New possibilities of higher-order mode filtering in large-mode-area photonic crystal fibers

We discuss both theoretical and experimental aspects of modal discrimination phenomenon that takes place in largemode- area photonic crystal fibers. A few special fiber designs providing efficient higher-order mode filtering were implemented and investigated. First adaptation had the core comprised of 7 elements (instead of 1) with a view to reduce the pitch, since smaller pitches correspond to lower bend-induced losses. That measure aided to realize a series of fibers with a 35-75 μm core diameter propagating only the fundamental mode within a wide spectral range due to embedded leakage channels for the higher-order mode which losses were rated to be above 1 dB/m. Second variation included the fiber with circularly distributed air holes surrounding a core of 30-50 μm in diameter. Circular geometrical configuration enabled leakage losses of the higher-order mode to be 120 times larger than leakage losses of the fundamental mode. Third adaptation had the alternation of large and small air holes (C6V symmetry converted to C3V symmetry) resulting in partial or complete delocalization of the higher-order mode power outward a core region. Fourth design represented the regular triangular-lattice structure with a core of 35-60 μm in diameter shifted from its usual location in the center of the lattice. The main idea consisted in provoking an enhancement of the higher-order mode discrimination, as higher-order mode has a larger field near to the air-hole silica interfaces compared to fundamental mode. Those fibers demonstrated distinguished bending resistance properties, since could be exploited with a bending radius of 2-3 centimeters.