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 simulation of the original Heisenberg-Langevin equations, and find them in excellent agreement. We also compare these predictions to those of a rate-equation model, which takes into account spontaneous emission. We show that our model gives more reliable results in the bad cavity limit at high intensities. Based upon these results, we propose a simple characterization of the thresholdless behavior. Finally, we apply our model to microsphere Nd-doped lasers at low temperatures, which are promising devices for a well-controlled thresholdless operation. [S1050-2947(99)10502-X].