  A charged particle passing through or by an optical fiber induces emission of
light guided by the fiber. The formula giving the spontaneous emission
amplitude are given in the general case when the particle trajectory is not
parallel to the fiber axis. At small angle, the photon yield grows like the
inverse power of the angle and in the parallel limiting case the fiber
Cherenkov effect studied by Bogdankevich and Bolotovskii is recovered. Possible
application to beam diagnostics are discussed, as well as resonance effects
when the particle trajectory or the fiber is bent periodically.
