Credit-card-sized storage devices based on holographic storage techniques are a likely future step in the data security industry. Key challenges to commercial adoption are size, cost, and complexity of the cards and the read/write systems. Recent advances in holographic storage using polarization techniques in thin polymer films at the Budapest University of Technology and Economics (Budapest, Hungary) are addressing these issues. "We have developed prototype systems for holographic reading and writing using these polymers in a credit-card-sized carrier, and have demonstrated error-free operation with high security," explains Pal Koppa, one of the lead researchers.
Traditional holography uses object and reference beams with the same polarization. In contrast, polarization holography employs beams of mutually orthogonal left- and right-circularly polarized beams. These beams do not create intensity interference fringes, but rather a spatially variable polarization that presents an optically induced anisotropy of the electric field in the storage media of azobenzene-side-chain polymers. Illuminating the hologram with the circularly polarized reference beam reconstructs the object beam.
The technique has a high diffraction efficiency and produces only the zeroth and first diffractive orders, which results in very low noise. The optical anisotropy induced by the spatial polarization distribution changes the orientational distribution of the side chains. This results in a birefringence that follows the spatial variation of the electric field. "We have found these holograms to be stable up to 160°C and the process is reversible, allowing read/write capabilities," says Koppa.
The read/write system uses the beam from a frequency-doubled (532 nm) neodymium-doped yttrium aluminum garnet (Nd:YAG) laser, split into the reference and signal beams. Data is impressed on the signal beam with a transmissive spatial light modulator. The beams undergo a series of optical Fourier filtering and beam-shaping steps to optimize the recording process, and the read/write head has the same general size as a CD pickup. Quarter-wave plates set the beams to mutually orthogonal polarizations at the recording plane of the card.
Reading the hologram requires only a short pulse of the reference beam, and this can be done over 1000 times without degrading the recording. The card is coated with a reflective layer that sends the reconstructed beam to a CCD array; a digital signal processor extracts the information. The team has used different data encoding and reading methods, demonstrating bit error rates of 10-12 and raw storage densities of 2 bit/µm3.
There are security advantages inherent in the polarization holography technique as well as methods for further improvements. Reading the data requires reproduction of both the wavefront and polarization. Page-oriented holographic storage records complex phase or amplitude of the entire page rather than bits of data spatially separated along a defined track. Random or deterministic phase modulation of the beams is a known technique for improved encryption, and the researchers have used it to demonstrate increased security in their system. A twisted nematic liquid crystal display in a purely phase mode encoded phase onto the beams during read/write, and results confirmed this approach works in a polarization-holography scheme.
"Most of the big companies have been advised that holographic data storage is the next step on the technology roadmap; what they are now trying to figure out is which technology to support," explains John Sheridan from University College (Dublin, Ireland). "I think the team's work is impressive since they have produced a card optical read/write system. The next thing to demonstrate is the high data densities promised by other techniques." The group is working to transfer the data density of 2 bit/µm3 to thicker films for increased capacity using a stack of multiple polymer layers. Issues like crosstalk and overwriting are being addressed with encouraging results. Maybe one day soon you'll have polarization holograms in your wallet protecting your data.