Interference and Shadow Effects in the Production of Light by Charged Particles in Optical Fibers

TL;DR

This paper explores how charged particles induce various types of light in optical fibers, analyzing interference, shadow effects, and potential applications in beam diagnostics, with a focus on the underlying physics and energy loss bounds.

Contribution

It introduces new insights into interference patterns, shadow effects, and energy loss bounds for particles interacting with optical fibers and related light sources.

Findings

Interference can produce quasi-monochromatic lines in fiber radiation.

Shadow effects reduce photon yield when particles pass parallel to a row of balls.

An upper bound on particle energy loss is proposed, applicable to various light sources.

Abstract

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.