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14 May 2010Optimal design of broadband photonic crystal fibre long-period gratings for evanescent absorption sensing
An index-guiding solid core photonic crystal fibre (PCF) is numerically optimized to achieve through an inscribed
long period grating (LPG) broadband coupling between the fundamental mode (LP01) and the first-order symmetric
cladding (LP02-like) mode. The vectorial finite element method and the Nelder-Mead simplex method
are used for the optimization of the PCF to get for LP01-LP02 mode coupling the phase matching curve with
the dispersion turning point located in the center of a selected range of resonant wavelengths. The bandwidth
of a LPG close to the dispersion turning point, where the resonance condition is nearly matched for multiple
wavelengths, is large. An evanescent power overlap of the LP02 cladding mode with the PCF's air holes is an
order of magnitude higher then that of the LP01 mode. We optimize the PCF with only five rings of hexagonally
arrayed air holes. By enlarging the air holes in the outmost ring the LP02 mode confinement loss is reduced to
a negligible value which allows to lengthen the cladding mode interaction length with an analyte infiltrated into
the air holes.