Refractive Index Determination and Air-Void Characterization of BNNT Assembly Using Optical Fiber Bragg Grating Sensors

The optical and structural properties of boron nitride nanotube (BNNT) assembly were characterized using optical fiber Bragg grating (FBG) sensors. FBGs were fabricated in tapered fibers with a diameter of less than
$30~\mu $
m. After tapered fibers were etched in hydrofluoric acid solution for a few minutes, BNNTs were then deposited on the fibers through a dip-coating process. The Bragg wavelengths of the FBGs were measured before and after BNNT coating such that the effective refractive index (RI) of the guided fundamental HE11 mode in the fiber was obtained. By numerical modeling of the dependence of HE11 mode effective RI on the RI of the surrounding material, both the RI and the air void content of BNNT coating were obtained. To verify the accuracy of the modeling results, a FBG sensor fabricated in a tapered fiber was immerged in various RI standard solutions and the corresponding Bragg wavelengths were measured and compared to numerical simulations. It was shown that the experimental data agreed well with simulation results.