The eigenmodes or "normal modes" of many common laser cavities are in fact eigensolutions of nonhermitian or non-self-adjoint operators, and this unfamiliar circumstance leads to distinctly "nonnormal" behavior for such lasers. Elementary examples include the transverse modes in gain-guided diode lasers and geometrically unstable optical resonators, the longitudinal modes in lasers with large output coupling at one end, and the polarization modes of twisted bire-fringent cavities. The modes of these systems are all nonorthogonal in the usual power-orthogonal or energy-orthogonal sense, and as a consequence many of the conventional conclusions of classical and quantum noise theory must be substantially modified. Laser oscillators having nonnormal cavity modes are subject in partically modified. Laser oscillators having nonnormal cavity modes are subject in particular to a so-called Petermann excess noise factor or a large excess spontaneous emission per mode. These excess noise properties have been decisively confirmed by observations of greatly increased Schawlow-Townes linewidths in such lasers.
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