The laser instability

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Abstract
Many physical systems exhibit a transition from a disordered state to an ordered one following the change of an externally controlled parameter. Among these cooperative phenomena are the phase transitions of matter, the development of a convection pattern in a fluid layer heated from below—and the onset of lasing in a laser. Ordering phenomena of systems in thermal equilibrium have been known for a long time; their properties near the transition point were studied intensely in the last decade. A wealth of experimental data has shown marked similarities among what appear to be very different phase transitions: A fluid near its critical point behaves very much like a ferromagnet near its Curie point, or to a binary liquid mixture near the consolution point. To see these similarities we must make the appropriate choice for the corresponding variables. A considerable amount of work has been devoted to explaining the simple, universal behavior of thermodynamic systems in the region of the critical point.