DVD (Digital Versatile Disk) (page 7)

Red laser vs. Blue laser

Current DVD drives use red lasers (630 to 650 nm), and the "easiest" way to increase areal density is to switch to shorter-wavelength lasers, i.e. blue or violet lasers with wavelengths as low as 400 nm. This will make possible about 15 GB of data per layer per side. To achieve, say, 45 GB of data per side per layer, even shorter, UV range lasers will be needed. Still, compact, reliable, and inexpensive short-wavelength lasers are hard to make.

Three primary blue-laser technologies are available now:

ZnSe lasers
GaN lasers
second-harmonic generation (SHG)lasers.

ZnSe lasers brought the first success to the field, but these lasers have problems with relatively short life-time at the required power levels, and also are at the green end of the blue range (460 to 520 nm).

GaN In-doped lasers have already demonstrated high reliability at wavelengths as short as 370 nm and are considered to be a very promising future technology.

SHG lasers offer the best durability at the moment. This technology either doubles the frequency of a given infrared laser or directly generates a second harmonic in the blue portion of the spectrum. For example, for a given infrared laser with a wavelength of 850 nm, this technology will double the laser light frequency (using a so-called distributed Bragg reflector or DBR), and produce blue light at 425 nm.

Another issue is manufacturing tolerances for 120 mm DVD disks, which are already very high. Blue lasers with smaller bits will make this issue even more complex. Also, servo-control with smaller spot size and higher (potentially) spin rates is another issue.

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