A semiconductor laser that emits light in the 400-450 nm range. Although it was very difficult developing blue lasers, which emit smaller wavelengths than other colors, blue lasers became commercially available in 2001. Blue diode lasers allow for smaller pits to be used in optical discs (CD-ROMs use 780 nm pits; DVDs are 630 nm). Blue light-emitting diodes (LEDs), a related technology, were also difficult to manufacture, but became practical and widely used after the turn of the century. See Blu-ray and UDO.