What is Holographic Storage?
Holographic storage was first proposed back in the 1960’s shortly after the invention of the laser. Holographic optical storage systems store data by recording the interference between the wave-fronts of a modulated optical field, containing the data, and a reference optical field, as a refractive index variation inside the storage media. It is this information containing refractive index variation that is the “hologram”. The stored data can then be retrieved by diffracting only the reference field off the hologram to reconstruct the original optical field containing the data.
Holograms can be created in polymer and electro-optic crystalline materials by exposing the material to a modulated optical field i.e. the interference pattern between the data and reference optical fields. In polymers the holograms are stored as a permanent change in the material and provide a Write Once Read Many (WORM) storage solution. Holographic storage in polymers for archival storage has been actively pursued as the successor to blue ray, however, this technology has yet to see commercial success.
In electro-optic crystalline materials the hologram is stored as a spatial variation in the distribution of the electron density inside the host crystal which due to the electro-optic effect, causes the refractive index to also vary spatially. The spatial distribution of the electrons can be changed by exposure to light of a lower energy wavelength, e.g. green light, to write holograms, and reset by exposure to a higher energy wavelength, e.g. UV, to erase the stored holograms. After UV erasure the media can be reused to write further holograms, thus electro-optic materials provide a Rewritable, Read Many media.
The hologram stores information in a 3D volume and thus provides 3 degrees of freedom when it comes to storing information. In polymer materials the thickness of the material is limited by scattering so typically thin layers of material are used which limits the capacity that can be achieved and necessitates the use of spinning media to achieve acceptable capacities e.g. 300GB in a CD size for factor. In electro-optic crystals large volumes (10s of cubic millimetres) can be used which allows the 3D nature of this storage technology to be fully exploited. In 2000 IBM were able to demonstrate impressive storage capacities in an electro-optic crystal, Iron doped Lithium Niobate (LiNbO3:Fe). However for the storage requirements of the time Hard Disk Drives proved to be a more compelling technology.
In Project HSD we are exploring the use of holographic storage in rewritable electro-optic materials for warm data storage to see if this technology makes sense in the cloud era.
How does holographic storage work?
This video shows how holographic storage works, using green light to write data as a persistent hologram inside an optical crystal. The data can then be read back from the hologram using another green light signal. The media is rewritable after erasure with UV light
The physics of hologram formation in iron doped lithium niobate
This video shows what is happening inside the lithium niobate media when a data page hologram is written.