  constitute an optical system  245 . A collecting mirror  250  is positioned to collect the non-linearly generated signal  116  and directs it to the detector  120 , positioned near the arm axis  230 , through a filter  152 . The laser drivers are not shown in  FIG. 2 .  
         [0057]     The large ratios between the radius of the motion of the optical unit  101  around its axis  230  and track radius on the one hand and the size of the spot and the distance between adjacent tracks and layers on the other hand allow the approximation that the motion controlled by the rotation of the optical unit  101  around its axis is essentially orthogonal to the track of the data sequence.  
         [0058]     To track, the system is provided with a tracking servo system shown generally as  125 , which feeds a correction signal to the magnetic coil  242  for moving the lens  240  under control of the tracking error signal to nominally position the beam spots at the center of the track so that the tracking error signal is zero. Coarse motion of spot is achieved by motion of the optical unit as a whole. Fine motion is achieved by the motion of the lens using the magnetic coil  242 .  
         [0059]     Although the tracking system  125  shown in  FIG. 2  serves to track the data sequence recorded on the specific disk  102  as described above, it is to be noted that the invention encompasses a novel tracking system, which is well-suited for use in the apparatus  100  described above with reference to  FIG. 2  although it is also suitable for use in other optical data retrieval systems. Likewise, it is to be noted that other tracking systems may be employed in the apparatus  100 .  
         [0060]     The improved tracking system according to the invention is described below with particular reference to  FIGS. 4   a ,  4   b  and  5  of the drawings. However, by way of general introduction there will first be described functionally with reference to  FIG. 3 a  read/write system  300  for a 3-D optical storage medium  102  having a tracking system. To the extent that the read/write system  300  includes components that are common to the apparatus  100  shown in  FIG. 2 , identical reference numerals will be employed.  
         [0061]     The read/write system  300  comprises a rotary shaft  302  driven by an appropriate driving motor  205  for rotating the optical storage medium  102  set thereon, and an optical unit  101  for reading information from one of the tracks in the optical storage medium  102 .  
         [0062]     The optical unit  101  comprises semiconductor lasers  104  and  106  for radiating a pair of intersecting light beams having a volume of intersection that forms a “spot”. Also included within the optical unit  101  is an optical system  245  for creating a focused spot whose location is controlled by an actuator  306 , which in the particular embodiment shown in  FIG. 2  is constituted by the magnetic coil  242 . The optical unit  101  is driven by a motor  307  so as to produce the required coarse and fine motion of spot described above.  
         [0063]     The system further comprises a laser driving circuit  308  for energizing the semiconductor lasers  104 , and  106  to emit the respective laser beams.  
         [0064]     In order to retrieve information from a desired track on the optical storage medium  102 , the optical focus  222  must be kept on the desired track. To this end, the system is provided with a tracking servo system shown generally as  125 , which feeds a correction signal to the lens actuator  306  for moving the optical system  245  under control of the tracking error signal to nominally position the beam spots at the center of the track so that the tracking error signal is zero.  
         [0065]      FIGS. 4   a  and  4   b  shows pictorially the effect of sinusoidally modulating the position of the optical focus  222  in the system of  FIGS. 2 and 3 . So far as the reading spot is concerned it trades data written into a continuous linear data track  321  while being subjected to spatial modulation that shifts its position continually from one side of the track to the other. Although the data is stored in tracks, the invention relies on the principle that even if the optical focus  222  is slightly off-center, data signal will still be read, albeit at reduced intensity. Thus, the further off-center the optical focus  222  is moved, the lower will be the magnitude of the data signal.  
         [0066]     Thus, with reference to  FIG. 4   a , consider the case where the tracking is perfect and the reading spot is symmetrical with respect to the data track  321 , its position being shown by the sinusoidal curve  322 . In this case, the average signal will be equal on both sides of the data track  321 . However, in the case of imperfect tracking as shown in  FIG. 4   b , the reading spot is asymmetrical with respect to the data track  321 , its actual line of symmetry being depicted by a dotted center-line  323 , shown to right of the data track  321 . The signal is inversely proportional, in perhaps a non-linear fashion, to the distance of the sinusoidal curve  322  from the data track  321 . Thus, in  FIG. 4   b  where the sinusoidal curve  322  is offset to the right of the data track, this results in a lower signal from samples made when the sinusoidal curve  322  is to the right of the center line  323 , thus indicating the spot is offset to the right of the data track  321  and must therefore be shifted to the left in order to correct the offset.  
         [0067]     The tracking operates on the principle that by continually reading the data and, at the same, continually modulating the position of the reading head, the resulting moving average signal intensity that is read may be used to indicate to which side, both axially and radially, the reading head is located. This having been determined, the reading head may then be moved in an opposite direction until it is found to be disposed symmetrically relative to the track in both axial and radial directions.  
         [0068]      FIGS. 5 and 6  are block diagrams showing functionally details of a tracking system  125  that is described in polar coordinates (r, θ, z) defining a position of the beams&#39; intersection in the optical recording medium. The tracking system behaves substantially identically for both radial and axial tracking. A modulator  332  spatially modulates the location of the optical focus  222  by a (r, z) modulation signal and feeds the resulting modulated data signal to the optical storage medium  102 . The modulation signal itself is fed together with the measured data signal to an error determination unit  333 , whose output is an error signal that is fed back to the optical unit  101  to correct the axial and radial offsets thereof. The modulator  332  in conjunction with the error determination unit  333  constitutes a tracking error correction unit. A rotation unit  334  provides a continuous change of θ.  
         [0069]      FIG. 6  shows in simplified form the principal functionality of the error determination unit  333  comprising a first 2-input multiplier  340  to whose first input the (r, z) modulation signal is fed and to whose second input is fed the data signal read by optical unit  101  at the position (r, θ, z) in the optical storage medium  102 . An output of the multiplier  340  is fed to a window integrator  341  which integrates the product of the data signal with the modulation signal so as to generate at its output a composite (r, z) error signal in the radial and axial directions.  
         [0070]     As described above by way of example with reference to  FIGS. 4   a  and  4   b  of the drawings, the modulation signal can be a sinusoidal function of the form m=(sin(ωt), cos(ωt)) t . The output of the window integrator may then be represented by:  
         err   ⁡     (   t   )       =       ∫     t   -   T     t     ⁢     m   ⁢           ⁢     I   ⁡     (   t   )       ⁢     ⅆ   t             
 
         [0071]     The intensity I(t) is inversely proportional in a non linear fashion to the distance from the center of the track. Thus, when the head is above the center of the track the modulation intensity is strongest, and it decays to zero when the head moves far from it. “T” represents the length of the time window of the integrator during which the modulated intensity is averaged. “T” should not be so large that it impacts negatively on the reaction time and creates distortions; but neither should it be too low since it is very difficult to construct a mechanical scanning system.  
         [0072]     However, the modulation signal can be any suitable cyclic function which serves to move the optical focus  222  on either side, in both axial and radial directions, of the reading spot. Thus, it can be a square wave function or any other suitable cycle function. It is assumed that the frequency of the modulation signal is much lower than the frequency at which data is read. The window integrator  341  thus operates as a low pass filter.  
         [0073]     The tracking operates on the principle that by continually reading the data and, at the same, continually modulating the position of the reading head, the resulting moving average signal intensity that is read may be used to indicate to which side, both axially and radially, the reading head is located. This having been determined, the optical focus may then be moved in an opposite direction until it is found to be disposed symmetrically relative to the track in both axial and radial directions.  
         [0074]     Whilst the tracking method has been described with particular regard to a tracking system for use with a 3-D optical storage retrieval system wherein data is stored at voxels written in the bulk of the material, it will be understood that the principles of the invention are equally applicable to other kinds of optical storage media where data is stored as a quasi-linear data sequence.  
         [0075]     The embodiment illustrated in  FIG. 2  is only one of many embodiments available to the artisan when designing an apparatus according to the present invention. Some non-limiting examples for variations from this embodiment include: the lens and the mirror may be replaced by any other optical means which is known to bring to the same result, the optical fibers may be omitted or replaced by any other wave-guide, the combination of a disk mount and an arm axis may be replaced by any other means for controlling the location of the common focus in the plains parallel to the disk&#39;s surface  102 ′, etc.  
         [0076]     It should also be noted that although the preferred embodiment is directed to retrieval of data, the tracking system according to the invention is equally suitable for use when writing data to the optical medium.