Radiation in waveguides and cavities: fundamental aspects of three-level system damping

A three-level ladder configuration is the most fundamental physical system required for cw laser action. In this paper we present an exact quantum calculation of three-level system radiation in simple waveguides and cavities which covers weak and strong coupling. A small effect exists even for a residual kind of cavity, a single reflecting wall. We show that lower level relaxation has the same effect on upper level spontaneous decay as cavity transmission losses. In contrast, the excitation probability of a strong two-level dipole subject to pure polarization noise differs from the three-level system. This suggests that the threshold behavior of microscopic lasers with small numbers of active atoms would depend not only on the amount but also on the origin of homogeneous broadening. Finally analytic expressions for the onset of strong coupling and the microlaser threshold are given and compared to the traditional laser threshold.