Getting the Most out of a Disc
Holographic Storage sounds like something you might find on Star Trek, but in fact it has actually been around a while. The first papers on holographic storage were published in 1963, and people have been working on it ever since. Unlike the optical storage technologies that we are all used to (CDs and DVDs), which store information in one or two layers in the disc, holographic storage uses the entire volume to store the data. How does it work, you ask? Well, to record information we take the beam coming out of a laser and we split it in two. One of the beams passes through a device called a spatial light modulator (SLM), which is a large array of apertures (maybe as many as a million!) where each aperture is open or closed to represent a one or a zero. The beam coming out of the SLM is then mixed with the other beam split from the laser to create a complex pattern. The pattern is incident on a storage material that is sensitive to light and captures the pattern. This pattern stored in the material is called a hologram. To read out the data, the original laser beam that had no data in it is bounced off of the stored pattern and recreates the data pattern, which can then be detected with a camera. The key to getting enormous amounts of data stored is that the patterns are very sensitive to things like the angle of the mixing beams. In fact, by adjusting the angle of one of the recording beams, a second pattern can be recorded directly on top of the first, and this process can be repeated as many as 10,000 times. Then we move to a new location on the disc and there might be as many as 1000 locations on the disc. So 1,000,000 bits per hologram, with 10,000 holograms recorded per location, and 1000 locations per disc – now that’s a lot of one and zeros!
You do not discuss the fact that the frequency of light can also be altered thus further increasing storage capacity.