Genetic-Algorithm-Based Design of Large-Mode- Area All-Solid Anti-Resonant Fiber With Normal Dispersion and Single-Mode Operation in the 2 μm Wavelength Region

Recent years have witnessed much progress in the development of fiber lasers in the 2 μm region. Yet, to date, their power levels are limited by modulation instability and soliton formation attributed to the strong anomalous dispersions of fused silica in this wavelength region. Further power scaling requires a novel design of an all-solid silica active fiber that features normal dispersion by compensating the material dispersion with the waveguide dispersion. At the same time, a large mode area, low losses, single mode operation and robustness need to be maintained. In this paper, we propose an all-solid anti-resonant fiber (AS-ARF) design that meets these demands. We demonstrate that normal dispersion can be achieved in AS-ARFs at 2 μm by exploiting the Kramers-Kronig relation. To balance the desired dispersion with the other performance parameters, we optimize the design of the AS-ARFs using a genetic algorithm. The optimized AS-ARF has a mode field area of 1170 μm ² and normal dispersion over the spectrum from 1.96 μm to 2.04 μm. Within this spectrum, the maximum confinement loss (CL) of the fundamental mode (FM) is 16 dB/km and the minimum CL of the higher order modes (HOMs) is over 100 dB/km. The HOMs can be easily coupled out by bending the fiber while the FM stays in the core. For example, the CLs are over 2 × 10 ⁴ dB/km for the HOMs and below 200 dB/km for the FM at 2 μm at a bending radius of 20 cm. Moreover, the properties of the proposed AS-ARF remain favorable even under large geometric variations, showing good tolerance to manufacturing errors. We expect the proposed AS-ARF to further stimulate the development of high-power fiber lasers in the 2 μm region.