Temperature and refractive index sensing with Al2O3-nanocoated long-period gratings working at dispersion turning point

In this work we discuss the effect of nanocoating a long-period grating (LPG) with aluminum oxide (Al2O3) on both its refractive index (RI) and its temperature (T) sensitivity. Two LPGs, one coated and one uncoated, were optimized to work at the dispersion turning point (DTP) of the higher-order cladding modes, where the sensitivities are the highest. The DTP was reached by two methods - wet etching of the fiber cladding and use of an optimized Al2O3 nanocoating applied by atomic layer deposition (ALD). In both cases we show a significant increase in RI sensitivity at the DTP. Thanks to an additional mode transition (MT) effect, the RI sensitivity of the coated LPG reaches about 8200 nm/RIU in the range nD = 1.333-1.345 RIU traced for only one resonance of the pair. This is more than 3.5 times higher than for the non-coated LPG. Moreover, T sensitivity for the Al2O3-coated LPG was found to be 2.5 times higher than for the non-coated LPG. When the MT effect alone is applied, i.e., the LPG works away from the DTP, the nanocoated LPG may offer higher RI and lower T sensitivity than a standard non-coated LPG working at the DTP. The differences in sensitivity are attributed to the order of coupled cladding modes and the thermo-optic coefficients of both Al2O3 and the external medium. We believe that by proper selection of the nanocoating material, the T sensitivity can be greatly reduced, a major advantage for future biosensing applications of LPGs.