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Laser - Wikipedia, the free encyclopediars6.5 Semiconductor lasers6.6 Dye lasers6.7 Free-electron lasers6.8 Exotic media
7.1 Examples by power7.2 Hobby uses
8 Safety9 As weapons10 Fictional predictions11 See also12 References13 Further reading14 External links
<span>Lasers are distinguished from other light sources by their coherence. Spatial coherence is typically expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can be launched into beams of very low divergence in order to concentrate their power at a large distance.
Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam. A
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