The modal cutoff of square-lattice photonic crystal fibers with a finite number of air-hole rings has been accurately investigated to our knowledge for the first time. By analyzing the leaky behavior of the second-order mode, we have obtained a phase diagram that describes the regions of single-mode and multimode operation as well as the endlessly single-mode regime. Furthermore, starting from these results, we have obtained the cutoff normalized frequency according to two different formulations of the V parameter previously adopted for fibers with a triangular lattice. A final comparison of the cutoff properties of fibers characterized by a square lattice and a triangular lattice has been carried out.

No abstract

No abstract

Triangular photonic crystal fibers with different core geometries have been investigated, in order to design single-mode large mode area fibers. In particular, 7-rod core fibers, obtained by removing the central air-hole and the first six surrounding ones in the transverse section, have been considered. By taking into account the leakage losses of the second-order mode, a phase diagram, which describes the boundary between the single-mode and multi-mode operation regimes, as well as the endlessly single-mode region, has been evaluated. Moreover, starting from this analysis, the cutoff normalized frequency has been calculated according to a formulation of the V parameter previously adopted for traditional 1-rod core triangular fibers. Simulation results have shown that, for a fixed air-filling fraction, 7-rod core triangular photonic crystal fibers are single-mode in a smaller wavelength range than 1-rod core ones. However, it is possible to obtain high effective area values, as well as single-mode operation, with 7-rod core fibers by considering low air-filling fractions and relatively small hole-tohole distances.

Raman amplifiers based on photonic crystal fibers, whose cross section can be designed to enhance the nonlinear properties, have been investigated in this work. The use of multiple pumps is studied in order to flatten the amplifier gain spectrum, by varying their number, power, and wavelength, yielding a flatness as low as 0.5 dB in the wavelength range between 1540 and 1572 nm.