  Recently it was suggested that Hawking radiation may be observed in a
nonlinear electromagnetic waveguide upon propagation of an optical pulse. We
show that the spectral characteristics of the Hawking effect in such a
waveguide are indistinguishable from the well-known effect of frequency
broadening of an optical pulse due to self-phase modulation. Furthermore, we
derive an estimate on the critical optical power at which Hawking effect is
dominated by the self-phase modulation. It appears that optical experiments
reported so far are clearly dominated by self-phase modulation.
