Abstract:
A system and method capable of providing high data rate data copying and recording in holographic data storage while maintaining the copied data&#39;s signal-to-noise ratio. This is achieved by using optically-based parallel signal processing. In one preferred embodiment, the present invention uses a plurality of optical resonating cavities to provide thresholding and gain functions via positive feedback.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to information storage using holographic media, and particularly to parallel recording and replication of holographically stored data. 
   2. Background of the Invention 
   Traditionally, high capacity data storage drives/media (e.g., magnetic tape, magnetic disk, optical disk and the like) has presented a scale of growth in data capacity that has outpaced the scale of growth in data transfer rates. Data transfer rate limitations primarily arise from the reading and writing of data in a substantially serial fashion. As a result, the time it takes to access all the data in a medium is exponentially increasing (e.g., in a StorageTek™ 9940 tape, with a capacity of 200 GB, and a data transfer rate of 30 MB/s, the resulting total access time is 1.85 hours). 
   This presents a significant problem for applications that require substantial copying of the contents of media, such as in the cases of archival duplication (safely storing an archive at an alternate location); system state snapshots (allowing the recovery of a computing system to a previous state); system mirroring (for quick disaster recovery); data distribution (allowing the distribution of a limited number of copies of the media); archive restoration (mass copying of an archive when the media approaches the end of its lifetime), among others. 
   One implementation of holographic data storage (HDS) provides a data handling advantage by storing data in the form of bit arrays termed “pages.” These data pages typically consist of thousands to millions of data bits which are written and/or read in a single step. Furthermore, HDS provides an advantage in storage density by multiplexing several data pages in the same volume of the medium. The optical multiplexing, fortuitously, also allows parallel access to multiple data pages stored in a given medium. Thus, the data can be massively parallel accessed either by illuminating a single location using all reference beams, or by illuminating the entire medium with a given reference beam at a time, the exact addressing method depending on the multiplexing scheme used during recording. 
   If data could be copied within this highly parallel optical domain, the data rate limitations of today&#39;s serial data handling storage devices would be overcome. However, such parallel copy processes would have limitations. 
   Similar to the process of photocopying paper documents, the copies (and copies of the copies) present a lower signal-to-noise ratio (SNR) than the original. Today&#39;s data storage systems provide electronic signal processing (typically by digital processes after an analog to digital conversion of the data has occurred) to maintain the integrity of the data by preserving its SNR. One alternative would be to apply this same type of electronic detection to every HDS bit before recording it optically. This method, however, suffers from the electronic transfer rate limitation described above. What is needed is a system and method to provide signal processing in the optical domain that maintains the HDS data rate performance advantage while also maintaining the SNR of the copied data. 
   SUMMARY OF THE INVENTION 
   The present invention uses optically-based signal processing to solve the problem of providing a data copy rate performance advantage through parallel HDS write processes while maintaining the copied data&#39;s SNR. 
   In a preferred embodiment, the present invention provides a system and method for achieving optically-based gain and thresholding during the recording and/or copying process of holographically stored data. In one preferred embodiment, the present invention uses a multiplicity of optical resonating cavities to provide thresholding and gain via positive feedback. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  shows a block diagram depicting a simple holographic storage system of the prior art. 
       FIG. 2  shows an exemplary optical system capable of performing parallel replication of data from a first holographic medium to a second holographic medium using angular multiplexing. 
       FIG. 3  shows an exemplary optical system of  FIG. 2  adapted for being capable of providing thresholding and gain during parallel replication of holographic media, consistent with a preferred embodiment of the present invention. 
       FIG. 4  shows a resonant cavity for providing thresholding and gain in the recording and replication of holographic media. 
       FIG. 5  shows a system wherein thresholding and gain are applied to a previously proposed holographic recording system. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a block diagram of a prior art holographic system  100  for storing and reconstructing information. To use volume holography for storage, the digital data to be stored in the holographic storage medium  102  is imprinted onto an object beam  104  for recording, then retrieved from the reconstructed object beam  106  during readout. The input device  108  for the system is called a spatial light modulator (SLM). The SLM  108  is, in one embodiment, a planar array containing a multiplicity of pixels, where each pixel is capable of independently modulating light that is transmitted through it or reflected by it (e.g., through phase modulation or amplitude modulation). The output device  110  is a similar array of detector pixels, such as a charge-coupled device (CCD) camera or CMOS APS (active pixel sensor) pixel array. There are also often lenses  112 ,  114  included in the system to image the SLM pixel array  108  onto the output pixel array  110 . Alternately, curved refractive or reflective surfaces may also be used to perform the imaging. In this example, the light from the SLM is the information beam  104 , and the reference beam  116  is shown arriving at the storage medium  102  at an angle. Both beams interfere at the holographic medium, capable of recording the resulting interference pattern. The Bragg selectivity of thick holographic media allows multiple holograms to be multiplexed in the same location. This can be done by a multitude of methods, usually accomplished by the variation of the angle or wavelength of the reference beam with respect to the information beam. 
   The exact method used for the parallel readout of holographically recorded data depends on the multiplexing method used to record the data in the holographic medium. Regardless of the means used to store the information, a preferred embodiment of the present invention includes the use of an optical resonant cavity to provide thresholding and gain via positive feedback during the recording and/or copying process. Thus, implementation of the present invention is independent of the multiplexing method used. However, the applicability of the present invention is easier to understand in the context of an example. 
     FIG. 2  shows an optical system  200  capable of performing parallel replication of data from a single holographic medium  218  to another holographic medium  222  using angular multiplexing. Lens  204  collimates a beam from laser  202  which is split into two beams by polarizing beam splitter (PBS)  208 . First waveplate (WP 1 )  206  is used to control the variable splitting ratio of the PBS  208  by controlling the polarization of the incident beam. For a laser producing linearly polarized light, for example, WP 1   206  could be a half waveplate and the splitting ratio would be controlled by varying the orientation angle of the fast axis of waveplate  206 . PBS  208  splits the beam onto an s-polarized reflected arm and a p-polarized straight propagating arm that functions as the reference beam in the holographic copying process. WP 2   210  is a half-waveplate oriented at 45 degrees converting the p-polarized beam onto a s-polarized beam, so that both arms of the interferometer are co-polarized. The reflected arm is directed by mirror  214  towards the original holographic medium HM 1   218 , reading the information recorded on it at a given angle θ. Mirror  214  is also translated in order to compensate for the lateral displacement of the beam at different readout angles, so that the readout beam is always overlapping with HM 1   218 . 
   Note that a large section of HM 1   218  is illuminated at any given time, meaning that a large number of sites (and a large number of data bits) are simultaneously accessed all the time. The diffracted beams are then imaged towards the replication media (HM 2 )  222  via a single lens  220 . Note that other imaging systems are possible, and the current description is only intended to serve as an example. The straight through arm from PBS  208  is reflected by mirror  224 , which is rotated in order to produce the same angle θ with respect to the diffracted beam at HM 2   222 , and which is also translated in order to keep the reference beam aligned at HM 2   222 . The information bearing beam and the reference beam interfere at HM 2   222 , producing a modulated interference pattern. The replication media records a volume grating between the two beams at angle θ by changing the index of refraction and/or the absorption of a photosensitive material proportionally to the intensity of the peaks and valleys of the modulated interference pattern produced by interference between the information and reference beams on the material of HM 2   222 . Beam blocks BB  242  and BB  244  are used to block the undiffracted beams going through the holograms. 
   The process is repeated over all multiplexed angles for a considerable increase in copying speed relative to electronic detection followed by holographic recording, since multiple sites recorded in the media HM 1   218  are accessed simultaneously at every angle without having to undergo electronic processing. The upper bound of the speedup provided by the example of  FIG. 2  compared to a page-by-page readout and recording is limited by the ratio between the resolution of the imaging system and the resolution of a single SLM. Assuming enough laser power is available so that the recording time is negligible compared to the time it takes to align the mirrors, the maximum throughput that can be achieved by a page-by-page copy would only be limited by the time it takes the SLM to receive and display new data. For more examples of parallel replication of data recorded holographically, see, for example, the following works which are hereby incorporated by reference: E. Chuang, H. Yamatsu and K. Saito, “Holographic ROM System for High-Speed Replication,”  Intern. Symp. on Optical Memory and Optical Data Storage , pp. 144–146, 2002; E. Chuang, US Patent Application 20030161246, “Holographic Disc Recording System,” Sony Corporation, August 2003. 
   Though the parallel replication scheme discussed above provides a considerable speedup in copying, it is still inferior to the page-by-page copy in one aspect: the noise added in the process deteriorates the BER, and the deterioration is cumulative with successive copies. In order to overcome this limitation, it is necessary to provide the system with thresholding in order to reconstruct the binary levels of the data.  FIG. 3  shows the same replication system as depicted in  FIG. 2 , with the addition of a partially reflecting mirror (PRM)  334 , a 100% reflective mirror (M 3 )  326 , and two waveplates  328 ,  330  in the reference arm. The two mirrors,  334 ,  326  form a resonant cavity around the holographic medium, similar to what has been shown in A. Sinha and G. Barbastathis, “Resonant Holography,”  Optical Letters , vol. 27, no. 6, pp. 385–387, 2002, which is hereby incorporated by reference. But in this case it is used for recording a hologram instead of reading it out. The thresholding operates as follows: the information is imaged onto the copy holographic media (HM 2 )  322  and produces a grating by interfering with the reference beam introduced into the resonant cavity. The grating records an initially weak hologram in the photosensitive media, usually a photopolymer. The weak grating is Bragg matched with the incident information beam, since this is one of the beams used to record it. Thus, when the information beam is diffracted by the grating, it provides us with a diffracted beam coherent with and propagating in the same direction as the reference beam, adding energy to the cavity and providing additional gain to the system. Also, as the media is bleached during exposure, it becomes more transmissive, reducing the loss in the cavity and increasing its Q-factor. This makes more energy available to the cavity, contributing to the holographic recording. This also provides the system with positive feedback, leading to a non-linear increase in the energy in the cavity and to the grating strength, in a process that is similar to what happens in optical bistability (see, for example, R. Boyd,  Nonlinear Optics , pp. 15–16, 1992, which is hereby incorporated by reference), except that, unlike a saturable absorber, the media becomes permanently bleached and the traditional histeresys curve is replaced by a one-way thresholding curve, as required for the reliable copying of binary data. 
   It is advantageous to monitor the energy level of the cavity by redirecting a fraction of its power towards a phototector. This can be done, for example, by using a low-reflectance beam splitter tilted with respect to the optical axis. Then, as the energy level of the cavity increases, more optical power can be redirected towards the readout of HM 1   318  by adjusting the ratio of energy split between the two outputs of the beam splitter  308 . This can be done by adjusting the orientation of WP 1   306 , and the adjustment should be done in a feedback control loop, having the effect of further speeding up the process of grating accumulation and, consequently, the transfer rate. 
   Two waveplates  328 ,  330  are needed when the media responds to both forward and backward propagating reference beams (i.e., transmission and reflection gratings at considerably different center spatial frequencies) in order to prevent both forward and backward propagating beams from recording gratings, thus consuming some of the available dynamic range of the material. The mirrors following the waveplates have the effect of inverting the handedness of the polarization (e.g., right hand circularly polarized light becomes left hand circularly polarized light, and vice versa) such that, once propagating again through the waveplates, light acquires a state of polarization orthogonal to that of the first pass. This has the effect of generating light that has orthogonal polarization states associated with the direction of propagation at HM 2   322 . By choosing the correct initial polarization states, orientation and retardation of the waveplates it is possible to certify that the reference beam will only be co-polarized with the information beam when it propagates in the desired direction. For example, waveplate  310  could be a quarter waveplate, rendering the reference beam initially right hand circularly polarized; waveplate  328  could be a quarter waveplate oriented at +45 degrees, providing us with a forward propagating beam that would be s-polarized, while waveplate  330  would be another quarter waveplate oriented at −45 degrees, providing us with a backward propagating beam that would be p-polarized, orthogonal with the information beam and, therefore, not producing a recordable interference pattern. 
   Note that one could also reflect back the transmitted information beam using a phase-conjugate mirror (PCM). This would have the effect of further enhancing the feedback providing thresholding even when the holographic material does not become increasingly transmissive during recording, but at the cost of increased complexity. Finally, note that the thresholding operation provided by the present invention could also be advantageously used in conventional holographic drives during recording, since this invention reduces the time it takes for the recorded hologram to reach a desired diffraction efficiency at a given laser power, thus improving the transfer rate of the drive. 
     FIG. 4  shows an alternative embodiment of a resonant cavity used for the recording and replication of holographic data with thresholding. It is shown as an alternative embodiment, without suggesting any limitations to the implementation of the present invention. In this example embodiment, information beam  402  originating from an SLM or from the readout of an original hologram (not shown), interferes with a reference beam  404  inside a resonant cavity  400 , thus producing the interference pattern necessary for recording a hologram in the holographic material  410 . In this embodiment, holographic material  410  is made from a photopolymer. Photopolymers present the property of becoming more transmissive (i.e., more transparent to light) once information is recorded in them. This is a similar property as that observed in saturable absorbers used in bistable optical devices, except that, in the case of the photopolymers, the material is permanently bleached. Note that even if the media did not become increasingly transparent during recording, positive feedback and thresholding could still be achieved by placing a saturable absorber close to an image plane of the media inside the cavity. 
   The cavity  400  is formed by partially reflecting mirror  406  and 100% reflecting mirror  408 . Other mirror setups are also consistent with the present invention, as long as a resonant cavity is formed that reflects light through the holographic medium  410 . In a preferred embodiment, the medium has the property of providing the feedback necessary for further bleaching the media and, as the media becomes increasingly transmissive, more positive feedback is provided, further increasing the strength of the reference signal and of the recorded information. Thus, gain is provided by the input reference beam, and thresholding is provided by the positive feedback. 
   In another embodiment, the transmitted information beam is also reflected back using a phase-conjugate mirror (PCM)  412 . This has the effect of further enhancing the feedback, providing the system with thresholding even when the holographic material does not become increasingly transmissive during recording, but at the cost of increased complexity. The PCM could be part of a second resonant cavity, providing the system with a second mechanism for positive feedback. 
   The implementation of the resonant cavity does not have to be limited to a classic Fabry-Perot resonator, since bistability can take place in any resonant cavity. Thus, in another embodiment the resonant cavity is composed or more than two mirrors forming a ring resonator or, in effect, in a resonant cavity of any kind. 
     FIG. 5  depicts another embodiment of the present invention, showing how thresholding and gain can be applied to a previously proposed holographic replication system. This is shown as an additional example of how to apply the present invention, without implying a limitation of its application, which could be similarly applied to other existing or future holographic recording and replication systems. 
   A collimated readout beam  502  is transmitted through and modulated by holographic media  1  (HM  1 )  504 , producing the modulated beam  1  (MB  1 )  506 . This beam  506  is first transmitted through a partially reflecting mirror (PRM  1 )  508  and then through a waveplate (WP  2 )  510  before reaching holographic media  2  (HM  2 )  512  as modulated beam  2  (MB  2 )  514 . The path between MB  1   506  and MB  2   514  should be short enough and the surfaces in between should be flat enough so that MB  2   514  closely resembles MB  1   506 , or else imaging and wavefront correction optics would have to be used to make MB  2   514  closely resemble MB  1   506 . At HM  2   512 , MB  2   514  interferes with the reference beam  522 , recording a hologram. The reference beam is initially formed from a collimated beam  516  which is transmitted through a hole in the center of waveplate  1  (WP  1 )  518  and highly reflecting mirror  1  (M  1 )  520 . Then, the collimated beam  516  is shaped by conic beam shaper  1  (CBS  1 )  524 , here depicted as a conic prism, but which could take any of the shapes described in “Holographic ROM System for High-Speed Replication,” referenced above. 
   The reference beam  522  not only records an initial grating by interfering with MB  2   514 , but it also reads out the weakly recorded grating, producing the modulated beam  3  (MB  3 )  526 , which coherently adds up with the transmitted part of MB  2   514 . MB  3   526  is transmitted through WP  1   518  and is reflected back by M  1   520 . M  1   520  and PRM  1   508  form the resonant cavity required for feedback. Note that as the grating becomes stronger as the media is bleached, MB  3   526  grows in intensity and the loss in the cavity is reduced, providing us with the positive feedback necessary for thresholding. The waveplates WP  1   518  and WP  2   510  are used in order to control the polarization of the modulated beams in the cavity and, that way, control the direction of the beams that record a grating at HM  2   512 . This is done by certifying that the reference beam  522  and MB  2   514  are co-polarized and, simultaneously, orthogonally polarized with respect to MB  5   528 . For example, the reference beam  522  could be s-polarized and the readout beam  502  could be right-hand circularly polarized and the waveplates could be quarter waveplates at the wavelength of choice. WP  2   510  would be oriented at +45 degrees in order to convert the right-hand circularly polarized light into s-polarized light. This way, MB  1   506  would become s-polarized after propagation through WP  2   510 , making MB  2   514  co-polarized with the reference beam. MB  3   526  (readout by the reference beam) would consequently also be s-polarized, until propagation through WP  1   518 , which would be a quarter wave retarder oriented at −45 degrees, and MB  4   530  would become left-hand circularly polarized. Reflection through M  1   520  followed by propagation through WP  1   518  would render MB  5   528  p-polarized and, thus, unable to record a grating neither with the reference beam nor with MB  2   514 . However, after propagation through WP  2   510 , followed by reflection by PRM  1   508  and propagation through WP  2   510 , it would once again be s-polarized, coherently adding with MB  2   514 . 
   Note that the total path length has to be short enough and the optical surfaces flat enough to guarantee that the resonant beams are close replicas of MB  1   506 , or else imaging and wavefront correcting optics have to be used. Also, to assure coherent addition of resonant fields in the cavity, the total path length has to be a multiple of the wavelength, as usually required in Fabry-Perot resonators. Finally, HM  1   504  could be replaced with a multiplicity of holographic media, each one recorded at HM  2   512  at an individual reference beam angle by varying the angle provided by CBS  1   524 , as explained in “Holographic ROM System for High-Speed Replication,” referenced above. This could be done while taking advantage of the thresholding operation just described in the present invention, without loss of generality. 
   The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiments were chosen and described in order to best explain the principles of the invention, the practical applications, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.