Blu-ray Disc uses a "blue" laser, operating at a wavelength of 405 nm, to read and write data. The diodes are GaN (gallium nitride) lasers that produce 405 nm photons directly, that is, without frequency doubling or other nonlinear optical mechanisms. Conventional DVDs and CDs use red and near-infrared lasers, at 650 nm and 780 nm, respectively.
The blue-violet laser's shorter wavelength makes it possible to store more information on a 12 cm CD/DVD-size disc. The minimum "spot size" on which a laser can be focused is limited by diffraction, and depends on the wavelength of the light and the numerical aperture of the lens used to focus it. By decreasing the wavelength, increasing the numerical aperture from 0.60 to 0.85, and making the cover layer thinner to avoid unwanted optical effects, the laser beam can be focused to a smaller spot. This allows more information to be stored in the same area. For Blu-ray Disc, the spot size is 580 nm. In addition to the optical improvements, Blu-ray Discs feature improvements in data encoding that further increase the capacity[citation needed]. (See Compact Disc for information on optical discs' physical structure.)
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