You need two lasers (as JesterK points out) to read CDs and DVDs on the same drive - for instance a DVD-ROM drive.
This is because a DVD laser reads the information from the disk using red lasers (630 to 650 nm); to increase areal density there would have to be a switch to shorter-wavelength blue or violet lasers with wavelengths down to 400nm.
This would make it possible to reach a total of approximately 15GB of data per layer per side. To achieve higher levels of data per side per layer even shorter UV range lasers would be needed.
It seems, however, that compact, reliable and inexpensive short-wavelength lasers are much more difficult to make.
A CD laser's wavelength is around 780mm but there is some good news on the horizon.
Matsushita has developed the first two-wavelengths laser array containing two semiconductor lasers.
One is a 100mW high-power infrared laser for CD-R recording at 8x speed and the secnd a 10mW low operating current red laser for DVD playback.
The technology enables integration of the optical pickup used for high-speed CD-R recording/playback and DVD playback.
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