We have measured gain profiles by light-beating techniques for an argon-ion laser operated above threshold. The observed behavior is interpreted by developing a theory which incorporates four-wave mixing of signal and image frequencies. The gain shows the typical characteristics of a second-order phase transition. We also discuss the implications for fundamental laser noise, and predict strong correlations between the noise contributions from either side of the laser frequency.

• Received 27 June 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.1743