  We present a theoretical and experimental study of multiple pulse formation
in passively mode-locked (PML) 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.
