Read/Write Holographic Memory versus Silicon Storage

Invited Paper
Read/Write Holographic Memory versus Silicon Storage
Wenhai Liu, Ernest Chuang and Demetri Psaltis*
Department of Electrical Engineering
California Institute of technology
Pasadena, CA 91125
This paper compares the read/write holographic memory with silicon storage on issues of cost, density, size
and speed. With a photorefractive crystal on top of a silicon interface, the holographic memory is of cost
efficiency, volume compactness and fast data accessing. Key challenges to implement the competitive
holographic memory are discussed.
Keywords: Holographic memory, silicon, data storage, SLM, detector
Over the past few decades, the personal computers and the internet have transformed the whole world as
people are able to store, retrieve and process more and more information easier and faster. All these
benefits inspire more scientific researches on faster, smaller, cheaper and more powerftul computer and
memory system. Semiconductor electronics have been and will continue to be the driving force in this
effort. According to the National Technology Roadmap For Semiconductors 7' the
semiconductor industry has maintained a 25-30% per-year cost reduction per function and the average
10.5%/year reduction rate in feature size throughout its history. It is projected to keep this historic trend for
another decade until it reaches physical limits as feature sizes approach lOOnm.
With a photorefractive crystal sitting on top of silicon, a read/write holographic memory is a potential
competitive technique to store more data with faster data access, smaller silicon area, lower cost and
smaller volume, compared with the traditional silicon Dynamic Random Access Memory (DRAM). Instead
of storing data on the silicon area, pages of data are stored as holograms inside the same crystal volume.
The silicon devices are only interfaces to read/write holograms to the memory.
In section 2, we will discuss the properties of the holographic memory. Section 3 will compare the
holographic memory with the silicon memory on issues of cost, density, size and speed. The challenges to
the device development, material research and algorithm of data organization for implementing a
competitive holographic memory system are addressed.
In a holographic memory, a page of data is recorded as phase gratings by interference between the spatial
modulated signal beam and a coherent reference beam inside a photorefractive crystal such as LiNbO3,
BaTiO3, etc. When the identical reference beam is brought back, the signal wavefront is reconstructed by
the diffraction and recovers the data. A large number of different holograms can be recorded in the same
volume of a photorefractive material by angle, spatial, fractal, wavelength, phase coding, peristrophic or
shift multiplexing. This leads to a very high data storage density in a crystal. If we assume each page of
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data has N by N binary pixels and M pages recorded in a crystal of volume V. then we will have storage
density MN V bits per volume. Typically Nl03. M=lO and V1 cm, yielding a density of l0 hitscm.
Two compact holographic memory designs with different detectors are shown in Figure I. l)iflrent pages
of data are angle multiplexed by a laser diode (LD) array. A diffirent one LD is chosen to record and
reconstruct one corresponding data page. The switching speed from one page to another can he as thst as
10 microseconds. After being collimated, the beam is separated into two branches. The signal branch goes
through Spatial Light Modulator (SLM) or Dynamic Holographic Refresher (DHR) hehre entering the
crystal. Instead of the identical reference beam, the phase conjugate of the reference beam is used for the
reconstruction of the signal beam. The volume grating diffraction reconstructs the phase conjugate signal
beam, which travels backward and self-focuses back to the original location of the SLM.
The phase conjugate reference beam is achieved by reflection of plane wave reference beams. For each 1.1)
cell, there is another cell symmetric to the optical axis of the collimating lens, which constructs the
conjugate beam in the crystal by reflection. Compared with using a real phase-conjugate mirror, using a
flat mirror is easy. compact and efficient. Simulation indicates that as long as the reference wavefront is a
plane wave within one-tenth of a wavelength, we can get up to 90% percent diffraction etficienc using
conjugate readout compared to the conventional architecture.
To detect the reconstructed signal. we can deflect the signal to a detector array with a heamsplitter as in
Figure 1 (b). The detector cell has the same physical size as the SLM pixel and is aligned pixel to pixel
with the SLM image.
Another method is to design a photo sensor cell next to each SLM pixel on the same chip. which leads to
the idea of the DHR chip. With the phase conjugate reconstruction, the image of each pixel is self-aligned
to its photo sensor, as shown in Figure I (a). This makes the system easier to operate and more reliable, at
the expense of data page density because of larger area fi.r each cell to contain both the dellector and the
Crs Section View of OFiC
- (,wr pIH,
Figure 2. The cross section of Opto-Flcctronic
IC. a DHR cell including a liquid crystal
controlled reflector and a photo sensor.
Figure 1. Architectures of phase conjugate
holographic memory with (a) DFIR chip; (b)
separated SLM and detector array. Crystal.:
photorefractive crystal; BS: Beam Splitter; M:
Mirror: L: Lens.
Figure 3. The conjugate reconstruction of 25
holograms h the Dl IR chip. I ) holograni t I after
I cycle recording: 2).3).4) hologram : I . 3,t25
after 100 cycle of refreshing.
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Figure 2 shows the cross section of one cell in a DHR chip. Each cell is of I 32x2 II
containing a
liquid crystal controlled reflector and a photo sensor. DEIR chips are fabricated with 24 by 20 cells on a
medium size chip by 2 m process. Figure 3 shows the experimental results of recording. reconstructing
and refreshing the holograms in a phase conjugate system with the DEIR chip.
Figure 4 shows a model of the holographic memory module with a DHR chip. where LI) array is not
included. In this module, one lx lxi cm' LiNhO3 crystal is used as storage medium on top of a lxi cm
silicon interface. With aggressive projection of one microns dimension for each SLM and Detector pixel.
this system can store 50 Gbits on 500 pages. Each page contains 10.000 by 10.000 binary pixels.
We assume that 100 photons are collected for each pixel to have a reasonable SNR detection. To achieve
the accessing time 25 M for each page. it requires a reconstructed power of 0.16 niW. For a material with
M/l0. the readout reference beam intensity must be at least 0.4W. This will give the data accessing
bandwidth as 4 Terabit/sec for each module. At present. a readout time of 25() tsec is lasiblc given the
power available from LDs.
Silicon(lxl cm2)
Liquid Crystal
Beamsplitters and lens
LD array (500)
$25 100
Table 1. The estimated cost of each component in
a holographic memory module.
Figure 4. A practical model of a phase conjugate
holographic memory module. It includes a I)HR
chip. one LiNhO crystal. two heamsplitters and
two mirrors.
To build a holographic memory competitive with silicon storage. it is essential to be more cost-efficient,
faster data accessing and smaller in volume. We will discuss these issues respectively and address the
advantages and drawbacks of the holographic module.
Cost model
For the holographic module, the cost includes mainly three parts: silicon interface Cs,, optical elements (()I
LD array CLD. where the LD cost is the most uncertain element. The cost far the optical elements is
well known. To compare with the cost of silicon storage DRAM. which is proportional to the silicon area.
we assume the same cost for the same silicon area in both holographic memory and DRAM. The cost ratio
per megabyte CR of holographic memory to the silicon storage will he:
CR =
C +C'+C
where the R is the pixel area ratio of' the SLM and detector to the silicon area of each hit on DRAM. NI is
the number of holograms multiplexed in the crystal on top of the silicon. With the lixed cost of silicon area
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Cs,. optical elements C0. and LD array C11). the key to have a small cost ratio CR is to have small R and
large M. which means a high storage density in holographic memory comparing with the DRAM.
The number of holograms to he recorded and readout with reasonable bit error rate. is limited by the
dynamic range and sensitivity, or the M# of the material. With M holograms recorded with exponential
schedule to keep each hologram the same intensity, the diffraction efficiency of' each hologram would he:
Recording and reading 10.000 holograms at one location of a LiNbO0 crystal was demonstrated with a
similar system.4 However limited by the material M/# and the LD array number and power. it is practical to
keep M below 1000.
For current commercial SLM and detector array, the pixel area is typically 4x4tm2. And the current
commercial DRAM is I (im/bit.' which leads R16. With typical M=l000. we have RIM=l.6. which
leads to a small and promising CR. However if the DRAM keeps the history trend as the NTR97
projected. the DRAM cell will h ()fl3tm.bit in 2006. To keep the R around 25. the pixel size of the
holographic data pages has to he Ix I tm or even smaller, which is achievahle for the holographic memory
Figure 5 shows the experimental demonstration of the recording and reconstruction of lx I pm' random
pixel mask as SLM. The phase conjugate reconstruction magnitied by a x80 microscope is shown in Figure
5 a). The intensity histogram in FigureS b) is sampled within a 30x30 super-pixel region. which gives Bit
Error Rate (BER) at 7xl0. This finite BER indicates the requirement for error correction coding for the
holographic memory.
, 30
"U 1ih
Intensity (Arh. Unit)
Figure 5. a) A phase conjugate reconstruction of the random lxi pm pixels. b) The intensity histogram fOr the
reconstruction and the Gaussian tittin. SNR4.. and BER='7x10.
Comparing the Cost per megabyte for the DRAM projection of 42 cents. Mbyte in 2006. we have the cost
estimation for the holographic module in table I. where we assume the same cost per area for silicon usage.
With the R25 for lx I
pixel size and M=500. the cost for holographic memory is around 4
centsMbyte. one order of magnitude lower than the DRAM in 2006.
However, if the DRAM feature size keeps decreasing beyond 0.O4pni'/hit with the historic trend, the
holographic memory would not be able to fOllow the pixel size decreases. The pixel size of lx I (tifl is
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already approaching the physical limit of the wavelength ofthe light. Therefore with the increasing ratio R,
holographic memory will lose its edge comparing with the silicon storage.
A key challenge to the small pixel size is to develop high resolution SLM and detector array to achieve the
pixel size as small as lxi tm2.
2. Volume Density
The volume density comparison is similar to the cost model.5'6 For the previous holographic memory
module, the silicon surface density will be up to 440 bits/tm2 due to the multiplexing M=500 and R25,
which is M/R times higher than the projected DRAM density 22 bits/pm2 in 2006. For matching the
capacity of a holographic module with certain silicon area, as much as M/R times silicon area are needed
for conventional silicon storage. These silicon area can be either fabricated on one silicon chip, or on
several chips, or combined on several layer by flip chip interconnect. With a factor M/R >20, the
holographic memory is expected to have a more compact volume than the silicon storage system.
3. Read/Write Speed
A holographic memory has a large writing and reading speed due to the intrinsic parallelism during
recording and reconstructing one full page of data each time. The data transfer rate is N2/r, where t is either
the reading time tR or the recording time 'rw for one page of data. For the previous holographic module,
N=104, and tR25tS, twlOOtS, it has reading rate at 4x10'2 bits/sec and writing rate at lO'2bits/sec.
Comparing with the projected l6Gbit-DRAM on a 790 mm2 silicon chip in 2006, which will have 1GHz
clock and 2000 pins, DRAM has a maximum read/write rate at 2x1012 bits/sec. The holographic memory
has faster accessing rate and compatible writing rate, although the latency for each page is relatively slow.
To increase the data transport speed, it is essential to increase the number of pixels in each page and
decrease the reading/writing time for each page. Both are limited by the power output from the LD array
and the M/# of the material. With higher power output and/or higher M/#, it can support a larger data page
and decreases the reading and recording time rR, rw. For the previous holographic module with M/1O and
the reading rate at 25 is for 108-pixel-nage, it requires the LD array output power as 0.4 W for each, which
is achievable for the LD array.
Another drawback for the holographic memory is the disparity between the recording speed and the reading
speed. The data accessing time depends on the diffraction efficiency of each hologram, or the M/# of the
material. Current photorefractive materials give M/# at the order of 1 . To achieve fast accesses to 500
pages, it is crucial to find materials of M/# around 1 0 or higher. For the recording process, the time to
record one hologram depends on the sensitivity of the material. Normally the recording speed is slower
than the access speed because of low sensitivity.
This raises another challenge to develop the advanced photorefractive material with high MJ# and
sensitivity. Current research on the doubly doped material for holographic memory provides a potential
solution.7 This work may provide a material with high sensitivity, large M/# and nonvolatility during
reading process.
4. Random Access
A holographic memory can randomly access any page of data recorded. However it is difficult to write a
new page of data onto an old page of data without changing other pages. The old data page has to be erased
before recording a new page on it. Experiments demonstrated that one page can be erased independently
and a new page is written at the same location without loss of other page of data.8 However, it is too
complicated to implement it into a practical compact holographic memory.
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In addition, the holographic grating recorded in the photorefractive material continues to decay during
reading and writing of other gratings at the same location. The data needs to be refreshed during the usage
to keep it above acceptable threshold intensity. This can be done by readout the page of data and record it
back to the original location to enforce the grating. Experiments are done to record 25 holograms and
refresh for 100 times, which demonstrated the ability to refresh with the DHR chip.9 Figure 3 shows the
samples of 25 data images stored and after refreshed 100 times. There is no bit error during these refreshing
A natural storage algorithm for the holographic memory is to keep recording new data into new pages
while refreshing old pages in a module. When most the pages of data in a module are obsolete, the whole
module is erased before reloading remaining data and new data into it. Considering the big capacity for
each module, this is inconvenient compared with silicon storage. In addition, the holographic memory
processes data in pages of size 100 Megabit, which is also considerable large as a basic data processing
unit. Therefore a practical memory system should combine several holographic modules with some silicon
storage as buffer. And special algorithms are necessary to organize and manipulate the data structure.
Compared with silicon storage, holographic memory has more cost efficiency than the traditional DRAM,
and comparable storage density and data transfer rate. It also has the shortcomings of low recording speed,
long page latency time, error correct coding requirement, random-page recording complication and large
data processing unit. A practical and competitive memory system should combine the holographic memory
of low cost and large storage capacity with the conventional flexible silicon storage as buffer. To
implement this competitive system, four key challenges have to be overcome: high resolution SLM and
detector array, high power high density LD array, advanced photorefractive materials and the data
organization algorithm.
Authors would like to thank Dr. J-J. P. Drolet and Dr. G. Barbastathis for their contribution on the
holographic memory system volume and cost models.
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