We introduce a time-domain model to study the dynamics of optoelectronic oscillators. We show that, due to the interaction between nonlinearity and time delay, the envelope amplitude of ultrapure microwaves generated by optoelectronic oscillators can turn unstable when the gain is increased beyond a given critical value. Our analytical predictions are confirmed by numerical simulations and experiments.

We show that when subjected to high gain, delay-line optoelectronic oscillators can display a strongly multimode behavior depending on the feedback bandwidth. We found that this dynamical regime may arise when the bandwidth of the feedback loop spans over several hundreds of ring cavity-modes, and also when the oscillator is switched on abruptly. Such a persistent multimode regime is detrimental to the performances of this system which is normally intended to provide ultra-pure and single-mode microwaves. We experimentally evidence this multimode dynamics and we propose a theory to explain this undesirable feature.