Abstract:
A system for phase detection in page-based optical data storage involves a number of features that may be used individually or in combination to provide optimizations and to facilitate the heterodyne detection of the modulated readout signal beam. In one aspect, alignment sensors provide signals indicative of angular misalignment between the heterodyne reference beam and the signal beam, and a feedback control system aligns the reference beam with respect to the signal beam. In another aspect, a differential detection approach is utilized. In yet another aspect, a scanning approach is used wherein the reference beam is scanned over approximately a wavelength of phase difference to cause interference fringes to shift over the photodetector pixel array. Each pixel detects the signal at the phase value that cancels out localized variations of the wavefront, thus increasing the tolerance of the system to phase errors and to angular misalignments.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to page-based optical data storage. The invention further relates to the readout of phase-modulated signals in page-based optical data storage applications. 
   2. Background Art 
   In page-based optical data storage, a page of data is read by imaging a two-dimensional data page onto a two-dimensional detector. The detector detects the image, and the image is subsequently processed to decode the data. 
   Page-based optical data storage technologies and, in particular, holographic data storage (HDS), have benefited greatly from the recent development of CMOS active pixel sensors (also known as APS detectors), such as the ones available today in high resolution digital cameras. These sensors, compared to traditional charge-coupled device (CCD) detectors, present the advantages of lower unit cost (due to the simpler design and manufacturing) and design flexibility provided by the CMOS process, allowing more functionality to be included in the detector chip. However, these sensors have invariably been optimized for consumer imaging and machine vision applications, not for data storage. 
   Most page-based optical data storage approaches currently pursued use amplitude modulation of the optical wavefront (i.e., on-off pixels). Significant improvements in storage density and BER can be achieved by using phase modulation (e.g., 0 to π phase modulation of the pixels followed by coherent PSK detection). Some of the advantages are: higher dynamic range (e.g., in holographic data storage, signal power falls as 1/M instead of 1/M 2 , where M is the number of the overlapped holograms, typically about 10 2 ), very low intensity of the DC spot in the focal plane (translating into more efficient use of the available dynamic range and lower chance of damage to the material), heterodyne gain at detection, 3-dB gain in SNR and common-mode noise rejection by using differential detection, lower BER for the same SNR by using coherent PSK detection (compared to incoherent ASK), and lower crosstalk between neighboring pixels and between collocated pages, due to noise cancellation. 
   These advantages are provided at the cost of higher hardware complexity and higher sensitivity to alignment. 
   Background information may be found in Paulo E. X. Silveira,  Optoelectronic Signal Processing Using Finite Impulse Response Neural Networks , Ph.D. dissertation, University of Colorado, 2001; Geoffrey W. Burr,  Holographic Storage , Encyclopedia of Optical Engineering, Marcel Dekker, Inc., 2003; Nicolas Blanc,  CCD versus CMOS—has CCD imaging come to an end ?, Photogrammetric Week ‘01’, D. Fritsch and R. Spiller Eds. Wichmann Verlag, Heidelberg, 2001; Jean Schleipen et al.,  Optical Heads , Encyclopedia Of Optical Engineering, Marcel Dekker, Inc., 2003; and Simon Haykin,  Digital Communications , John Wiley &amp; Sons, 1988. 
   For the foregoing reasons, there is a need for an improved sensor that is optimized for phase detection in page-based optical data storage and that addresses the challenges posed by heterodyne phase detection. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an improved system for page-based optical data storage including a sensor optimized for phase detection in page-based optical data storage. 
   The invention comprehends a system for page-based optical data storage. A photodetector sensor array is positioned to receive a data page when the data page is optically read from the storage medium. The sensor includes a pixel array, and preferably is in the form of a CMOS APS array. A processing arrangement processes the data from the sensor array to determine the most likely correct value of the bits read. 
   According to the invention, the photodetector sensor array is improved in that it is optimized for phase detection in page-based optical data storage applications. The invention involves a number of features that used singly, or in various combinations, provide an improved sensor. 
   In carrying out the invention, a system for phase detection in page-based optical data storage is provided. The system comprises a storage medium, a photodetector sensor array, and a coherent phase-shift keyed detection system. The photodetector sensor array is positioned to receive a data page when the data page is optically read from the storage medium (e.g., by optical diffraction, reflection or transmission through the storage medium). Phase detection is typically performed by aligning a signal beam with a reference beam and coherently mixing the two, resulting in a mixed signal proportional in amplitude to the phase of the signal beam, in a process commonly known as heterodyne detection. In the invention, the photodetector sensor array includes a pixel array responsible for detecting the phase of each corresponding pixel in the data page. The coherent phase-shift keyed detection system is coupled to the photodetector sensor array and is responsible for processing the data and determining the most likely correct value of the bits read. 
   The system further comprises a feedback control system for aligning the reference beam with respect to the signal beam. A plurality of alignment sensors are positioned about the pixel array. Differences in sensor signals are indicative of angular misalignment between the reference beam and the signal beam. The feedback control system processes the differences in sensor signals and aligns the heterodyne reference beam with respect to the signal beam. 
   The individual pixels in the photodetector sensor array may be configured to filter a DC bias resulting from the use of the coherent phase-shift detection system. In a preferred approach, the plurality of alignment sensors includes four alignment sensors with an alignment sensor in each quadrant of the photodetector sensor array. 
   Further, in carrying out the invention, a system for phase detection in page-based optical data storage is provided. The system comprises a storage medium and a beam-splitter. A signal beam is generated by reading out the storage medium. The beam-splitter aligns the signal beam with a heterodyne reference beam. The system further comprises a first photodetector sensor array and a second photodetector sensor array. Each photodetector sensor array receives one of the beams from the output ports of the beam-splitter. A differential detection system processes the outputs of the first and second photodetector sensor arrays and determines the most likely correct value of the bits read. The first and second photodetector sensor arrays may be configured to cancel a DC offset by balancing the inputs to the differential detection system. 
   Still further, in carrying out the invention, a method for phase detection in page-based optical data storage is provided. The method is used with a system including a storage medium, a photodetector sensor array positioned to detect a data page when the data page is optically read from the storage medium by aligning a signal beam with a reference beam, and a coherent phase shift-keyed detection system. The coherent phase-shift keyed detection system is coupled to the photodetector sensor array for phase detection to process the data to determine the most likely correct value of the bits read. The method comprises scanning over a wavelength of phase difference with the reference beam to cause interference fringes in the reconstructed object beam to shift over the pixel array. The method further comprises capturing multiple data points for each pixel during the scanning. In a preferred approach, for each pixel a most readable phase difference is determined and the user data at each pixel is determined as the data point at the most readable phase difference. 
   The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a page-based optical data storage system made in accordance with the invention; 
       FIG. 2  illustrates differential heterodyne phase detection; 
       FIG. 3  illustrates alignment detectors placed at the corners of the active pixel area to provide a differential feedback signal that grows with the relative angle between the reference beam and the signal beam; and 
       FIG. 4  illustrates a modulator ramping the phase of the reference arm while the photodetectors sample the ramp. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  depicts the readout operation of a generic page-based optical data storage system  100  (e.g., implemented holographically, diffractively, etc). Optical medium  110  is illuminated by light source  120 , providing a plane  130  of modulated signals representing the stored data. The modulated signals and the optical heterodyne reference signals  134  are made collinear by using beam steering optics (e.g., using the beam-splitter  136 ). Then, they are coherently interfered and detected by the photodetector array  140 , which is optimized for phase detection in page-based optical data storage. From the photodetector array, the data readout  150  is transferred out to the controlling firmware  160 . The controlling firmware also transmits control feedback signals  170  to the photodetector array, directing it on how to adjust its optical detection parameters. The photodetector array  140  also transmits feedback control signals  170  to the feedback control system  190 . The feedback control signals provide information about the misalignment (e.g., by conveying a voltage signal with amplitude proportional to the amount of misalignment, and a sign proportional to the direction of the misalignment). The feedback control system  190  uses the feedback control signals to adjust the relative alignment between the modulated signal beam  130  and the reference beam  134  (e.g., by realigning the beam-splitter  136 ). The details of the operation of the page-based optical data storage system are readily understood by one of ordinary skill in the art.  FIG. 1  illustrates page-based data readout in general. It is appreciated that the invention may be implemented in other ways differing from that shown in  FIG. 1 . 
   An optically phase modulated bit signal read out from a two-dimensional page at row i, column j, can be represented by 
             E   ij     =           η   ⁢          E   0            2     ⁢     exp   ⁡     [     ⅈ   ⁡     (         2   ⁢           ⁢   π   ⁢           ⁢   nct     λ     -         2   ⁢           ⁢   π   ⁢           ⁢   n     λ     ⁢         k   ^     ij     ·     z   ^         +     ϕ   ij       )       ]         +     c   .   c   .             
where |E 0 | is the amplitude of the optical beam used for readout, η is the amplitude modulation efficiency of the storage medium, λ is the wavelength of light, c is the speed of light in a vacuum, n is the index of refraction of the medium,
 {circumflex over (k)} ij ·{circumflex over (z)} 
is the cosine of the angle between the direction of propagation of the signal beam and the direction of the detector  30  with reference to  FIG. 2 , and φ ij  is the pixel phase modulated by the optical storage medium.
 
     FIG. 2  illustrates a differential heterodyne detector at  20 , where the signal beam  22  is combined with a heterodyne reference beam  24  in a beam-splitter  26 . The combined beam is then imaged towards one (simple heterodyne detection, not specifically illustrated) or two (differential heterodyne detection, illustrated in  FIG. 2 ) output sensor arrays  30  and  32 . The laws of conservation of energy dictate that the signals  34  and  36  at the output ports of the beam-splitter must be 180° out of phase, requiring the calculation of the signal difference  30  and  32 , but providing 3-dB gain and common-mode noise rejection. When the heterodyne reference wave is sufficiently coherent with the signal wave, the output voltage for pixel ij is given by 
             ν   ij   1     =     α   ⁢     {                  E   r          2     +            E   0          2         4   ⁢   N       +                E   r          ⁢          E   0              2   ⁢   N       ⁢     cos   ⁡     [       ϕ   ij     -         2   ⁢   π   ⁢           ⁢   n     λ     ⁢       (         k   ^     ij     -       k   ^     r       )     ·     z   ^           ]           }             
where α is a constant of proportionality, N is the number of pixels on the spatial light modulator (SLM), |E r | is the amplitude of the reference wave (with a spatially flat phase, assumed to represent the zero phase), and
 {circumflex over (k)} r    
is the direction of propagation of the reference beam (aligned by adjusting the beam-splitter). In the case of perfect alignment,
   {circumflex over (k)}   ij   −{circumflex over (k)}   r =0, 
the output signal voltage reduces to
 ν ij   1 =β 1   I   DC +β 2  cos φ ij    
where β 1  and β 2  are proportionality constants and I DC  is a constant DC intensity that should be filtered out, or which is cancelled out by the differential detection. In the case of differential detection, there is an intrinsic 3-dB gain due to the phase difference between the two arms of the heterodyne interferometer  20 , yielding
 Δν ij =2β 2  cos φ ij    
   Note that cos φ ij =±1, and this provides a bipolar signal representation instead of the unipolar signal provided by amplitude modulation. In order to keep all of the pixels in the photodetector aligned with the reference wave, the system must have a maximum angular misalignment δ given by
 
 √{square root over (N)}p  sin δ&lt;&lt;λ/ n  
 
where p is the pixel pitch and, for p=5 μm, the system must have δ&lt;&lt;20 arc sec. Note that this is about the same order of magnitude of the angular resolution required for the angular multiplexing of holograms used in holographic data storage, putting it within the range of capabilities of commercially available galvanometers used for angular control of optical components, but requiring active control of the alignment.
 
   With reference to  FIGS. 3-4 , the preferred embodiment of the invention comprehends a number of features in a photodetector array that optimize the sensor for holographic data storage using phase detection. 
   As best shown in  FIG. 3 , a sensor  50  (preferably a CMOS APS array) includes an active pixel area  52 . Sensor  50  includes symmetric and diametrically opposed detectors  54 ,  56 ,  58 ,  60 . The detectors feed differential amplifiers  62 ,  64 ,  66 ,  68 . In this way, the detectors provide a voltage difference proportional to the cosine of angular difference in the x-axis (Detectors  54  and  58  feeding Differential amplifier  64 , or Detectors  56  and  60  feeding Differential amplifier  68 , or a combination of detected signals) and y-axis (Detectors  54  and  56  feeding Differential amplifier  62 , or Detectors  58  and  60  feeding Differential amplifier  66 , or a combination of detected signals). More specifically, when the angular misalignment between the heterodyne reference and signal beams is small (less than a wavelength), the difference signals from the differential amplifiers  62 ,  64 ,  66 ,  68  can be used by a feedback control system  190  to align the reference beam with respect to the signal beam. In a preferred approach, the detectors  54 ,  56 ,  58 ,  60  are larger and may be slower than the photosensors in the active pixel area  52 . Uniform illumination by the reference beam alone can be used to calibrate the gain of the alignment detectors, triggered by an external calibration trigger  70 . 
   In addition, each detection pixel in sensor  50  may be configured to perform on-chip filtering of the DC bias resulting from the use of heterodyne detection. The filtering can be done prior to analog-to-digital conversion, thus better using the available dynamic range of the analog-to-digital converter. The chip also decodes the detected data, preferably using coherent phase-shift keyed binary detection. 
   As best shown in  FIG. 4 , the preferred embodiment of the invention may utilize two detectors  30 ,  32  and a beam-splitter  26 . In this way, the invention comprehends a compact module suited for differential heterodyne detection with less degrees of freedom required for alignment. In a preferred approach, on-chip gain controlled by an external input signal is provided. This feature may be used to balance a differential heterodyne detection and to cancel out the DC offset, with the added benefit of removing common-mode noise (e.g., relative intensity noise of the laser source, fluctuations of the electrical power supply, etc). 
   With continuing reference to  FIG. 4 , the reference beam is indicated at  24  while the signal beam is indicated at  22 . Differential detection system  40  calculates the differences between the outputs of the first and second heterodyne detectors and processes the data to determine the most likely correct value of the bits read. 
   In a preferred embodiment, and as best illustrated in  FIG. 4 , in a calibration step a phase ramp  42  is used to scan over about a wavelength of phase difference in the reference beam while capturing multiple data points of a signal beam that is known a priori. The phase ramp can be produced, for example, by a Pockel&#39;s cell, or by a piezo-mounted mirror which alters the optical path length. This causes the interference fringes to shift over the surface of any detectors, assuring that each pixel captures at least one valid data point. Prior knowledge of the correct pixel values used in the calibration step is used to determine the correct phase difference at which each pixel value should be read. This phase difference is individually detected and recorded for each pixel location. Next, when an unknown signal beam is read, the phase is once again ramped and each photodetector pixel is read whenever the phase ramp reaches the same phase difference pre-recorded in the phase calibration step. This allows for the compensation of imperfections over the phase front of the diffracted wave, as well as for alignment compensation over a wider range of angles. 
   It is appreciated that embodiments of the invention provide a number of features for phase detection in page-based optical data storage. These various aspects of the invention may be used individually or in various combinations to achieve improved sensors optimized for phase detection in page-based optical data storage. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.