Fiber optic pH sensor with self-assembled multilayer nanocoatings on tilted FBG

A novel fiber optic pH sensor is experimentally demonstrated by coating multilayers of poly(acrylic acid) (PAA) and poly(diallyldimethylammonium chloride) (PDDA) on a tilted fiber Bragg grating (TFBG) with the layer-by-layer (LbL) electrostatic self-assembly method. The utilization of PDDA/PAA pH sensitive polymeric coating presents a variation of the thickness with the pH of the solution, which introduces the wavelength shift and transmission change in the spectrum of TFBG. Using fast Fourier transform (FFT) analysis on a selected bandwidth of the transmission spectrum, it is found that the peak amplitude of the dominant spectral fringes increases with the increasing pH value with an near-linear sensitivity of 117 arbitrary unit (a.u.)/pH unit (in the range of pH 4.66 to pH 6.02). The dynamic response of the sensor also has been studied (10 s rise time and 18 s fall time)