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We experimentally demonstrate critical behavior of a passively mode-locked laser with properties that are similar to those of gas-liquid and magnetic spin systems. The laser light modes provide a special nonthermodynamic many-body system where noise takes the role of temperature. It is also a rare opportunity of an experimental pure one-dimensional system. As theoretically predicted, we identified in the laser light-mode system two thermodynamiclike phases, one characterized by spontaneous pulses and the second by field-induced parapulses, separated by a first order phase transition boundary that is terminated by the critical point. We also measured the critical exponents, β≈0.52, γ≈1, and δ≈3.1, which are close to the mean field values that are exact in the laser system.

We present a theoretical and experimental study of multiple pulse formation in passively mode-locked lasers. Following a statistical-mechanics approach, the study yields a thermodynamic-like "phase diagram" with boundaries representing cascaded first order phase transitions. They correspond to abrupt creation or annihilation of pulses and a quantized rf power behavior, as system parameters (noise and/or pumping levels) are varied, in excellent accordance with the experiments. Remarkably, individual pulses carry an almost constant quantum of energy.