Radiation Induced by Charged Particles in Optical Fibers

The electric field of a charged particle passing through or near an optical fiber induces a transient charges and currents in the fibrer medium (1; 2). These charges and current radiates electromagnetic waves, both outside the fiber (free light) and inside (guided light). This chapter is devoted to the guided light, which will be referred to as PIGL, for Particle Induced Guided Light. If the fiber radius is large enough and the particle passes trough it, as in Fig. 1, both PIGL and oustide radiation can be considered as transition radiation and becomes Cherenkov radiation when the particle velocity exceeds that of light in the medium. This is the basis of the quartz fibre particle detectors (3–5). Let us mention two other uses of optical fibers as particle detectors : (i) as dosimeters, through the effect of darkening by irradiation (6); (ii) in scintillating glass fibers for particle tracking. Here we will consider fibers of radius a comparable to the wavelength, in which case the standard OTR or Cherenkov descriptions are not appropriate. Two types of PIGL have to be considered : Type I : The particle passes near or through a straight or weakly bent part of the fibre, far from an extremity. Translation invariance along the fiber axis is essential. Type II : The particle passes near or through an end of the fiber or an added structure (e.g., metallic balls glued on the fibre surface), which is not translation invariant.