  Light is shown to exhibit critical and tricritical behavior in passive
mode-locked lasers with externally injected pulses. It is a first and unique
example of critical phenomena in a one-dimensional many body light-mode system.
The phase diagrams consist of regimes with continuous wave, driven para-pulses,
spontaneous pulses via mode condensation, and heterogeneous pulses, separated
by phase transition lines which terminate with critical or tricritical points.
Enhanced nongaussian fluctuations and collective dynamics are observed at the
critical and tricritical points, showing a mode system analog of the critical
opalescence phenomenon. The critical exponents are calculated and shown to
comply with the mean field theory, which is rigorous in the light system.
