Same space, more data: A new generation of DVDs

The days of the videotape are long gone; these days Compact Discs (CDs) and Digital Versatile Discs (DVDs) are the technologies of choice not only in the entertainment industry in particular, but also in the world of data storage in general.
Nowadays it is not surprising to be able to store around 4GB of data in a single DVD using equipment that has become ubiquitous in offices and even households. Nonetheless, there is a very vivid interest for increasing the amount of data that can be stored in these media.
In recent years, new advances in optical technology have become promising new avenues to improve data storage. In particular super-resolution seems to be a viable way to increase the density of data that can be stored in a disc without having to change the architecture of existing appliances.
To explain how super-resolution helps in this task let us imagine that our media is a simple A4 page and that data is written using an ordinary pen. For the sake of argument let us imagine that on average 800 words per page cab generally be written. We are now faced with the question of fitting more words per page without having to change the size of our A4 standard. One way of achieving this is to change the size of the font. If we keep on making the size smaller and smaller we finally come across two problems. On the one hand, the tip of our pen has to be small enough to let us write small letters and on the other hand, we should still be able to read what is written on the page. In the optical equivalent of this situation a laser beam takes the place of the pen and a disc that of A4 page.
The resolution in an optical system is limited by the colour of the light used (characterised its wavelength) and the capacity of the system to accept that light (the numerical aperture of the system). In this fashion, by changing the traditional red laser for a blue-violet one, Blu-Ray discs are able to hold 25-50 GB. Super-resolution techniques try to address these problems by exploiting the changes in the optical properties of the disc under the influence of the light, giving rise to nonlinear optical effects that have to be better understood.