Fiber-Optic Viscometer with All-Fiber Acousto-Optic Superlattice Modulated Structure

We present a viscosity sensing method based on a fiber Bragg grating (FBG), which is longitudinally excited by an acoustic transducer. The reflection spectrum of the FBG is broadened by spatial modulation of the FBG caused by the applied acoustic wave. The reflection bandwidth of the acoustically modulated FBG decreases with increasing viscosity of the liquid surrounding the FBG. The sensing mechanism relies on the reduction in spatially periodic strain in the FBG caused by the viscosity of the surrounding fluid. Experimental results show that the bandwidth of the FBG changes from 0.41 to 0.27 nm as the viscosity increases from 1.0 to 5.5 mPa·s.