An evanescent field sensing platform is being pursued through excitation of cladding modes using long-period (LPGs)
inscribed in an endlessly single-mode photonic crystal fiber (ESM PCF) by CO2 laser irradiation. Core-cladding mode
coupling and recoupling has resulted in significant improvement in the evanescent field overlap throughout the cladding
air channels in the PCF-LPG, compared to the PCF alone. Our numerical simulation has shown that design optimization
of the PCF-LPG configuration can lead to a field power overlap as high as 23% with a confinement loss of less than 1
dB/m in the cladding mode.
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.
In this paper, we present the numerical analyses of modal coupling properties and resonance spectral response of longperiod
gratings (LPGs) in solid-core photonic crystal fibers (PCFs) with respect to their sensitivity to refractive indices
of measurands in the air channels using a full-vectoral mode solver combined with frequency-domain method. The
calculated results show that the wavelength resonance of a PCF-LPG can be extremely sensitive to the refractive index
change. The PCF-LPG refractive index sensors, inscribed by residual stress relaxation using a scanning CO2 laser, are
also experimentally investigated in terms of resonance shift with the PCF-LPG structure filled with sodium chloride
(NaCl) solutions at concentrations ranging from 0-26% (w/w). The spectral features were greatly improved by inscribing
the LPG structure in PCF prefilled with water. The PCF-LPG sensors are shown to be able to detect the change of ~10-7
refractive index unit (RIU) in the index range of 1.33 to 1.35, in a good agreement with the numerical simulation.
Long-period gratings (LPGs) have been fabricated in endlessly single mode photonic crystal fibers (ESM-PCFs) utilizing focused CO2 laser irradiation and residual stress relaxation technique. The responses of the ESM-PCF-LPGs to external refractive index and applied bending curvature have been investigated. As compared with the conventional fiber-based LPGs inscribed under the same condition, the ESM-PCF-LPGs exhibit higher sensitivity to external refractive index change and macro-bending, making them attractive candidates for chemical and biological sensing applications.
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