Single-mode high-power fiber laser system

Abstract

A monolithic fiber is configured with a double bottleneck-shaped multimode (MM) core capable of supporting substantially only a fundamental mode at a given wavelength and having opposite end regions, frustoconically shaped transformer regions, which run inwards from the respective end regions, and a central uniformly dimensioned region, which bridges the transformer regions. The MM core has a refractive step-index profile which is configured with a centrally positioned dip having a variable width along the length of the fiber. The width of the dip is relatively small at the end regions of the MM core so as to support only the fundamental mode with a Gaussian profile. As the dip becomes larger along the input transformer region, it gradually shapes the Gaussian profile into the ring profile of the fundamental mode, which is guided along the central region of the MM core. The dip gradually becomes smaller along the output transition region so as to shape the ring profile back into the substantially Gaussian profile of the fundamental mode radiated from the output end region of the MM core. The refractive index profile has a ring area doped with one or more rare-earth elements and configured to amplify substantially only the fundamental mode.