Optimal 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.