Accessing another dimension

17 Jun 2002

Swedish company Optilink claims that it is the only firm to have developed a portable holographic data-storage system. The technology promises to give the consumer gigabytes of storage capacity on a smart card. Nadya Anscombe reports.

From Opto & Laser Europe January 2002

Imagine you could store all of your medical records on a smart card that could fit in your wallet - data storage in doctors' surgeries would never be the same again. While today's electronic smart cards are used as telephone cards and in transport systems, they do not have enough memory to store large amounts of information. Swedish company Optilink believes its holographic data-storage technology is the answer.

Optilink has developed a polarization holography system that can read and write 10 x 10 mm chip holograms and has a data capacity of 30 Mbyte per chip. Each square centimetre of a 2 µm-thick polymer film deposited on a credit-card-sized substrate can hold 1600 holograms. After error-code correction - which is at a level comparable to that of a CD - the whole area of the smart card can hold up to 1 Gbyte of data.Optilink's intellectual property and programme manager, Sune Montán, says that the company made considerable progress last year. He said: "We have signed a deal with a Korean firm that wants to use our system in security applications, such as access control. Data that are held in holograms are more difficult to manipulate than electronic data, making the system more secure. We still believe that data storage will be the biggest market for our technology, but security applications will be the first."

The company has built a demonstrator system for its technology, placing the firm ahead of its competitors. Montán told OLE: "We are the only holographic data-storage firm that has a portable demonstrator."

Optilink was founded in Sweden in 1991 by Hungarian electronics engineer Peter Toth who had an idea for an optical high-speed memory module. He discovered that holographic-materials studies were being carried out at Risø National Laboratories, Denmark, and links between Optilink and Risø were soon established. Optilink now employs three people in Denmark and leases premises from Risø.

Toth also had strong links with Laszlo Gazdag a scientist at the Technical University of Budapest in Hungary, and in 1998 Optilink set up a subsidiary there. The firm now has 22 employees in Hungary, where it carries out all of its systems research.

The business's headquarters are in Lund, Sweden, where it has five employees. Montán commented: "We are a truly European firm and have chosen our locations because of the quality of the research staff there."

Current product development is based on two material systems: peptides and liquid-crystalline polyester. The polyester is a photochromic material, containing the functional group azobenzene, which reorientates itself to form the holograms. It then reverts back to its original state when it is irradiated with polarized light. Raman Ramanujam, one of Optilink's researchers based in Denmark, says that the main advantage of this system is that the same wavelength of light can be used to read and write the holograms.

"We use one frequency-doubled YAG laser at 532 nm and split the beam into a reference beam and an object beam. Data are written on the card in a parallel way using liquid-crystal spatial modulators to encode the object beam. This is Fourier transformed by a custom objective lens and interferes on the card with the orthogonally polarized reference beam."

Peptides store holograms by a different molecular mechanism. They are photopolymers and form new bonds when irradiated. This process is usually irreversible, but some bonds are weak enough to break. The advantage of the peptide polymers, says Ramanujam, is that they are sensitive to ultraviolet light, allowing a higher storage density than the polyester which is sensitive to visible light. It is predicted that the peptide materials could enable a storage capacity of more than 20 Gbyte per layer on a CD-type substrate. Using greyscale and recording on two sides with double layers would give a CD a storage capacity in excess of 300 Gbyte.

However, this is a long way off because the firm's peptides research only began last year. The commercial viability of Optilink's product also depends on the systems group in Hungary being able to develop a portable device that is acceptable to the consumer.The optical-engineering problem of fitting the array of optics needed for holographic data storage into a casing the size of a shoe box is not trivial. Ramanujam said: "The challenge now is to improve and miniaturize all of the optics and electronics needed for a system. Getting hold of good spatial light modulators has been a limiting factor in the development of holographic systems. Devices with a high enough resolution are not currently available on the market."

But the company has overcome many of these challenges and, with its demonstrator, Optilink is ahead of its rivals. And it faces some strong competition: German firm Optostor is developing holographic systems based on lithium niobate crystals and IBM in the US is investigating crystals and polymers. However, Optilink is confident that its device has advantages over those of its competitors. Ramanujam said: "Our method is simple and cheap, and we use only one laser."

The year ahead should be an interesting one for Optilink, as it finally starts to ship its holographic data-storage systems and gets its product known in the market-place. Who knows, one day you could be carrying an Optilink smart card in your wallet. A hologram is a recording of the optical interference pattern that forms at the intersection of two coherent optical beams: the object beam carries information; and the reference beam records and reads out the hologram. Data are imprinted on the object beam by shining the light though a pixelated device, such as a spatial light modulator.

The reference beam overlaps the object beam on the storage material and the interference pattern is stored as a change in the absorption, refractive index or thickness of the medium. A pair of lenses images the data through the storage material onto a pixelated detector array.

Optilink's technology uses photochromic polymers that store data via the reorientation of the polymer's side chains. This is a stable process but can be slow. The stored holograms can be erased using circularly polarized light.

A unique aspect of holographic data storage is that an archive is searched by content rather than by bits, bringing data searching into the hardware device. The system supports massive parallelism, for example, every record in a database can be searched simultaneously. This, together with a terabyte storage capacity on a medium the size of a CD, makes holographic data storage ideal for all storage applications.