Efficiency enhancement of cladding-pumped fiber laser structures

Cladding-pumped fiber lasers have been developed in the last years to high-power laser sources (more than 100 W) with excellent beam quality. The efficiency of these lasers depends mainly on the index geometry of the used fibers. A disadvantage of most structures is their noncircularity, which makes coupling to round standard fibers difficult. In this paper, we first classify experimentally different cladding structures by carefully analyzing absorption measurements, secondly we propose and test a new possibility to enhance the mode coupling properties maintaining the round fiber structure, namely the use of a spatially varying index profile in the cladding region.