Holographic storage

Have you ever been to a museum or novelty store that displayed holographic images or perhaps even offered them for sale? It's amazing how three-dimensional
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Have you ever been to a museum or novelty store that displayed holographic images or perhaps even offered them for sale? It's amazing how three-dimensional a hologram can appear.

Well, as NAB2006 showed, holography is more than just 3-D pictures. In fact, the technology has now entered the storage arena. The company InPhase demonstrated working versions of its Tapestry HDS-300R drive, which is being proposed as tomorrow's next-generation storage platform.

The benefit isn't 3-D images, but rather storage densities never before possible. The InPhase holographic drive can store 515Tb per square inch. This results in a storage capacity of 300GB per optical disc. The company predicts disc capacity to grow to 1.6TB in the near future.

The key to such storage densities isn't simply making the bits smaller. The solution is in how the bits are stored — in three rather than two dimensions. In other words, like a hologram.

Some history

Holographic technology isn't new. In fact, it's older than I am. Never mind that's already really old. The basic theory of holography was developed by a Hungarian physicist, Dennis Gabor, in 1947. He was working on improvements for electron microscopes, not storage. Gabor was trying to increase the resolving power of his microscopes. He proved he could do so with a light beam and ended up producing the first hologram.

Unfortunately, it took until the 1960s to develop the laser to make holograms precise enough to create clean images. Two engineers at the University of Michigan, Emmett Leith and Juris Upatnieks, developed the first device that created the kind of 3-D images many of us have seen. Today, holography is used in a variety of ways, from creating counter-proof images on credit cards to three-dimensional magazine covers.

How it works

Magnetic tape and optical and magnetic discs store data in an X-Y, two-dimensional, sequential manner. By adding a Z, or depth plane, holography allows the storage of far more data in the same space.

The storage process relies on creating a unique pattern signal, called interference, which represents the actual data to be stored. (See Figure 1.) To generate the interference pattern, the holographic recorder uses two beams of light.

A single coherent light source, a laser, is first separated into two beams with a beam splitter. One beam is called the signal beam (the data-carrying beam); the other is called the reference beam. The signal beam — passed through a Spatial Light Modulator (SLM), which is an array of pixels — is fed digital data. (See Figure 2.) This data can be any type of digital information, digital video, financial records or e-mail.

The SLM is responsible for encoding (modulating) the bits onto the signal beam. The chip converts the data into a visual display of light and dark pixels, which are illuminated by the signal beam. Typical SLM pixel counts are in the 1 million range. As the signal beam passes through the SLM, it is modulated and continues on toward the storage medium.

The modulated signal beam crosses the reference beam near the surface of the recording medium. The interference signal — created by the intersection of the reference beam and modulated data beam — gets recorded.

To move the storage around the recording surface, the reference beam's angle or media position is changed. This produces the many different holograms (called data pages) that can be recorded in the same physical place in the medium without interfering with each other. The result is a disc with high storage densities because each single physical location can hold multiple holograms.

To recover the stored data, it's a simple matter of shining the reference beam onto the stored hologram. (See Figure 3.) The reflection of the reference beam from the stored interference pattern is projected on a CMOS camera detector array, which recovers more than 1 million bits of data in parallel or with one exposure of the laser.

The parallel output of the CMOS array allows an entire page of information to be read at one time. Typically, a detector chip is capable of outputting 500 frames or pages of data per second.

The key to recovering the data from the holographic storage is precise alignment of reference beam with respect to the storage surface. This beam must precisely match the original angle of the recording beam. A difference of one thousandth of a millimeter in the beam's angle will result in a failure to recover the data.

The storage medium

The holograms are stored on photopolymer media. Today's DVDs provide about 120Mb/in2 of storage capacity. The holographic disc will provide 515Gb/in2 of storage capacity.

The two beams write the pages of data in the 1.5mm recording layer of the 3.5mm-thick disc. This layer is filled with two photopolymers sandwiched between the upper and lower 1mm substrates. The InPhase Tapestry recorder uses a 407nm blue laser, providing average record and readout times of 2ms.

More than 300 pages are recorded in a single location, and this collection of pages forms a book. Each page can hold 1.2Mb of user data.

The first question asked by potential users is: How much storage can holographic technology provide? Figure 4 compares the holographic storage capacities for some typical applications. Depending on the task, even small form factors can provide high capacity and performance. For instance, a 2cm postage-stamp-sized recorder (e.g., a Flash-like device) could provide from 2GB to 20GB of storage. A Maxell optical disc provides enough storage to hold 462 CD-ROMs. Or, the same optical disc can store up to 24 hours of SD video or seven hours of HD video.

The second question from potential users concerns transfer rates. Here again holographic performance is good. A Tapestry disc can provide a transfer rate of 20MB/s, which equates to 160Mb/s.

Other vendors

While InPhase was the first to launch the technology as a professional product, other vendors are vying to produce similar items for consumer products.

Last year, Toshiba, Intel Capital and others invested in another holographic optical storage developer, Optware. Optware is proposing a Holographic Versatile Disc (HVD), which is the same size as a DVD. The HVD will store 1TB of information. That's 200 times more data than what a typical DVD holds. The company claims transfer rates of 1Gb/s, or about 40 times a DVD's throughput.

While still a holographic storage technology, the Optware system relies on coherent-path laser beams. This means the reference and data beams move along the same axis. The company believes this will allow the development of more consumer-friendly (i.e., low-cost) players. Optware has proposed the HVD be declared a standard with the modest capacities of 100GB for ROMs and 200GB for cartridge-enclosed HVD-R products.

Broadcast applications

While all this may sound like future hardware, it's not. Pappas Telecasting has announced it will be the first to integrate holographic storage into a broadcast facility. The company is building new, state-of-the-art studios to serve KAZR-TV/KREN-TV in Reno, NV.

The company's new automated master control facility, called the Crystal Palace, is being constructed at an indoor Reno shopping mall. The facility will house two local high-definition news studios, one for each station. The HD studios will integrate the InPhase holographic storage platform into an automated program archive system.

Just when you thought the stoage race was only between Blu-ray and HD-DVD, something new and potentially better pops up. Stay tuned; the storage front is getting exciting.


“Holographic Memory,” Gregory T. Huang, Technology Review, Sept. 2005

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