Monochromaticity.Whatever the nature of the active medium, the fact that the laser cavity is resonant only for definite frequencies) limitsthe wavelength range. 3. Brightness. This is defined as the power emitted per unit area of the output mirror per unit solid angle and is extremely high compared with that of a conventional source. The reason for this is that, the solid angle over which it is distributed is very small. 4. Coherence. Conventional sources of radiation are incoherent, which means that the electromagnetic waves associated with any two photons of the same wavelength are, in general, out of phase. The coherence of laser radiation is both temporal and spatial, the coherence lasting for a relatively long time and extending over a relatively large distance. Coherence of laser radiation is responsible for its use as a source of intense local heating, as in metal cutting and welding, and for holography. Its military potential is being exploited, particularly in guiding missiles.Surgeons performing operations have found the laser as a surgical knife. There are suggestions that laser beams may ultimately replace cables in telecommunications. Finally, a whole new area of optics is being opened by lasers.
v Comprehension Check
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N. G. Basov A. M. Prokhorov Charles Hard Townes
