Holographic storage is starting to move out of the high-profile academic hothouse project environment and into the real, even the usable, world. It sounds theoretical and far fetched but, at the end of the day, it is just another optical drive with read:write technology using lasers.

The technology
To state the obvious, holographic storage is a form of optical storage. It's promise is very high storage densities, fast reading of data and thirty-year stored data life spans.

It starts with a laser. DVD-R (red 680nm) and DVD-B (blue 405-407nm) can be used. This shines a beam of coherent light to a splitter which divides the beam into two: a reference beam; and a signal beam.

Digital data needing to be stored, the string of bits, is arranged into pages or large arrays. The 0s and 1s are translated into pixels in a spatial light modulator (SLM) that either block or transmit light.Thus the signal beam, shining through the SLM has the data to be stored encoded into it in a sort of checkerboard pattern. The SLM is implemented as a digital micro mirror.

The modified signal beam then meets the reference beam, both being directed to the same point by a mirror, and an interference pattern is created which is stored by a photosensitive medium on the recording surface of a disk. So far so standard in hologram terms except that the image to be stored is not of an object but of a pattern created from the sequence of 1s and 0s, the bit pattern of the data to be stored.

The recording medium has a refractive index which affects any light shone on it such that the refracted light has certain characteristics. The interference pattern, mentioned above, causes modulations, termed diffracted volume gratings, in this refractive index which can be detected by a pixelated detector array, similar to the CMOS light detector chip in a digital camera. The stored array of 1s and 0s can be reconstructed from this, a page or array at a time.

The data is read in parallel and provides a fast data read rate in the 10-100+MB/sec region.

A fascinating aspect of this technology is that by varying the characteristics of the reference beam, its angle of incidence or its wavelength, the recorded interference pattern is made different. We can record sequential pages of data in the same recording medium by sequentially varying the reference beam characteristics to generate a different interference pattern.

These different patterns can be recovered by reading the diffractive volume gratings with the right character reference beam, one with the same angle of incidence or wavelength, or both, of the original reference beam used when the data was recorded.

This is a multiplexing process and is key to the large storage capacity of holographic technology. The ways used to multiplex or overlap holograms determine the architecture and complexity of holographic drives.

Drives are beginning to appear in PCMICIA, 3.5 inch disk, 4.7 inch CD/DVD and 5.25 inch MO form factors.

Potential suppliers
InPhase Technologies is a start-up developer of holographic storage devices. Much of the foregoing discussion is courtesy of that company. It is developing rewritable holographic data storage media through a project grant from the Advanced Technology Program (ATP), part of the US National Institute of Standards and Technology (NIST).

The two-year project started in October, 2002 and includes the development and demonstration of rewritable recording materials for holographic data storage systems that offer 'ultra high storage density and data access rates'.

As we might expect IBM has mentions of holographic storage projects in its Almaden Research Centre but these appear to be fairly old.

The company patented a holographic storage method in 2000. In its statement at the time it said, "In holographic data storage, entire pages of data are committed to and read from a photosensitive material by a laser. Each page consists of an array of "bright" and "dark" pixels, resembling a checkerboard. This invention improves the contrast between the bright and dark pixels by subtracting unwanted electronic signal from the dark pixels as each hologram is recorded. By making the dark pixels darker, errors are reduced and capacity can be increased."

Since then there has been no mention of the technology by IBM which suggests the company is not that enthusiastic about it.

Aprilis produces HMD and HMC holographic media. These consist of a recording medium sandwiched between two unformatted glass substrates. HMD series media comes in 120 mm disk format and in 50 x 50 mm. card format.

The Aprilis Vulcan DHD 1000 System has a capacity of 200 Gbytes per media cartridge and delivers data at 75 MBytes/sec. Aprilis claims it 'provides users with the highest performance of any removable technology available today.'

How does it compare?
Both UDO and PDfD use DVD-B or Blue Ray laser technology. Plasmon's UDO has its first generation capacity of 30GB, with capacity reaching 120GB by the third generation. First generation UDO has 15GB per side, compared to MO's 9.1GB. The average seek time is 25 msec and the sustained read and write transfer rates are 8 and 4 MB/secs respectively.

Sony's Professional Disk for Data (PDfD) holds 23GB in its first generation (compared to DVD's 4.7GB). The roadmap has a doubling of capacity every two years leading to 46GB, using multi-layering, and then 92GB, using both sides - Sony refers to 50 and 100GB - in 2005 and 2007 respectively.

It looks as if PDfD and UDO will be in the 100-120GB capacity area by 2007. If holographic storage can arrive by then and have a 200GB capacity and 75-100MB/sec transfer rate then it should have a role to play as another optical storage medium. The prospects of PCMCIA holographic storage for notebook computers has interest but it's hard to see actual business needs requiring this.