6.4 Fibre-optic models
A recent implementation of analogue models based on electrodynamics is that based on
fibre-optic engineering [504
, 505
]. The basic idea in this case is to use long dispersive light
pulses (solitons), generated with a suitable laser, to create a propagating front at which the
refractive index of the fibre changes suddenly (albeit by a small amount). Basically, the refractive
index of the fibre,
, acquires a time- and position-dependent correction
, which is
proportional to the instantaneous pulse intensity
at a give space-time position,
. The
wavefront at which this change in the refractive index occurs will move naturally at a speed close
to the speed of light (and fibre optic engineering allows one to control this feature). If one
now sends a continuous wave of light, what we might call a probe, along the fibre in such a
manner that the probe group velocity in the fibre is arranged to be slightly larger than the pulse
group velocity, then it will be possible to obtain horizon-like effects. In fact, as the probe wave
reaches the back of the pulse, the increase in the refractive index will slow it down, until the
probe group velocity will match the pulse one. Effectively, the rear end of the pulse will act as a
white hole for the probe wave. Similarly, there will be a point on the front side of the pulse
where the two group velocities will match. This will be the equivalent of a black-hole horizon for
the probe wave. In [504, 505] the behaviour of the probe waves at the pulse was investigated,
and it was shown for the white hole case that the expected classical behaviour is theoretically
reproduced. Since this behaviour lies at the core of the mechanism responsible for the mode
conversion underlying the Hawking effect, it is then expected that the quantum counterpart
should also be reproducible in this manner. Indeed, very recently Belgiorno et al. have reported
experimental detection of photons from a black-hole–white-hole configuration possessing a “phase
velocity horizon” [66
]. The underlying theory behind their specific experiment is considered
in [63, 64].