Continuous and Discontinuous Phase Transitions in the Laser with a Saturable Absorber

Abstract

It is well known theoretically that the laser with a saturable absorber (LSA) is capable of exhibiting nonequilibrium phase transitions, analogous to the mean field first-order (discontinuous) and second order (continuous) phase transitions in equilibrium systems1,2. At the intersection of the first order and second-order line the LSA provides an example of nonequilibrium tricritical behavior2,3. Experimentally, the tricritical behavior in the LSA has not been observed. Some evidence for its existence comes from the experiments of Ruschin and Buer4. Evidence for first-order phase transition has come from the observed hysteresis in the output of the laser5. However, because of long relaxation times of of metastable states involved the first-order phase transition has not been observed directly. The second-order phase transition in the laser has already been observed6. By adding an intracavity Ne absorber tube in the He:Ne laser we have succeeded in observing both the first-order and the second-order phase transitions in the LSA. With our system we have also been able to study the tricritical behavior in the LSA.