  We analyze theoretically and numerically the nonlinear process of pulse
formation in mode locked lasers starting from a perturbation of a continuous
wave. Focusing on weak to moderate dispersion systems, we show that pulse
growth is initially slow, dominated by a cascade of energy from low to high
axial modes, followed by fast strongly nonlinear growth and finally relaxation
to the stable pulse waveform. The pulse grows initially by condensing a fixed
amount of energy into a decreasing time interval, with peak power growing
toward a finite-time singularity that is checked when the gain bandwidth is
saturated by the pulse.
