Aperture, attenuation, and far-field radiation diagrams have been studied on short lengths of commercial polymethyl methacrylate- (PMMA-) core optical fibers. The spectral attenuation of the PMMA constituting the core has been carefully measured and compared with the attenuation that may be calculated from optical properties of PMMA. Accurate extrinsic attenuation spectra have been obtained. Moreover, the dependence of the far-field radiation diagram on the fiber length and on the launching incidence of a laser beam has been studied on this side of the mode-equilibrium length. The analysis of these diagrams, first performed with the Gloge mode-coupling model, has been improved with the hypothesis that the mode-coupling processes are a result of light diffraction by structural anomalies in the core. The average size and form of these structural anomalies has been evaluated. They may be longitudinal microcracks of the PMMA coming from stress relaxation, which occurs during the fiber-drawing process.
The variation with illumination of the grain boundary (GB) barrier height EB and of the effective recombination velocity Seff is calculated by means of a self-consistent procedure which takes into account the bending of the minority carrier quasi-Fermi level in the GB space-charge region and in the GB quasi-neutral region. The GB interface states have been assumed to be uniformly distributed in a half-filled band whose width and position in the band gap can vary. Seff is nearly proportional to exp(EB/kT), only when EB is sufficiently low. For high EB, Seff is limited by the thermal velocity. The influence of the density of interface states and the grain doping concentration has been studied. The experimental results obtained with Silso–Wacker polycrystalline silicon show that the grain boundaries present different behaviors.
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