Detailed observation of modulation instability on THz NovoFEL

Detailed experimental observations of the modulation instability in the frequency and time domains were made on the THz NovoFEL by a Fourier spectrometer and an ultrafast Schottky diode detector. The regime stabilized by negative detuning between the electron and light frequencies is shown to transform into a multimode one with coherent modulation of the light pulse. Further decrease in the stabilization factor leads, for a high gain, to an unstable regime at the resonance frequency. In this regime, a light pulse splits into incoherent sub-pulses with a duration equal to the slippage length divided by the light velocity. The generation of one short sub-pulse is possible for small positive detuning.