On page 28 of the April 2007 issue, Popular Science published an article on holographic storage. This article is essentially identical, if not as in depth, to one I wrote for this website in 2005 (Holographic Data Storage). I sincerely doubt the author read my article so I am not about to accuse him of plagiarism. There was one noteworthy point of deviation which I feel is worth further examination.
The article mentions that the technology allows information to be printed on the same point on the disc but at a different angle. While it may seem like this will allow for a near infinite amount of information, it does not actually increase the theoretical limit for information density.
Consider how holographic storage works on the level of a single bit: A laser prints a point onto the surface of the disc. This point will either reflect light and be read as a 1 or not and be read as a 0. In a sense, you can think of each bit as a small mirror to reflect the read laser.
If this mirror is tilted in the wrong direction, the read head will not be able to receive information from the point. This light does not just disappear though, it is just re directed somewhere else. If the read head is in that somewhere else, it will have no trouble reading that point.
Now imagine you take mirror at the point and split it in two. The right half remains horizontal and the left is tilted by a few degrees. The incident read laser will be split into two direction. If you have a read head at both points, you can read the bit at each. There is no reason why the left part of the mirror needs to be encoded with the same bit as the right. The left could represent a 1 and the right could represent 0.
This would seem to double he amount of information you could put on a disc. In truth this does little to increase the theoretical maximum amount of information that can be stored on a disk. As I have mentioned in my holographic storage article, information density is limited by the signal to noise ratio. Splitting the mirror in half halves the amount of light it will return. The minimum size (and hence maximum density) of a readable point remains the same.
While this technology does little to increase the theoretical maximum information density, it does have practical benefits. What it does is separate the returned signal from each side of the point. When you are dealing with real lenses and real read heads, this makes each point easier to read. So in effect writing adjacent points at different angles increases the real world signal to noise ratio.
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