Radiative effects driven by shock waves in cavity-less four-wave mixing combs.

We investigate the frequency comb spectrum produced in an optical fiber via multiple four-wave mixing pumped in the normal group-velocity region close to the zero-dispersion wavelength. We show that the dynamics are strongly affected by shock formation. In this regime, the resonant radiation emitted by the shock waves correctly explains the enhanced spectral peaks in the comb. The resonant frequencies found by means of perturbation theory accurately fit those observed from the numerical simulation based on the generalized nonlinear Schrödinger equation.