Numerical aperture of optical fibers based on high-purity As2S3 glass: Unsteady-state modal distribution

The numerical aperture of core-clad multimode optical fibers with the core made from high-purity As2S3 glass is measured by a far-field technique usin g a cw CO laser (5.3-6.3 mu m). The lengths at which a steady-state modal d istribution is reached are determined for optical fibers with metallic coat ings, tetrafluoroethylene/difluoroethylene copolymer coatings, or immersion . Various factors affecting the numerical aperture of fibers with an unstea dy-state modal distribution are examined.