Materials for TMI mitigation

Power demands from high beam quality fiber-based lasers have reached levels where nominally weak optical nonlinearities now limit continued scalability. Amongst the parasitic nonlinearities, transverse mode instability (TMI) is especially problematic because it is the dominant scaling limitation. To manage optical nonlinearities, the fiber laser community has singularly focused on large mode area (LMA) designs to spread the optical power out over a larger cross-sectional area and reduce the effective intensity to increase nonlinear thresholds. Such LMA designs are necessarily multimode and, so, TMI, while not necessarily predictable, was not surprising in hindsight. This paper will focus on an alternative and complementary approach; one where nonlinearities are managed materially through understanding and judicious design of the glass compositions from which the fiber is comprised. Indeed, optical nonlinearities are fundamentally light-matter interactions and so attacking them through the 'matter' component is the purest approach. A further benefit of a materials approach to mitigating nonlinearities is that multiple nonlinearities, e.g., SBS and TMI, can be simultaneously reduced while permitting a much simpler fiber design, which aids in manufacturability and cost. In other words, discussed here, is a materials approach to larger core, simple step-index fibers bypassing TMI. This paper highlights several material approaches to specifically mitigating TMI including < 1% quantum defect fiber laser compositions, power-scaling in intrinsically low thermo-optic core fibers, and novel fullypassive "thermally self-single moding" LMA fibers that intrinsically become single moded as the fiber lases and reaches its operating power and temperature.