Working Of Holographic Memory - Online Article

Properties of Holograms

A hologram is a block or sheet of photosensitive material which records the diffraction of two light sources. To create a hologram, laser light is first split into two beams, a source beam and a reference beam. The source beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting diffraction of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the source beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material which can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors.

Holographic Memory

Holographic Memory

Diagram of Holograms

Working

A hologram is a block or sheet of photosensitive material which records the diffraction of two light sources. To create a hologram, laser light is first split into two beams, a signal beam and a reference beam. The signal beam is then manipulated and sent into the photosensitive material. Once inside this material, it intersects the reference beam and the resulting diffraction of laser light is recorded on the photosensitive material, resulting in a hologram. Once a hologram is recorded, it can be viewed with only the reference beam. The reference beam is projected into the hologram at the exact angle it was projected during recording. When this light hits the recorded diffraction pattern, the source beam is regenerated out of the refracted light. An exact copy of the signal beam is sent out of the hologram and can be read by optical sensors. For example, a hologram that can be obtained from a toy store illustrates this idea. Precise laser equipment is used at the factory to create the hologram. A recording material which can recreate recorded images out of natural light is used so the consumer does not need high-tech equipment to view the information stored in the hologram. Natural light becomes the reference beam and human eyes become the optical sensors.

holographic-memory

In a holographic memory device, a laser beam is split in two, and the two resulting beams interact in a crystal medium to store a holographic recreation of a page of data

Spatial Light Modulator (SLM)

A spatial light modulator is used for creating binary information out of laser light. The SLM is a 2D plane, consisting of pixels which can be turned on and off to create binary 1s and 0s. An illustration of this is a window and a window shade. It is possible to pull the shade down over a window to block incoming sunlight. If sunlight is desired again, the shade can be raised. A spatial light modulator contains a two-dimensional array of windows which are only microns wide. These windows block some parts of the incoming laser light and let other parts go through. The resulting cross section of the laser beam is a two dimensional array of binary data, exactly the same as what was represented in the SLM. After the laser beam is manipulated, it is sent into the hologram to be recorded.

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A Spatial Light Modulator implemented with a LCD panel

The device first splits a blue argon laser beam into separate reference and object beams. The object beam that carries the information gets expanded so that it fully illuminates a spatial light modulator (SLM). An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels.

Recording Data

Digital data to be stored are "encoded" onto the signal beam via a spatial light modulator. The data or strings of bits are first arranged into pages or large arrays. The 0's and 1's of the data pages are translated into pixels of the spatial light modulator that either block or transmit light. The light of the signal beam traverses through the modulator and is therefore encoded with the "checkerboard" pattern of the data page. This encoded beam then interferes with the reference beam through the volume of a photosensitive recording medium, storing the digital data pages.

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Recording data on Holographic Memory

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Patterns on the data page

The interference pattern induces modulations in the refractive index of the recording material yielding diffractive volume gratings. The reference beam is used during readout to diffract off of the recorded gratings, reconstructing the stored array of bits.

Reading Data

An advantage of a holographic memory system is that an entire page of data can be retrieved quickly and at one time. In order to retrieve and reconstruct the holographic page of data stored in the crystal, the reference beam is shined into the crystal at exactly the same angle at which it entered to store that page of data. Each page of data is stored in a different area of the crystal, based on the angle at which the reference beam strikes it. During reconstruction, the beam will be diffracted by the crystal to allow the recreation of the original page that was stored. This reconstructed page is then projected onto the charge-coupled device (CCD) camera, which interprets and forwards the digital information to a computer.

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Reading data from Holographic Memory

The key component of any holographic data storage system is the angle at which the second reference beam is fired at the crystal to retrieve a page of data. It must match the original reference beam angle exactly. A difference of just a thousandth of a millimeter will result in failure to retrieve that page of data.

By varying the reference beam, for example by changing its angle of incidence or wavelength, many different data pages can be recorded in the same volume of material and read out by applying a reference beam identical to that used during writing. This process of multiplexing data yields the enormous storage capacity of holography.

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