Holographic Data Storage
Digital data storage using volume holograms offers high density and fast readout. Current research concentrates on system design, understanding and combating noise, and developing appropriate storage materials. Possible applications include fast data servers and high-capacity optical disks.
Has the rapid increase in available storage capacity fueled the
demand for storage or vice versa? It’s hard to say: Computer users’ hard disk
drives are perpetually overflowing with data, even though a year earlier the
same-size disk seemed more than sufficient. Research into and development of
data storage devices is a race to keep up with this continuing demand for more
capacity, more density, and faster readout rates. Improvements in conventional
memory technologies—magnetic hard disk drives, optical disks, and semiconductor
memories—have managed to keep pace with the demand for bigger, faster memories.
However, strong evidence indicates that these two-dimensional surface-storage technologies are approaching fundamental limits that may be difficult to
overcome, such as the wavelength of light and the thermal stability of stored
bits. An alternative approach for next-generation memories is to store data in
three dimensions.
Storage of Data
HOLOGRAMS
A hologram is a recording of the optical interference pattern that
forms at the intersection of two coherent optical beams. Typically, light from
a single laser is split into two paths, the signal path and the reference path.
Figure 1 shows this holographic recording arrangement. The beam that propagates
along the signal path carries information, whereas the reference is designed to
be simple to reproduce. A common reference beam is a plane wave: a light beam
that propagates without converging or diverging. The two paths are overlapped
on the holographic medium and the interference pattern between the two beams is
recorded. A key property of this interferometric recording is that when it is
illuminated by a readout beam, the signal beam is reproduced. In effect, some
of the light is diffracted from the readout beam to “reconstruct” a weak copy
of the signal beam. If the signal beam was created by reflecting light off a 3D
object, then the reconstructed hologram makes the 3D object appear behind the
holographic medium. When the hologram is recorded in a thin material, the
readout beam can differ from the reference beam used for recording and the
scene will still appear.
Storing and retrieving digital data
To use volume holography as a storage technology, the digital data
to be stored must be imprinted onto the object beam for recording, then
retrieved from the reconstructed object beam during readout. The input device
for the system is called a spatial light modulator, or SLM. The SLM is a planar
array of thousands of pixels; each pixel is an independent optical switch that
can be set to either block or pass light. The output device is a similar array
of detector pixels, such as a charge-coupled device (CCD) camera or CMOS pixel
array. The object beam often passes through a set of lenses that image the SLM
pixel array onto the output pixel array.
How its Work
Microsoft's case, the company is using a lithium niobate (LiNBO3)
crystal, with an added iron dopant. This brings an additional electron donor
level and a deep trap state to the electronic energy levels of the LiNBO3.
Conduction Band
To store data, two green-light beams are used to illuminate the
crystal - one a data-storage carrying beam, and the other a reference beam.
Where the two beams cross an interference pattern is created.
In the bright regions of the interference pattern, the crystal
absorbs light causing electrons to be excited from the iron donor level to the
conduction band where the electrons are free to move around the crystal
lattice. These electrons then preferentially decay into the deep iron trap
level where they remain trapped.
This results in a spatially varying distribution in electron
density and its associated electric field that stores the data as a hologram.
The data can then be read out by diffracting the reference beam off
of the stored hologram, and capturing the data image on a camera.
Exposing it to UV light excites electrons out of the deep iron trap
level, where they then preferentially decay into the iron donor level,
resetting the medium to be used all over again.
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