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Non Volatile Rad-hard Holographic Memory

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					        Non Volatile Rad-hard
        Holographic Memory


               Presented to
Non Volatile Memory Technology Symposium 2001

               Presented by
            Dr. Tien-Hsin Chao

        Jet Propulsion Laboratory
    California Institute of Technology
    4800 Oak Grove Drive, Pasadena
             California, 91109


                                                1
JPL Holographic Memory Team Member



        •   Hanying Zhou
        •   George Reyes
        •   Danut Dragoi
        •   Jay Hanan




                                     2
    Objectives and Performance Specifications

•   Objectives :
     – Develop innovative memory technologies to enable large-
       capacity, high-speed, read/rewrite of image and digital data in a
       space environment
     – Demonstrate key capabilities:
          > Ultra High data/image storage capability (1TB)
          > High-speed random access data transfer (1GB/s)
          > Nonvolatility
          > Radiation-resistance
•   Performance Specifications
     – A compact holographic data storage with 10 GB non-volatile random
       access memory per cube
     – Up to 10 x 10 cubic memory can be stacked into an ordinary memory
       board size to achieve a storage capacity of 1TB
     – Read/rewrite, rad hard, high transfer rate

                                                                           3
                             System Schematic of an
                           Advanced CHDS Architecture
                                                           Unique Advantages
                  Photodetector                            •   Very compact
                      Array
    Laser                           Data                        – Cubic package with the size of a
    Diode                           SLM                            cigarette box
                                           Laser Diode     •   Massive data storage
 Beam
Steering                                         Beam           – store up to 104 pages of hologram
LC SLM                                          Steering           with 10 GB capacity
                                                LC SLM
                                                           •   High-speed
                                                                – current throughput 200 Mbytes/sec
                                                                  achieved with using a LC Beam
                  Photorefractive                                 Steering Device. Could be 10x faster
                     Crystal
                                                                  if FLC is used
                                     Read Module           •   Device/components maturity
            Write Module
                                                                –   Use two single diode lasers that are
                                                                    commercially available at low cost
                                                                –   Beam Steering Device is a emerging
                                                                    technology. JPL is actively engaged with
                                                                    BNS in developing the next generation
                                                                    high-speed version
                                                                                                               4
    Liquid Crystal Beam Steering Device
- Beam steering based on optical phase modulation




                                    Surface phase-modulation profile
                                        of a beam steering device

                                                  θ
                         θ




                    d                         d

 Optical phase profile (quantized multiple-level phase grating) repeats
 every 0-to-2π ramp w/ a period d which determines the deflection angleθ
                                                                           5
        Performance Characteristics of
          LC Beam Steering Device


• Number of pixels: 4096 Reflective
• VLSI backplane in ceramic PGA carrier
• Array size: 7.4 x 7.4 mm
• Pixel size: 1µm wide by 7.4mm high Pixel pitch:
  1.8 µm
• Response time:
   – 200 frames/sec with Nematic Twist Liquid Crystal
   – 2000 frames/sec with Ferroelectric electric Crystal (under
     development)



                                                                  6
         Liquid crystal phased array beam steering device
                   - Performance specifications



•   Diffraction efficiency:

                                         n: number of steps in the phase profile
                   sin (π n ) 
                                   2

               η =
                   π n                e.g., η ~ 81% for n =4, η ~95% for n =8
                              

•   Deflection angle:

           θ = sin −1 (λ d )             for the first order diffracted beam

    Number of resolvable angles:

           M = 2m / n + 1                m:pixel number in a subarray
                                         n: minimum phase steps used

    e.g., M = 129 for m =512, n = 8 with a 1x4096 beam steering device




                                                                                   7
   LabVIEW Based Controller for Beam Steering


• Use LabVIEW to calculate the theoretically correct
  beam steering profile (I.e. sawtooth wave).
• Optimize the diffractive efficiency and suppress the
  spurious high orders
• A hardware-in-the-loop routine has been developed to
  customize the driving voltage for each and every beam
  deflection angle
• A nonlinear waveform of the driving voltage profile is
  obtained for good performance



                                                           8
                                       Sawtooth Profile


•   The resulting profile (using an input value or N* of 57):




    *The input value is proportional to the number of gratings on the device.



                                                                                9
                           Tangent Profile

• For optimal results, parameters must be chosen such that the entire range of 0-256
  is used with 0 and 256 occurring with a consistent period.
• The selected parameters are unique for each angle.




                                                                                       10
             Resulting Diffraction Patterns


•   The spurious higher orders of
    diffraction are nearly eliminated
                                        Device Off
    by using the nonlinear voltage
    driving waveform to the liquid
                                        Increasing
    crystal BSSLM
                                            N




                                                     11
                  Liquid crystal phased array
                     beam steering device

•   Cascaded beam steering architecture:




        Input beam


                                                          M1xM2 1-D or 2-D
                                                          output beam directions


                     M1-angle 1-D          M2-angle 1-D
                     beam steerer          beam steerer


    A total resolvable angels of more than 10,000 can be easily achieved.



                                                                                   12
                    New 512 x 512 Grayscale
                     Spatial Light Modulator




    Photo of the new FLC SLM, much
                                         A high-quality grayscale image readout from the SLM
          smaller than a dime
•   New Grayscale SLM has been developed by the Boulder Nonlinear System
    Inc. under a NASA/JPL SBIR Phase II program (T.H. Chao is the JPL
    contract monitor
     –   512 pixel x 512 pixel, 7- µm pixel pitch, 3.6 mm x 3.6 mm aperture size
     –   High-speed at 1000 frames/sec
     –   Enable high-density, high transfer rate data storage
     –   Enable further system miniaturization
                                                                                       13
            Book-sized Holographic Memory Breadboard

 Input Spatial
Light Modulator




                                                                Beam Steering
                                                                   Device




                                             LiNbO3
                                      Photorefractive Crystal                   CCD Detector

              Photograph of a JPL compact holographic memory breadboard developed
                               under the sponsorship of NASA ESTO

                                                                                           Dr. Tien-Hsin Chao
                                                                                                                   14
                                                                                           Advanced Holographic Memory Task
Holographically Retrieved Grayscale Images
            - Asteroid Toutatis




Experimental results showing retrieved holographic images
                       of a Toutatis Asteroid
                                                            15
     Nonvolatile Two-photon (or Gated) Recording


Recording                                            Readout
–   First photon (e.g., uv, green) excites an        –   Readout by a single photon (e.g., red)
    electron to an intermediate state                    ==> insufficient energy to promote
–   Second photon (e.g., red, near-IR) further           electron to C.B., no photoexcitation
    promotes it to the conduction band               –   No erasure of data
–   The electron then migrates & gets                –   To erase: use both photons
    trapped to record the interference pattern

                                  e


                  Conduction band                                    Conduction band

                    hν2


   Intermediate                                       Intermediate                     hν2
   state                                              state
                    hν1
                                         Trap site                                           Acceptor site
                                                       Donor site
    Donor site




                          valence band                                    valence band




                                                                                                             16
    Nonvolatile Two-photon (or Gated) Recording


•   To achieve two-photon recording, materials must have:
     – Deep traps that are partially filled w/ electrons, and
     – Shallow (intermediate) traps to trap photogenerated electrons w/ sufficiently long
        lifetime

•   Materials for two-photon recording:
     – Pure (undoped) PR crystals, e.g. LiNbO3
           » Intrinsic defects (bipolarons induced by reduction) as intermediate states
           » large dynamic range, low sensitivity
           » Gating light: blue laser(476nm) , ~ 0.2 W/cm2
           » Writing light: near-IR (800nm) Ti:sappire, ~ 6 W/cm2

     –   Doped PR crystals, e.g., Fe:Mn:LiNbO3
          » Extrinsic dopants (Fe2+, Mn2+) provide intermediate states
          » High sensitivity, small dynamic range
          » Gating light: UV (365nm) mercury lamp, ~20 mW/cm2
          » Writing light: red HeNe laser, ~300 mW/cm2


                                                                                            17
      Nonvolatile Two-photon (or Gated) Recording

  •          Comparison of gate-on and gate-off readout*


               –          Readout with gate off:
                              –        no erasure
               –          Readout with gate on:
                              –        erasure



               * undoped LiNbO3,
                 blue gating light,
                  ~0.2W/cm2
                 IR writing light, ~6W/cm2

-----------------------------------------------------------------------------------------
*          L. Hesselink et al, Science v.282, p1089, 1998




                                                                                            18
       Nonvolatile Two-photon (or Gated) Recording

  •          Different readout/erasure methods in two-photon recording*


               –          Erasure w/ UV and red
               –          Erasure w/ UV only:
               –          Readout w/ red only
                          (partial erasure), then UV
                          only (erasure)


               * Fe:Mn: LiNbO3,
                 UV gating light,
                  ~20mW/cm2
                 red writing light,
                  ~0.3W/cm2
-----------------------------------------------------------------------------------------
*          D. Psaltis et al, Opt. Lett. 24, p652, 1999




                                                                                            19
               Radiation Sources in Space


•   Major types of radiation that are potentially hazardous to
    memory systems (electronic and holographic) include:
    – higher energy photon (X-ray, gammas),
    – neutrons,
    – and charged particles (electrons, protons, alpha particles,
      heavy ions)
•   The parameters, which determine the amount of damage
    introduced by a particle,
    – Rest mass (e.g. zero for protons),
    – Energy and the charge state (e.g. electronics are negative,
      protons and alphas are positive. Ions can even be multiply
      charged).



                                                                    20
           Doubly Doped LiNbO3 PR Crystal


•   Recently, more doping ions have been investigated for
    nonvolatile performance in a doubly-doped (2-color) LiNbO3
    crystal
    – Iron group (Ti, Cr, Mn, Cu)
    – Rare-earth element ions (Nd, Tb)have been investigated for
      nonvolatile performance in a LiNbO3 crystal. To date, it has
      been reported that doubly doped Cr:Cu:LiNbO3 as well as Fe:
      Tb:LiNbO3 are effective in nonvolatile holographic recordings.
•   A holographic memory testbed will be assembled for
    testing the nonvolatile data storage capability of candidate
    2-photon PR crystals
    – Fe:Mn: LiNbO3, Cr:Cu: LiNbO3, and Fe:Tb:LiNbO3, and Ce:Mn:
      LiNbO3)



                                                                       21
•   Recently, more doping ions have been investigated for nonvolatile
    performance in a doubly-doped (2-color) LiNbO3 crystal
     – Iron group (Ti, Cr, Mn, Cu)
     – Rare-earth element ions (Nd, Tb)have been investigated for
       nonvolatile performance in a LiNbO3 crystal. To date, it has
       been reported that doubly doped Cr:Cu:LiNbO3 as well as Fe:
       Tb:LiNbO3 are effective in nonvolatile holographic recordings.
•   A holographic memory testbed will be assembled for testing the
    nonvolatile data storage capability of candidate 2-photon PR
    crystals
     – Fe:Mn: LiNbO3, Cr:Cu: LiNbO3, and Fe:Tb:LiNbO3, and Ce:Mn: LiNbO3)




                                                                            22
               Radiation Sources in Space


•   Major types of radiation that are potentially hazardous to
    memory systems (electronic and holographic) include:
    – higher energy photon (X-ray, gammas),
    – neutrons,
    – and charged particles (electrons, protons, alpha particles,
      heavy ions)
•   The parameters, which determine the amount of damage
    introduced by a particle,
    – Rest mass (e.g. zero for protons),
    – Energy and the charge state (e.g. electronics are negative,
      protons and alphas are positive. Ions can even be multiply
      charged).



                                                                    23
       Radiation Damages to LiNbO3 PR Crystal
                  - Previous Studies

        Types of
          Damages
                                                           Spectral
                    Refractive index   Density changes     absorption
Source of
                    (∆no) changes
    Radiation


X-rays, g-rays      ∆(no) increases    None                Spectral
                    with dose                              absorption in blue
                                                           spectral region
                                                           observed


Neutrons/charged    Decrease with      Volume increases    None
particles           dose               at very high
                                       nuclear deposited
                                       energy




                                                                            24
                          RAD-HARD NONVOLATILE
                         HOLOGRAPHIC MEMORY TEST
Comprehensive radiation test of several doubly doped LiNbO3 PR crystals will be performed by
measuring the following key photorefractive (PR) parameters before and after the radiation tests.

      •    Photorefractive sensitivity S:
      For holographic application, it is defined as the energy density needed to give rise to a
      1% diffraction efficiency for a 1mm-thick storage medium:

                                         1 d (η1 2 )
                                     S=              ,
                                        I r L dt
      where η is the diffraction efficiency, Ir is the total recording beam intensity, L is the
      thickness of the crystal, and t is the recording time.

      •     Recording time-constant τ
      This determines the speed of a PR hologram formation, which is crucial to any real-
      time applications of the PR crystal. It can be calculated from the time evolution of the
      diffraction efficiency
                                η (t ) = η max (1 − e − t τ ),
      Where ηmax is the saturation value of the diffraction efficiency.
                                                                                                  25
    Material dynamic range (or maximum refractive index change) ∆nmax
This determines the maximum number of holograms that can be stored in a crystal. It is
related to the steady-state diffraction efficiency ηmax by
                                  − αL  2  πL∆nmax 
                     η max = exp        sin         
                                  cosθ       λ cosθ 
Where α is the optical absorption coefficient, λ is the recording wavelength, and 2θ is the
internal angle between the two incident beams.

•    Measure M/# as an alternate parameter of “dynamic range”:


                          M /# = η max τ r τ e ,
where τr and τe are recording and erasure time constants, respectively. Then, the number
of holograms recordable M is linked to the target (equalized) diffraction efficiency ηh by:
                                              2
                                      M /# 
                               ηh =        .
                                      M 
                                                                                              26
               Summary and Future Work


•   We have developed (with BNS Inc.) a new liquid crystal beam
    steering device for high-speed, random access beam steering
    for angularly multiplexed hologram recording
•   We have developed a compact CHDS breadboard and
    demonstrated grayscale holographic data storage/retrieval
•   We will continue to integrated a 2-D angularly multiplexing
    scheme to achieve > 10,000 page of holograms store per PR
    cube
•   We will investigated non-volatile hologram storage using 2-
    wavelength PR crystal
•   We will started radiation tests of holographic data stored in a
    LiNbO3 PR crystal


                                                                      27

				
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Description: Holographic Memory is to use holographic technology to achieve the principle of data records. This concept is Dennis Gabor in 1984 to improve the resolution of electron microscopy made of. His biggest advantage is the high density, only that, Holographic Memory also has great potential to improve, as long as the controller chip has a sufficiently strong data processing capabilities, Holographic Memory techniques can even provide up to 1000TB of capacity. In contrast, the current maximum capacity of the hard disk only 2TB, the Holographic Memory capacity is only equivalent to the "cube candy" offered by a small fragment of storage capacity.