Fused silica photonic crystal fiber with heavily germanium doped microinclusion in the core dedicated to couple, guide and control LP higher-order mode

We report on modeling, development, and optical characterization of fused silica photonic crystal fiber with germanium doped microinclusion placed in the middle of the core. The fiber is designed to efficiently couple and guide LP mode. It offers high optical density in the center region, large mode separation, low losses, and small dispersion with relatively flat profile for both LP and LP modes in 1-1.6 µm wavelength range. We demonstrate that by changing geometrical and material parameters of the inclusion partially independent tuning of propagation constants of individual modes is possible, what might be found is a variety of potential applications, e.g., in nonlinear optics. We also show that diffraction-limited propagation of LP mode in free space can be exploited in microscopy or lab-on-a-chip systems, where the proposed fiber can be used for light delivery.