Accurate determination of the thermal variation of the aperture of step-index optical fibers.
Thermal variations of the aperture of step-index optical fibers have been studied by measuring the transmitted power when a laser beam is launched with a variable incidence. For not too-long fibers only the rays launched with the higher incidence are affected. The variations of their transmittance have been accurately related to the refractive-index ones. Silicone clad silica fibers are sensitive to temperature because the silicone index varies much more the the silica index. On the contrary, due to a compensation phenomenon between index variations of core and cladding materials, PMMA core fibers have apertures that are nearly insensitive to temperature.
Mode-coupling processes in polymethyl methacrylate-core optical fibers.
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.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.