Comparison of zoned microstructure fiber geometries for low-dispersion waveguiding

This paper presents a comprehensive analysis of zoning in microstructure fiber (MSF) in which parabolic and Gaussian graded-index (GRIN) refractive-index profiles are compared in both continuous and zoned geometries. Ray trajectories were calculated using Fermat's principle of least time and the paraxial approximation. Optimization of the zoned MSF refractive-index profile revealed that a piecewise Gaussian refractive-index profile exhibits aberration of just 10-nm on-axis focal variation, compared with 40 nm in the zoned parabolic case. In addition, a quarter-period length of the Gaussian-zoned MSF has a 630-nm theoretical spot-size, thus offering efficient coupling between standard single-mode fiber and photonic-crystal devices. A preliminary analysis of a binary radially chirped Bragg fiber geometry is performed using an eigenmode expansion of Maxwell's equations. Its simpler geometry offers fabrication advantages, but its spot size is closer to 1.2 /spl mu/m due to its 41-/spl mu/m quarter-period focal length, and it suffers greater waveguide dispersion compared with the optimized zoned GRIN MSF geometry presented here.