How Holographic Versatile Discs Work

Basics of Holographic Memory

3-D image of the Death Star created by holography
3-D image of the Death Star created by holography
Photo courtesy Wolfgang Wieser

The first step in understanding holographic memory is to understand what "holographic" means. Holography is a method of recording patterns of light to produce a three-dimensional object. The recorded patterns of light are called a hologram.

The process of creating a hologram begins with a focused beam of light -- a laser beam. This laser beam is split into two separate beams: a reference beam, which remains unchanged throughout much of the process, and an information beam, which passes through an image. When light encounters an image, its composition changes (see How Light Works to learn about this process). In a sense, once the information beam encounters an image, it carries that image in its waveforms. When these two beams intersect, it creates a pattern of light interference. If you record this pattern of light interference -- for example, in a photosensitive polymer layer of a disc -- you are essentially recording the light pattern of the image.

To retrieve the information stored in a hologram, you shine the reference beam directly onto the hologram. When it reflects off the hologram, it holds the light pattern of the image stored there. You then send this reconstruction beam to a CMOS sensor to recreate the original image.

Most of us think of holograms as storing the image of an object, like the Death Star pictured above. The holographic memory systems we're discussing here use holograms to store digital instead of analog information, but it's the same concept. Instead of the information beam encountering a pattern of light that represents the Death Star, it encounters a pattern of light and dark areas that represent ones and zeroes.

DVD vs. HVD: Recording-layer depth DVD vs. HVD: Recording-layer depth
DVD vs. HVD: Recording-layer depth
Courtesy Optware

HVD offers several advantages over traditional storage technology. HVDs can ultimately store more than 1 terabyte (TB) of information -- that's 200 times more than a single-sided DVD and 20 times more than a current double-sided Blu-ray. This is partly due to HVDs storing holograms in overlapping patterns, while a DVD basically stores bits of information side-by-side. HVDs also use a thicker recording layer than DVDs -- an HVD stores information in almost the entire volume of the disc, instead of just a single, thin layer.

Volumetric recording method Volumetric recording method
Volumetric recording method
Courtesy Optware

The other major boost over conventional memory systems is HVD's transfer rate of up to 1 gigabyte (GB) per second -- that's 40 times faster than DVD. An HVD stores and retrieves an entire page of data, approximately 60,000 bits of information, in one pulse of light, while a DVD stores and retrieves one bit of data in one pulse of light.

Now that we know the premise at work in HVD technology, let's take a look at the structure of the Optware disc.