A quantum theory of the spectral width of a laser with a satu-rable absorber is presented. We perform a density matrix calculation yielding the off-diagonal element ρ↓(n,n+l)(t) which is proportional to the average value of the electric field <E(t)> associated with the laser field having a large photon number n. The general linewidth theory is applied to any general laser which has saturable absorber features for a portion of the building of the laser field. We have found that the saturable absorber action contribution to the linewidth of such a laser can be substantial.#1998 Society of Photo-Optical Instrumentation Engineers. S0091-3286(98)02306-X Subject terms: quantum theory; laser; density matrix; linewidth; phase diffusion; saturable absorber. Paper 27107 received Oct. 20, 1997; revised manuscript received Jan. 30, 1998; accepted for publication Jan. 31, 1998.
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