External refractive index sensitivity of weakly tilted fiber Bragg gratings with different coating thicknesses

In this paper, we present an experimental analysis of the effects induced in the transmitted spectrum by depositing a uniform high refractive index (HRI) thin coating on weakly tilted fiber Bragg gratings (TFBGs). Experiments were conducted on two weakly TFBGs presenting different tilt angles (5 degrees and 7 degrees) covered by two different coating thicknesses using the dip-coating technique. The transmission spectra of the obtained structures were collected as the surrounding refractive index (SRI) changed in the range 1-1.47. The coated gratings present some interesting differences in their spectral evolution in comparison to bare gratings. An important effect of HRI coatings is indeed to enhance the SRI sensitivity of the spectral position of the dips corresponding to cladding modes couplings, leading to an improvement of the efficiency of previous demodulation techniques exploited in the case of bare TFBGs.