Experimental investigation of cutoff phenomena in nonlinear photonic crystal fibers.

Modal cutoff is investigated experimentally in a series of high-quality nonlinear photonic crystal fibers. We demonstrate a suitable measurement technique with which to determine the cutoff wavelength and verify the technique by inspecting the near field of the modes that may be excited below and above the cutoff. We observe a double-peak structure in the cutoff spectra, which is attributed to splitting of the higher-order modes. The cutoff is measured for seven different fiber geometries with different pitches and relative hole sizes, and very good agreement with recent theoretical work is found.