Holographic Data Storage

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The optical limits of semiconductor fabrication I discussed in my previous article (HERE) apply equally to optical media. Unlike semiconductors, there are some workarounds – namely, holography.

Standard optical media stores information as pits in the disc’s data layer. A laser then reads these pits. The size (and density) of the pits is limited for the same reasons as semiconductors. You can think of it as Morse code – the shorter the blips, then the more information you can pack into a given length of time.

When you decrease the size of the dots by half, you can quadruple the amount of information the disc can hold in a given area. Holographic storage still encodes the information as dots but it allows considerably higher efficiency.

First a little on how a hologram is made:

Before I get into lasers, light diffraction and spatial light modulators, imagine a perfectly calm lake at the break of dawn. Aside from being fantastic imagery, it is the perfect analogy for holography.

As the wind begins to pick up, small identical and perfectly straight waves will begin to form on the surface and hit the shore (we will assume the waves don’t reflect off the shore). Now imagine a rowboat moving across the water producing another perfectly straight set of waves of exactly the same size and frequency forty five degrees from the waves made by the wind.

As the waves interfere with one another, what you will see is a crosshatch pattern where the waves cross and interfere with one another. Along the beach you will see parts where the wave goes twice as high and other places where there is no wave at all. This pattern will not be at all random; in fact, it will be perfectly symmetrical from wave to wave.

The exact same effect can be applied to waves of light.

Imagine a perfect beam of light (such as what you get from a laser) with a perfectly flat wave front of an exact wavelength hitting a piece of paper. What you will see is a perfectly illuminated sheet.

Now imagine an identical beam hitting the sheet from an angle. Now you will see perfectly straight lines alternating between light and dark. Take this one step further and imagine that the second beam of light has a spherical wavefront instead of flat. What will appear is a series of concentric rings alternating from light to dark (as if it were a topographic map of a sphere).

If a beam of laser light bounces of an object, first you will end up with a complex pattern of fringes. If you replace the sheet of paper with a piece of film, the pattern will expose it. If you illuminate the developed film with the first beam (reference beam), it will recreate the second through diffraction (Diffraction). It will look as though the object has reappeared.

Instead of an object, let’s say you illuminate an SLM (spatial light modulator [the image element in a digital projector]) and instead of film you make your hologram on the recording surface of an optical disc. You end up with an image of the SML. If you illuminate this image with the reference beam, you can then image the hologram with a digital sensor (CMOS or CCD).

At its most basic level, a holographic drive splits the single laser used in your regular optical drive into a million individual lasers (typically a 1024 x 1024 grid). The process used is holography, but you can think of it as printing a photograph onto the back of an optical disc, rotating the disc a little, then printing another photo.

When you want to read the data, all you need to do is take a digital snapshot of each photo. The data being transferred is quite literally an uncompressed video file. The data rate is like having one disc drive per pixel in the photo. If you use a standard 1024 x 1024 camera, it is like having 1,048,576 disc drives running in RAID 0. It won’t be quite that fast, but for a given data rate the disc won’t need to spin as fast.

Myths and Techno Babble

Servo Layer:

A little techno babble associated with holographic storage is the servo layer. This is an irrelevant piece of information for the consumer. For each of these discs there are at least three layers:

  • The bottom layer holds the data;
  • The middle layer is the interference filter. This is a reflective coating on the top of the data layer that will reflect the laser used to read the data layer but allow the laser used to read the servo information to pass through;

  • The top layer is the servo layer. This tells the computer what part of the disc is being read. This is like printing a number on the back of each photo (in this case the photos printed onto the info layer) so you know which one you are looking at.

Density:

The theoretical density of data (bits per square inch) on holographic media is identical to standard media. The minimum resolvable spot size (and hence physical density) is determined by the size of the lens and the colour of the light being used. Holographic media may require less space between the tracks, but other than that, all of the inherent advantages are in speed.

The information is stored in 3D?

This is another useless piece of techno babble. When you read that Holographic storage is 3D, what you are being told is that the information can be stored on multiple layers, just like a standard DVD. The theoretical maximum density for the layers (or the minimum distance between them) is one wavelength of light. Don’t hold your breath to see anyone achieve this density.

Why cartridges?

A disc cartridge is less a necessity for holography than it is for any short wave (blue, purple, or ultraviolet) optical media. The smaller each point of data is, the more likely it is to be obscured by a scratch or piece of dust. This is not much of a problem with IR lasers because in a normal house or office, the size of the dust and scratches that accumulate on the surface will not interfere with the laser. When blue or ultraviolet lasers are used, seemingly minor scratches and invisible dust particles are more than enough to obscure the data on the disc.

Ian Anderson

Custom Optical Systems

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