HVD: Holographic Versatile Disc

[Couperin]

Has anyone here heard about the new data storage medium that is being researched? Apparently it'll be able to store up to 3.9 terrabytes of data, which is insane.

Anyway, I checked out this article on Wikipedia to see how it worked...

http://en.wikipedia.org/wiki/Holographic_Discs

...and I don't quite get it.

Here's what I can gather... there are two layers on the HVD, one which is read by red laser light and contains 'servo' data, and above this is a layer with normal data on it, read by a green-blue light. The red light passes through the normal data layer, so there's no interference or other weirdness.

But what I don't get it... how does simply storing servo data on a seperate layer manage to free up 3.9 TERRABYTES of data? The green-blue light has a longer wavelength than the blue laser light used to read Blu-ray discs, and they can't store anything near the amount as HVDs will.

So what's going on? And what *exactly* is servo data?

[Mech_Engineer]

Your answer lies in the very first paragraph of the Wiki article:

It employs a technique known as collinear holography, whereby two lasers, one red and one blue-green, are collimated in a single beam. The blue-green laser reads data encoded as laser interference fringes from a holographic layer near the top of the disc while the red laser is used as the reference beam and to read servo information from a regular CD-style aluminium layer near the bottom. Servo information is used to monitor the position of the read head over the disc, similar to the head, track, and sector information on a conventional hard disk drive. On a CD or DVD this servo information is interspersed amongst the data.

It would seem to me that the advantage of having the servo information on a different layer is it doesn't take up space on the data layer, making it possible to store more "real" data on a layer.