High-sensitivity refractive index sensor based on low order cladding mode resonance in long-period grating with high refractive index coating

Abstract

Deposition of a high refractive index (RI) overlay on long-period grating (LPG) was recently studied to enhance the sensitivity of LPG sensor to the surrounding ambient RI changes and offset the sensitive operational range of LPG RI sensor to low ambient RI. The enhancement mechanism relies on cladding mode reorganization due to the existence of the overlay. In this paper, the reorganization of the cladding mode in a cladding radius reduced LPG coated with high RI overlay is studied with full vector transfer matrix method. The dependence of the cladding modes effective indices on the fiber parameters and ambient RI is systematically investigated. Based on the simulation result, an LPG refractive index sensor optimization procedure is proposed, in which the grating period is set as an adaptive parameter and the cladding radius of the LPG is reduced. It is found that by using the proposed optimization method, the notch band corresponding to low order cladding mode resonance has higher sensitivity to the ambient refractive index. It is shown that an ambient refractive index sensitivity as large as 4012 nm /RI can be achieved, representing a 3-fold sensitivity enhancement as compared to the best result obtained from the reported structure in which the high order cladding mode resonate notch wavelength was employed in sensing.