This article is an extention of my previous article Holographic Storage. Rather than repeat half of it as exposition I will assume that you have read it.
A number of prominent people have suggested that optical storage is on its way out. Unless there is a sudden increase in the size of hard disks and the speed of internet connections, I sincerely doubt that will happen any time soon. Eventually it will happen. There are practical limits for information density on optical storage media. Fortunately though, we are a long way from hitting this limit.
Placing the data layer on the bottom of the substrate rather than inside it will allow engineers to increase information density even further. This is what is known as a ‘first surface’ optical disc.
The reflective substance on a bathroom mirror is placed on the back of the glass to protect it from damage. A ‘first surface’ mirror is one where the reflective coating is placed on the outside of the substrate so as to be the first surface the light encounters. I doubt this type of disc will be on sale in the next ten years. Hopefully this article will give you an idea of why this technology will eventually become necessary.
As we all know by now, information on an optical disc, standard or holographic, is stored as a series of dots. We also know the dots are stored on one of what could be many layers of a certain minimum thickness within the disc. On CDs the data layer is directly beneath the label. The advantage of this is CDs can be replicated inexpensively by stamping them like vinyl LPs.
On DVDs it is somewhere in the middle of the disc. There are two advantages to this:
First, the data layer is better protected from damage to the label surface.
Second, it is possible to make a double-sided disc.
On HDDVD, Blueray, and holographic discs the layer is within microns of the bottom surface. This holds the same advantages of DVDs but the dots can be packed more densely. The amount of substrate between the information and the laser affects the signal to noise ratio. The lower the S/N ratio, the more read errors the computer will encounter. If you can increase the ratio, then you can also increase the information density.
There are a number of reasons this happens. Unfocused light will reflect off the bottom of the substrate. From high-school physics class, we know light reflects off the interface between two materials of different refractive indexes, in this case polycarbonate and air. This reflection returns to the read head as pure noise. This accounts for roughly a 5% drop in the S/N ratio each time light encounters a surface. Fortunately, the amount of light that returns is a known value and, to a certain extent, can be compensated for electronically.
Surface imperfections on the other hand cannot be compensated for. The laser read head will read a pit on the surface of the substrate the same as a pit on the data layer. The surface will not be optically perfect. It is theoretically possible to make a better surface but, for reasons already discussed (HERE), this is not economically feasible.
Similarly, on a microscopic level all optical substances contain imperfections. For optical discs these can take two forms: inclusions and poor homogeneity.
An inclusion in this sense is a dust particle that makes it into the material. Like a pit, this will be read as a false data point. To understand homogeneity, imagine mixing chocolate syrup into a glass of milk. If you don’t stir it enough, some parts of the milk will be more chocolatie than others.
Instead of chocolatieness, an imperfect melt of polycarbonate will have varying refractive indexes within it (in spite of being a single chemical rather than a mixture). To the read laser these imperfections will form small lenses between it and the data layer. The less material you have between the laser and data layer the fewer imperfections a laser is likely to encounter.
The final major obstacle to information transfer is light absorption. Even in a perfect system, some light will be absorbed by the substrate. All optical materials will absorb a certain amount of light per unit thickness. The amount of light absorbed is based on the type of material and the frequency of light in question.
All materials have a “band-pass window” – this is the range of frequencies the material is transparent to. Polycarbonate has little absorption in the visible frequencies but the absorption increases rapidly toward the blue end of the spectrum. This is the main reason data layers have been getting lower in the substrate. Otherwise, it would take too much energy to get a useable signal. You could make a more powerful laser but it would be expensive.
Thus, data layers have been getting steadily lower in the disc substrate. Eventually the data layer will need to be placed on the bottom to avoid these problems entirely. From a consumer standpoint, the disadvantage of first surface optical storage is the fact that the data is unprotected.
A first surface disk will be as fragile as a hard disk platter. Any dust particle or scratch larger than a few nanometers will corrupt the entire disc. This is to say nothing of what a finger print or drop of water could do.
The simple solution is to put it in a cartridge. We have seen these before for early CDs and DVDs but the expense simply could not be justified from a consumer standpoint. In the future it will become a necessity. Of course, this is assuming you will eventually require a disc large enough to hold the collected works of David Hasselhoff on a single disc without compression.
Ian Anderson – Custom Optical Systems