Background light scattering in optical fibers.

Abstract

At low frequencies the forward light scattering spectrum of optical fibers is dominated by several series of lines associated with thermally excited elastic eigenmodes of the cylindrical fiber structure. However in fibers where these lines are narrow, i.e., in aluminum jacketed or stripped fibers, a background light scattering component can be detected between them. This background component has been isolated and studied quantitatively. Typical homodyne-detected backscattering spectra are presented. The temperature dependence of the backscattering at frequencies of 5, 11, and 39 MHz is shown and discussed. The results suggest that the dynamic structural relaxation processes that have been implicated in ultrasonic attenuation could be directly responsible for the observed light-scattering spectrum. Fibers with germania-doped cores show up to two orders of magnitude more light scattering than ones with pure silica core and depressed index cladding. This is consistent with a modification of the glass network by the dopant, which affects the dynamics of the relaxational modes.