Mode area scaling for high-power fiber lasers with all-solid photonic bandgap fibers

There are very strong interests for power scaling in high power fiber lasers for a wide range of applications in medical, industry, defense and science. In many of these lasers, fiber nonlinearities are the main limits to further scaling. Although numerous specific techniques have studied for the suppression of the wide range of nonlinearities, the fundamental solution is scaling mode areas in fibers while maintaining sufficient single mode operation. Here the key problem is that more modes are supported once physical dimensions of waveguides are increased. There are two basic approaches, lower refractive index contrast to counter the increase of waveguide dimension or/and introduction of additional losses to suppress higher order modes. Lower index contrast leads to weak waveguides, resulting in fibers no longer being coil-able. Our research has been focused on designs for significant higher mode suppression. In conventional waveguides, modes are increasingly guided in the center of the waveguides when waveguide dimensions are increased. It is hard to couple the modes out to suppress them. This severely limits the scalability of all designs based conventional fibers. In an all-solid photonic bandgap fiber, modes are guided due to anti-resonance of cladding photonic crystal lattice. This leads strongly mode-dependent guidance. Our theoretical study has shown that it can have some of the highest differential mode losses among all designs with equivalent mode areas. Our design and experimental works have shown the potential of this approach for all-glass fibers with >50m core which can be coiled for high power applications.