  The theory of Hawking radiation can be tested in laboratory analogues of
black holes. We use light pulses in nonlinear fiber optics to establish
artificial event horizons. Each pulse generates a moving perturbation of the
refractive index via the Kerr effect. Probe light perceives this as an event
horizon when its group velocity, slowed down by the perturbation, matches the
speed of the pulse. We have observed in our experiment that the probe
stimulates Hawking radiation, which occurs in a regime of extreme nonlinear
fiber optics where positive and negative frequencies mix.
