Currently, the power handling capability of optical fibers is primarily limited by glass damage thresholds and induced nonlinearities, including stimulated Brillouin scattering and stimulated Raman scattering. In order to mitigate unwanted nonlinear effects, a majority of high power delivery fibers have increased core sizes, which are generally used near the threshold of multimode operation. Under high power, thermal changes lead to transverse mode instabilities which degrade the overall beam quality. We have been investigating hollow core fibers based on the anti-resonant effect (ARHCF) due to their excellent guiding properties, such as low loss, large core sizes, wide transmission windows, and significantly increased optical nonlinearity and damage thresholds. Anti-resonant HCFs have significantly simpler designs compared to other microstructured fibers, namely photonic bandgap fibers, which leads to more flexibility and less complex fabrication. An ARHCF design was optimized in Comsol Multiphysics for single mode operation, low propagation loss, and low bending loss. The ARHCF was fabricated at the University of Central Florida. Initial testing has shown that power handling up to 170 W input, 0.7 GW/cm2 at the fiber facet is possible with no damage to the fiber.
Hollow core fibers have been investigated for several use cases relating to both single mode and multi-mode operation. Single mode, low-loss operation is a desired commodity in telecommunications and high-power delivery applications. Hollow core fibers can be designed with a structure that guides multiple modes in the core at low loss while also exhibiting strong stress sensitivity. In these anti-resonant hollow core fibers, perturbations to the structure such as micro-bending can efficiently couple core guided modes in short sections of fiber. This high sensitivity to structural distortion can be exploited for higher order mode generation, sensing, and for developing multimode nonlinear light sources. This work presents an investigation on using anti-resonant hollow-core fibers as a higher-order mode converter by inducing mechanical stress on the structure of the fiber.
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