The road to holography

Holographic storage has long held promise as a technology with vast capacity and high data rates. However, its commercialization depends on viable storage material, high density recording strategies and high-performance optical components. WLODEK MISCHKE, Director of Research at DaTARIUS, says these challenges have been overcome.

Movies, music, digital photos, x-rays, financial records, emails – more data will be generated in the next several years than has been generated in the last thousand. The challenge for business and home consumers is where to put it and how to keep it safe, and current storage technologies are struggling to keep up with this demand for increasing capacity and decades of archive life. Magnetic hard disk drives with 1TB (terabyte) of capacity are now coming to the market.

Impressive as the capacity may be, the life of the drive is still five years, and then the data must be migrated. Magnetic tape with capacities of hundreds of gigabytes has historically been used for longer term back-up and archive, but the media life beyond seven years is a problem.

New optical formats such as Blu-ray and HD DVD were thought to be the answer, but when dealing with petabytes and exabytes of data, their limited capacities make media management a problem. So, the question remains. What future storage technology has the potential for storing exabytes of data both economically and safely for decades?

Holographic storage has long held promise as a technology with the potential for vast capacity and high data rates. However, the major challenges facing commercialization were the development of a viable storage material, recording strategies to achieve very high data densities, and high-performance optical components.

The fundamental materials research, started at Lucent’s Bell Laboratories, and completed at InPhase Technologies, has now resulted in the development of Tapestry holographic media, a robust photosensitive plastic. Additional breakthroughs in key optical components such as high-speed CMOS detectors used in digital cameras, digital modulators used in TVs and projectors, and new blue lasers have also been incorporated into the first holographic drive from that company. These recent advances are moving holographic storage from concept to reality.

Conventional storage records on the surface of the material. On the other hand, holographic storage records data in three dimensions through the thickness of the recording material which results in terabytes of data being recorded on a single disk. In addition, 1.4 million bits of data are recorded and read in one millisecond exposure, rather than one-bit-at-a-time as is done in all other data storage devices.

This massively-parallel record and read results in transfer rates that are significantly higher than any other optical technology. The resulting high storage densities and fast transfer rates, along with durable, reliable, low-cost media, makes holography a compelling choice for the data intensive and rich media applications.

RECORDING AND READING HOLOGRAPHIC DATA

Data are recorded when light from a single laser beam is split into two beams – the data carrying signal beam and the reference beam. The process for encoding data onto the signal beam is accomplished by a device called a spatial light modulator that translates the electronic data of 0s and 1s into an optical ‘checkerboard’ pattern of 1.4 million light and dark pixels.

The hologram of 1.4 million bits is recorded as the result of a photochemical reaction in the light-sensitive material where the reference beam and the signal beam intersect. In order to achieve very high data densities, hundreds of holograms are recorded into the exact same location, but each hologram is written at a unique angle.
To read the data, the reference beam deflects off the hologram, and reconstructs the stored data. This hologram is then projected onto a detector that reads all 1.4 millions bits simultaneously. This parallel read-out of data provides holography with its fast transfer rates.

HOLOGRAPHIC STORAGE MEDIA

The key invention that has made holographic storage possible is the development of Tapestry media from InPhase Technologies, which satisfies all the stringent criteria for commercialization such as media robustness, environmental stability, high capacity and performance, and low cost. When compared to the incumbent technologies the case is very compelling. One Tapestry disk holds the equivalent of 462 CDs or 64 DVDs, and has an extra layer of media protection because it is protected by a cartridge.

When compared against tape, the disk capacity is competitive with tape, but offer much faster time to data – milliseconds and not minutes. Better yet, because data on a holographic disk is file-based it can be read directly from the disk by any application and does not require to be restored onto hard drives. This makes it the perfect solution for integration into automated library systems that provide cost-effective access to petabytes of data.

Archive life is also very critical in today’s world and 50-year archive life holographic media has been substantiated by extensive testing. This is done through a process known as accelerated life testing, where long-term behaviour is simulated by subjecting the media to short-term environmental conditions far more severe than would ever be encountered in actual usage. Specifically, InPhase holographic media is placed in a special environmental chamber where the temperature is raised to 80°C with 95% relative humidity.

Periodically, the optical quality of the media is measured to determine any degradation in the ability of the media to store data. Typical 50-year archive tests for optical media require the media to maintain optical quality for 1,000 hours under these elevated temperature and humidity conditions. InPhase media has shown stable optical properties beyond 3,000 hours of testing at 80°C with 95% relative humidity.

Media durability is further extended because there is no physical contact between the media and the drive. The only thing ‘touching’ the media is the light from the laser. Tens of millions of reads will not cause any loss of data. No need to worry about magnetic interference/demagnetization, re-tensioning of tape, or maintaining strict temperature and humidity controls. Tapestry media requires no special handling and can be stored safely in standard office environments.

The first InPhase holographic product is Tapestry 300r drives and media, in a Write Once Read Many (WORM) format that holds 300 gigabytes of user data and has a transfer rate of 20 megabytes per second. The drive connects to the computer through SCSI, fibre channel, gigabit Ethernet, SAS, iSCSI and others interfaces.

In addition, in order to facilitate inte- gration into a broad range of applications the drive emulates a variety of existing storage devices such as LTO tape and optical WORM drives. The result of these standard interfaces and drive emulations means that existing applications can easily interface to the holographic drive.

Subsequent generations of devices will increase capacity to 800 gigabytes and a transfer rate of 80 megabytes per second. This is followed by a third generation that holds 1.6 terabytes on a single disk and records and reads 120 megabytes per second. InPhase is also developing rewritable holographic media, which will further expand the applications that can take advantage of this storage breakthrough. Shipments to customers will begin in the second half of 2007....

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