Design of photonic crystal fiber long-period grating refractive index sensor

Numerical optimization of photonic crystal fiber (PCF) structures for refractive index sensors based on long period gratings inscribed in PCFs has been performed. The optimization procedure employs the Nelder-Mead downhill simplex algorithm. This direct-search method attempts to minimize a scalar-valued nonlinear function of N real variables (called the objective function) using only function values, without any derivative information. An inverse design approach utilizes the objective function constructed using desired sensing characteristics. For the modal analysis of the PCF structure a fully-vectorial solver based on the finite element method is called by the objective function. The dispersion optimization of PCFs is aimed at achieving a high sensitivity of measurement of refractive index of analytes infiltrated into the air holes for the refractive index and probe wavelength ranges of interest. We have restricted our work to the index-guiding solid-core PCF structures with hexagonally arrayed air holes.