  A charged particle passing through or near a narrow optical fiber induces, by
polarisation, coherent light guided by the fiber. In the limit of zero crossing
angle, the radiation tends towards a Cherenkov radiation with a discrete
spectrum, studied by different authors. If the particle crosses a bent fiber at
regularly spaced points, interference gives quasi-monochromatic lines. If the
particle passes near an end of the fiber, light is produced by the capture of
virtual photons through the end face. An alternative way consists in sticking a
metallic ball to the fiber: the passing particle induces plasmons which are
then evacuated as light in the fiber. Interferences can occur between lights
from several ends or balls. Applications of these various light signals to beam
diagnostics are discussed. The shadow effect, which reduces the photon yield
when the particle runs parallel to a row of balls, is pointed out and an upper
bound $-dE/dz\le C(Ze/b)^2$ for the particle energy loss is conjectured ($Ze$
is the particle charge, $b$ the impact parameter and $C$ a numerical constant).
This bound should also apply to other kinds of light sources, in particular to
Smith-Purcell radiation.
