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The most compact of lasers, the semiconductor laser usually consists of a junction between layers of semiconductors with different electrical conducting properties(1). The laser cavity is confined to the junction region by means of two reflective boundaries(1). Gallium arsenide is the most common semiconductor used(1). Semiconductor lasers are pumped by the direct application of electrical current across the junction, and they can be operated in the CW mode with better than 50 percent efficiency. A method that permits even more-efficient use of energy has been devised. It involves mounting tiny lasers vertically in such circuits, to a density of more than a million per square centimeter(4). Common uses for semiconductor lasers include compact audio digital disk (CD) players and laser printers(6).
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