Spectral Properties of Tricritical Lasers

Abstract

It is by now well-known that the laser threshold can be considered a non-equilibrium analogy to a second-order phase transition point in equilibrium systems.¹ With the addition of a saturable intracavity absorber, this behavior may be replaced by optical bistability, in which a hysteresis cycle is observed²; this has been explained theoretically by several authors, for standing-wave Doppler-broadened lasers³ and for running-wave homogeneously broadened lasers⁴, on the basis of semiclassical theory. It has been shown, however, that the bistable behavior is, strictly speaking, a transient phenomenon and that the presence of noise sources leads to behavior that is analogous to a first-order phase transition.^5,6 The field statistics have been modelled on the basis of both Fokker-Planck equations^4,7,8 and birth-death equations⁹ to demonstrate this analogy. It is important to point out, however, that the relaxation to this steady state takes place very slowly^7,10, through “tunneling”, and only the hysteresis-cycle behavior has been observed experimentally.