Mode-area scaling of helical-core, dual-clad fiber lasers and amplifiers using an improved bend-loss model

Abstract

For small-bend radii, the waveguide condition for total internal reflection is violated in a large angular spread of incident angles at the interface of the fiber core. To account for this, we derived an improved semianalytic bend-loss model that allows for the propagation of radiated fields outside the plane of the fiber bend. This new model is applied to large-mode-area helical-core fibers (which require small-bend radii) for use as high-power fiber lasers and amplifiers. In particular, the limits of scaling the mode area while maintaining good beam quality are explored. Single-mode operation is expected for a 40 μm core with a 0.10 numerical aperture, and scaling to a 100 μm core diameter is shown to be possible. Additionally, helical fibers are shown to readily perform where conventional coiled fibers are not appropriate to operate long term.