Effective Single-mode Fibers with Large Mode Areas Through Intermodal-coupling

Abstract

With the rapid progress of fiber lasers, there is an increasing demand to develop optical waveguides with large mode area to reduce non-linear affects such as stimulated Raman scattering. This is most easily attained by increasing the core size. However, a larger core supports more higher-order modes which in turn worsen the beam quality. The inclusion of a ring surrounding the core (and within the cladding), combined with coiling, is one known method for removing higher-order modes. In this work, we further this approach by first coupling the core modes to the ring, and subsequently inducing losses through implementation of an absorbing material. However, such single-ring design only allows for coupling with one of the two core LP11 modes, leaving the second unaffected. We proposed a windmill fiber design that is able to simultaneously couple to both LP11 modes. It consists of four annular sectors that strongly couple to the targeted core modes, resulting in an effective single-mode fiber. Importantly, the windmill design is optimized to increase its tolerance to fluctuations in the refractive indices to within current fabrication limits of 1x10-4, and proves itself to be more effective than the single-ring design for out-coupling of higher-order modes.