Analysis of the effect of air-hole shapes on the propagation properties of photonic crystal fibers with square lattice

Abstract

The solid core square lattice Photonic Crystal Fibers (PCFs) with circular, elliptical and rectangular air holes have been studied using Full Vectorial Finite Element Method in this research work. The PCFs studied in this work have a core formed by a missing air hole at the center in a silica background and a photonic crystal cladding with a square lattice having 5 rings of air holes around the core. The dispersion, effective mode areas and losses are computed and compared for each structure with the fixed hole dimension and fixed pitch length. Same pattern of dispersion curve is observed for each of the structures. Investigated mode field in all cases shows that the fundamental mode is always confined in the fiber core. In the circular hole PCF, the zero dispersion point is shifted very near to 1550nm when its pitch and diameter are fixed at 2.0 μm and 1.15221 μm i.e.it has minimum value of dispersion which is -0.3998 ps/nm-km. at 1550nm. Thus it can be used as dispersion shifted fiber to allow a communication system to possess both low dispersion and low attenuation.At 1550nm wavelength, the value of chromatic dispersion obtained for circular holes PCF with a pitch 2.0 μm and a hole diameter 1.158 μm is 1.5277 ps/nm-km. For the same value of pitch, the obtained chromatic dispersion is 2.0369 ps/nm-km when hole diameter is 1.16379μm. For an elliptical hole PCF, dispersion of 2.5199 ps/nm-km is obtained at the same wavelength when pitch is 2.1μm, major and minor axis values are .8μm and .42μm respectively. For rectangular hole PCF, dispersion of 3.8256 ps/nm-km is obtained at the same wavelength when pitch is 2.1μm, length and width are 1.62 μm and .65 μm respectively at 1550 nm. These low but positive value of dispersion can be used to avoid unwanted effects of four wave mixing between WDM channels and thus these fibers can be used as “Non-zero dispersion-shifted fibre” (NZ-DSF). Birefringence and nonlinear coefficients were calculated for each structure. The structures show a very low value of birefringence and very high value of nonlinear coefficient at lower wavelengths. The highest value of nonlinear coefficient obtained in this study is 24.1658W-1km-1 which is found for PCF with elliptical holes.