Quantum theory of a thresholdless laser

We develop a quantum theory of a single-mode thresholdless laser. We start from basic Heisenberg-Langevin equations of motion for the field and atomic operators, and obtain an approximate analytical solution to these operator equations. We compare the predictions of this model for the intensity and power spectrum of the field to the results of a Monte Carlo numerical simul ation of the original Heisenberg-Langevin equations, and find them in excel lent agreement. We also compare these predictions to those of a rate-equati on model, which takes into account spontaneous emission. We show that our m odel gives more reliable results in the bad cavity limit at high intensitie s. Based upon these results, we propose a simple characterization of the th resholdless behavior. Finally, we apply our model to microsphere Nd-doped l asers at low temperatures, which are promising devices for a well-controlle d thresholdless operation. [S1050-2947(99)10502-X].