Holographic storage
Aug 1, 2006 12:00 PM, BY BRAD DICK, EDITORIAL DIRECTOR
Figure 1. (A) Creating a hologram relies on the generation of an interference pattern from crossing of two laser beams. One beam contains the data, the other the reference signal. (B) A photosensitive medium is used to record this interference pattern, or hologram. (C) The data hologram is the image of the data pattern stored within the photopolymer medium. (D) The original data can be recovered by shining the reference laser at the stored hologram at precisely the original angle. Figures courtesy InPhase Technologies.
Click image to enlarge.
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.
Figure 2. To record the data, a single laser beam is first split into two beams: a signal beam, which carries the data, and a reference beam. The data hologram is formed where these two beams intersect.
Click image to enlarge.
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.
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