Abstract
In the experiments using incident optics which have been developed for launching high power YAG laser light into a fiber, the entrance portion of the fiber is overheated under some experimental conditions. In order to analize this overheat phenomena we propose expressions that the maximum laser power possible to launch into a fiber is limited by the core diameter and the NA (numerical aperture) of the fiber in terms of the maximum beam divergence angle αmax using ray matrices. The experimental αmax, above which overheating takes place was found to be larger than the calculated one by 15mrad. This difference suggested the existence of a cladding mode transmission in the fiber of the launching experiments. The experiments of cladding mode stripping proved the existence of this mode transmission. These calculations and experiments lead to a conclusion that the overheat of the fiber is attributed to the laser light escaping through the removed edge of silicon primary coat which refractive index is degraded by the heat of removal.
