How Holographic Storage Works

The Chef writes: "Tweak3D.net has yet another excellent article for nerds -- err, I mean, guys looking to fill their brain with technical know-how. This time it's on holographic storage for PCs. Yeah, that's right -- storing files using holography! Go here for the story." This is something that gets mentioned in passing frequently but it's nice to have the technology explained nicely. Thanks for the overview!

Comments

Bad Vibrations

[NaughtyEddie]

A lot of people are asking why we don't have this technology yet.

One possible answer is because of the sensitivity of holographic equipment to vibrations. A hologram encodes phase differences between laser beams. Errors in the phase encoding mean errors in the data retrieval - you get a blurry or disjoint hologram, or you lose your data.

Light is in the hundreds of nanometers range of wavelength. This means a vibration in the equipment (a movement of one part relative to another) of only a tenth of a micron can completely throw the phase encoding out of alignment. Imagine a tape deck whose heads needed positioning to submicron precision.

Making holographic images is therefore rather difficult if, say, a large lorry rolls past your window. A hard-drive with the same problem would be absolutely useless.

So until a suitably hard substrate can be found on which to engineer this equipment, it's only a pipedream. Maybe nanotechnology will create such a material ... I doubt it'll happen before then.

I'm more jaded by the day, here's why

[orpheus]

I do not in any way mean to detract from the accomplishments presented here (though, of course, it's been presented in varying shades of "Any time now" for decades. I made my first hologram as a Explorer scout in 1977 or so, and at that time it was already a staple of such classics as "The Adolescence of PI")

However, the storage capacity cited is 10GB/cm^3 cubic centimeter) not cm^2 (square centimeter) as you stated. By comparison, given how thin the magnetic films are, I doubt the 75GB HDD even has 1 cc of active storage volume so holograms do not approach the volumetric density of magnetic media.

The key of course is that holograms offer the promise of true volumetric storage, where magnetic media is limited to the thin film [though who can forget the hedelberg group who used a thin film -- namely a roll of commercial adhesive tape to as an optical medium for up to 10GB a few minths ago?)

So, since it comes down to form factor, I'm not excited. The problem with the HDD is the overall associated mechanism, and with 1 GB matchbook 10G-resistant HDDs out there *today*, I'm not sure when I'll ever be excited. There is no reason to expect we'll be carrying 'naked' (or packaged) holographic media, any more than we carry naked (or packaged) *high density* platters today -- and holographic drives may well be larger, more expensive, more fragile, etc. than HDDs in 2003, as well. In 2003, you won't be able to *buy* a new HDD as puny as 10GB, if indeed they are still maanufacturing that size, today.

The external hologram surface may turn out to be sensitive to damage (though I can hope for the use of confocal optics, etc. to image the volume despite surface imperfections or contamination) and the volume may be vulnerable to sunlight.

The hologram technology used here showed promise because it can be multiplexed with different laser colors and at different angles, but the 'clarity' of the signal goes down with the square of the number of channels, until it is unintelligible. This does not bode well for rapid breakthroughs (though if we could predict them, they wouldn't be breakthroughs). Precise alignment is necessary to assure high density, reliable readings. it seems likely that the 2003 holographic drive will be larger, more expensive, and offer no appreciable advantage (aside from ?magnetic insensitivity?, if that counts)

Aside from the probability of actually seeing a production drive someday, I think that several other holographic technologies are more promising. and none of the holographic technologies show signs of exceeding the practical capacities of straight optical media in the predictable future -- i.e. the next three years. Standards, not technical capabilities, block DVD-R from coming out *this year*

Hey, I want my petabyte encrypted keychain as much as the next guy... but, you know, 'fire' still has many unparalleled uses, in the nuclear age. And I'd rather grill than irradiate my dead cow this weekend anyway