Analogy between the Laser and Second-Order Phase Transitions: Measurement of "Coexistence Curve" and "Susceptibility" for a Single-Mode Laser near Threshold

We report the first experimental determination of the laser "coexistence curve" and "susceptibility" below threshold. Both quantities are derived from an accurate measurement of the average output intensity as a function of the normalized net gain. Our data confirm the predictions of laser theory and give a quantitative support to the recently proposed analogy between the laser near threshold and second-order phase transitions.