  We theoretically and experimentally investigate the chaotic regime of optical
frequency combs generated in nonlinear ring microresonators pumped with
continuous wave light. We show that the chaotic regime reveals itself, in an
apparently counter-intuitive way, by a flat top symmetric envelope of the
frequency spectrum, when observed by means of an optical spectrum analyzer. The
comb demodulated on a fast photodiode produces a noisy radio frequency signal
with an spectral width significantly exceeding the linear bandwidth of the
microresonator mode.
