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DVDs in the spotlight


Nowhere is the relevance of physics to everyday life more obvious than with the success of the DVD in the home-entertainment market. Sales of DVD players and recorders are booming, and last year consumers around the world spent over $20bn on DVD disks. In the UK, for instance, sales of DVD disks were more than twice those of video cassettes.

While physicists might sometimes complain about the "bad physics" in the movies that people watch on DVD, when making a case for the importance of their subject they should also stress that the relentless march of the DVD into homes is based on lots of "good physics" - as do the lens and visual-effects tools that are used in the film-making process.

DVD players and recorders, for instance, rely on an impressive mix of optics, electronics and mechanics to read to and write data on a plastic disk that is spinning at a frantic 10 800 revolutions per minute. The disks themselves are good examples of materials science in action, especially the DVD-RW disks that allow data to be overwritten again and again.

[The challenge is] to burn submicron marks with nanometre precision onto a disk that is spinning at nearly 200 kilometres per hour. This feat can be likened to the challenges of building a Formula 1 racing car -- the use of the latest technology in pursuit of speed. And like Formula 1 teams, the manufacturers of high-speed DVD drives have entered a fierce race to make sure that it is their name that is on the world's fastest DVD recorder.

However, there is clear finish line to this race: beyond a rotation speed of about 10 000 rpm the centrifugal force will cause a standard disk to explode. Current DVD drives are already able to read disks at speeds close to this mechanical limit, which is equivalent to a data rate of 176Mb per second. Writing information at such speeds, however, is a bigger challenge because the writing process is more complicated.

Electronics companies want to reach the maximum possible recording speed. Various tricks are also being used to increase the storage capacity of disks; these tricks include using lasers with shorter wavelengths and lens with higher numerical apertures to read and write the data.

When writing data at these speeds it is not enough to quickly turn the laser on and off in the hope that it has left ones and zeroes in the correct places: the way that the power of the laser varies with time has to be controlled very carefully, otherwise a whole host of problems will arise. So companies are developing materials that can melt and then crystallize on timescales of 10 ns for DVD-RW disks.

Meanwhile, electronics companies are preparing to go beyond DVD, and, as often happens, at least two competing technologies - HD-DVD and Blu-Ray - are jockeying for position. Both approaches rely on gallium-nitride lasers operating at a wavelength of 405 nm in the violet-blue part of the spectrum. Light-emitting gallium-nitride devices have become a billion-dollar industry since they were first demonstrated by Shuji Nakamura at the then little-known Japanese company Nichia in the mid-1990s; this growth looks set to continue. The use of violet-blue lasers will allow HD-DVD and Blu-Ray disks to store an enormous 50 GB of data.

Although these new disks will not be compatible with existing DVD systems, it looks as if both HD-DVD and Blu-Ray recorders will be able to play CDs and DVDs.

In the July issue of Physics World Jochen Hellmig of Philips Research Laboratories in Eindhoven in the Netherlands describes these latest developments in detail. May the approach with the best physics win.

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