How can a fricken' blue laser make DVD more powerful? Ty Pendlebury investigates blue diode laser technology, which promises to give DVDs 120GB storage capacity.
VHS versus Beta
The media loves a format fight. Whenever two or more formats are introduced into the market, you can bet we'll be asked to swear our allegiance to one or the other almost immediately -- and it will cost us. The recent DVD+R/DVD-R battle is the latest in the format war, and the reason for the clash is straightforward: with the runaway success of the DVD format, everyone wants a piece of the licensing pie.
If you thought the battle was over when the DVD Forum recognised DVD-R as the official format, you were mistaken.
DVD-RW backups are increasingly unfeasible thanks to ballooning hard drive sizes and HDTV has created the need for recordable media that can hold large volumes of data, mainly due to HDTV's relatively high bandwidth of 12-14Mb/s.
As a result, there are two major 'next-generation' technologies in development:* Blu-Ray -- a joint effort from nine companies, including Philips and Sony; and* AOD, Advanced Optical Disk System -- proposed by DVD founder Toshiba and partner NEC.
Both systems use blue lasers and have recording capacities of 50GB or over. Blue lasers are the future of DVD recording, according to the makers, and this is due to the inherent properties of blue light.
Red laser / blue laser
Blue laser beams have a smaller 'spot' size and shorter wavelength than the traditionally-used red lasers, therefore the corresponding data pits can be much shorter and thinner, resulting in more information on a disk.
A 'blue' laser assembly mightn't be blue -- it can also use indigo or violet lasers from the visible spectrum of light of red, orange, yellow, green, blue, indigo and violet. Infrared has the longest wavelength and ultraviolet has the smallest. Therefore, violet lasers would enable even smaller pit sizes again.
The advocates of Blu-Ray say they want to eliminate the compatibility nightmares that have characterised the competing writeable DVD formats. But like Rocky of Bullwinkle fame used to say, 'That trick never works'. It is simply one of several burgeoning technologies competing to become the second generation of DVD, with the DVD Forum currently "considering" the contenders.
This is a repeat of events from the mid-1990s when two standards competed for the domination of the market: Super Density from Toshiba and Panasonic, and Multimedia Compact Disc from Sony and Philips Electronics. The two rivals consulted with the movie studios, and as result a compromise format was conceived: DVD. This led, quite naturally, to the establishment of the DVD Forum, and the release of the first DVD player in March 1997. The Forum now includes over 200 companies involved in the sale and manufacture of DVDs, with Toshiba as the head.
ABOVE: All CDs are a recordable layer (usually Aluminium) packed between two polycarbonate layers. With DVDs, there are a mulitple semi-transparent recording layers.
How blue laser works
Blue diode lasers work with similar media to that used by CD for the last 20 or so years. The disks are a sandwich of injection-moulded polycarbonate plastic, Aluminium and acrylic as a combination of a singular read surface or several semi-transparent ones in the case of DVD. The readable surface of all optical media consists of a series of bumps in a spiral from the inner to outer edge, divided into pits (the bumps) and lands (the spaces between).
In pre-recorded media, the pits are imprinted into the outer polycarbonate, which is also called the substrate. A thin layer of Aluminium is then sprayed over the top and then capped with a protective vinyl coating.
As the laser travels across the pits it is converted into pulses of light of varying intensity, which are translated into a digital signal.
Recordable CD and DVD media use a similar system, but instead of pits and lands they use a light-sensitive dye, which is sandwiched between the vinyl layer and the substrate. When this dye is heated it becomes opaque, causing the laser to reflect differently. Rewriteable media, on the other hand, uses a dye that changes states dependent upon the intensity of the laser that writes to it. This is why most CD players can read CD-Rs, while CD-RWs require newer, more sensitive pickups.
Since the CD was invented the widths and lengths of the pits has been whittled down significantly from an initial minimum pit size of 500 nanometers (nm) width x 830nm length x 125nm height (a nanometer is 1 x 10-9 meters -- damn small!). DVDs use a slightly shorter wavelength again and reduce the size to 320nm (w) x 400nm (l) nanometers, with 740nm between each track.
Blue lasers have shrunk the minimum length per pit to 138nm, with 320nm between each track. Yet as the distance between tracks shrinks, the likelihood of crosstalk also increases, bringing its own headaches.
Fingerprints and scratches obviously cause problems with optical media and even more so for blue laser media. However, one of the less obvious problems affecting media is disk tilt, which can be caused by warping or an uneven spindle. This can cause distortion or even the disk to come grinding to a halt. CDs are very susceptible to it. DVDs overcame this problem by using two layers of polycarbonate instead of one, each at a thickness of 0.6mm. The steeper angle created by the shallower substrate is therefore less affected by 'wonky' media.
The final factor that determines recording density is the numerical aperture (na) of the lens: the higher the aperture, the higher its resolution. CDs use 0.5na, DVDs 0.6na and Blu-Ray 0.85NA.
Blu-Ray
Blu-Ray was officially announced in February 2002 as the culmination of several companies' efforts to develop blue laser technology. Nichia Industries invented blue diode lasers in 1995 and its technology-partner Sony unveiled it to the public in 2000 under the codename 'DVR-Blue'.
Blu-Ray disks will come in two formats: the rewriteable Blu-Ray RE and the read-only Blu-Ray ROM that comes without a cartridge.
So will DVDs become obsolete in 12 months, and will you need to buy a new player? No. Blu-Ray disks are unlikely to replace commercially produced DVDs, as even Sony is calling Blu-Ray a recording format...
Unlike DVDs, Blu-Ray disks use a 0.1mm-thick read layer which, according to its makers, diminishes disk tilt aberration and allows for a better readout and increased recording density. This also means the Aluminium layer is a lot closer to the surface than in DVDs or CDs. As a result, the disks are very sensitive to dirt and grime. Due to the shallow 0.1mm optical layer the writeable disks need to be housed in a protective shell, as any particles between the laser and the disk surface will render it almost usele
Issue: 137 | June, 2012