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It has been suggested that this article be merged into Holographic memory. (Discuss)

Theory behind Two-Colour Holographic Recording

Creating holograms is achieved by means of two coherent beams of light split from one laser source, one being the reference beam and the other the signal beam. When both these beams interfere with one another, a resulting interference pattern is formed which encompasses the pattern both in amplitude and phase information of the two beams. When an appropriate photorefractive material is placed at the point of interference, the interference patterns are recorded inside the material. When the reference beam illuminates the material in the absence of the signal beam, the hologram causes the light to be diffracted in the same direction of the initial signal beam and all the information of the original signal beam is reconstructed.

For two-colour holographic recording, the additional fact is that the beams would be defined as follows, the reference and signal beams shall be fixed to a particular wavelength (green, red or IR) and the sensitizing/gating beam shall be of another shorter wavelength (blue or UV). The sensitizing/gating beam is used to sensitize the material before and during the recording process, while the information is recorded in the crystal via the reference and signal beams. It shall be shone intermittently on the crystal during the recording process for measuring the diffracted beam intensity. Readout is achieved by illumination with the reference beam alone. Hence the readout beam with longer wavelength would not be able to excite the recombined electrons from the deep trap centres during readout, as they need the sensitizing light with shorter wavelength to erase them.

Usually, for two-colour holographic recording, two different dopants are required to promote trap centres, which belong to transition metal and rare earth elements and are sensitive to certain wavelengths. By using two dopants, more trap centres would be created in the Lithium Niobate crystal. Namely a shallow and a deep trap would be created. The concept now is to use the sensitizing light to excite electrons from the deep trap farther from the conduction band to the conduction band and then to recombine at the shallow traps nearer to the conduction band. The reference and signal beam would then be used to excite the electrons from the shallow traps back to the deep traps. The information would hence be stored in the deep traps. Reading would be done with the reference beam since the electrons can no longer be excited out of the deep traps by the long wavelength beam.

Sensitivity refers to the extent of refractive index modulation produced per unit of exposure. Diffraction efficiency n is proportional to the square of the index modulation times the effective thickness. The dynamic range determines how many holograms may be multiplexed in a single volume.

Effect of Annealing

For a doubly doped LiNbO3 crystal there exists an optimum oxidation/reduction state for desired performance. This optimum depends on the doping levels of shallow and deep traps as well as the annealing conditions for the crystal samples. This optimum state generally occurs when 95 – 98% of the deep traps are filled. In a strongly oxidized sample holograms cannot be easily recorded and the diffraction efficiency is very low. This is because the shallow trap is completely empty and the deep trap is also almost devoid of electrons.In a highly reduced sample on the other hand, the deep traps are completely filled and the shallow traps are also partially filled. This results in very good sensitivity (fast recording) and high diffraction efficiency due to the availability of electrons in the shallow traps. However during readout, all the deep traps get filled quickly and the resulting holograms reside in the shallow traps where they are totally erased by further readout. Hence after extensive readout the diffraction efficiency drops to zero and the hologram stored cannot be fixed.

External links

• Daewoo Electronics Develops the World’s First Holographic Storage Device
• First commercial application of HDS by InPhase

• Computer storage