QUANTUM-NOISE IN 2-LEVEL AND 3-LEVEL MODELS OF THE LASER

We compare the quantum fluctuations in two commonly used models of the laser, the Haken-Lamb model [H. Haken, Laser Theory (Springer-Verlag, Berlin, 1984)] and the Lax-Louisell model [W. H. Louisell, Quantum Statistical Properties of Radiation (Wiley, New York, 1973)]. In the Haken-Lamb model the laser medium is taken to consist of two-level atoms whereas in the Lax-Louisell model it consists of three-level atoms. We compare the predictions of both models for the output statistics of the field in the limit where the atomic variables may be adiabatically eliminated. Our comparison is valid from near threshold to far above threshold. While the three-level model is the more realistic of the two, we find that the simpler two-level model gives good agreement for the amplitude noise, provided this decouples from the phase noise in a given system.