Compact dip-style viscometer based on the acousto-optic effect in a long period fiber grating

An acousto-optic modulator (AOM) is used to tune the spectral properties and add a dynamic component to a long period fiber grating (LPG). Most of the physical parameters that an LPG is sensitive to, can be obtained through passive analysis of the spectral changes. Here, we show that by adding a temporal response component to the grating sensitivity, dynamic parameters such as fluid viscosity, can be extracted. By measuring the modulation signal rise time and/or the acousto-optic efficiency, we demonstrate a compact, dip-style, multi-parameter sensor, which is able to measure, among various parameters, the viscosity of a fluid in frequencies up to 700 Hz. Further, we show that taking the temporal response into consideration, the sensor has a linear response against viscosity, in contrast to average measurements based on spectral properties. The non-linear component of the average measurements arises from a relaxation of the initial acoustic perturbation on a timescale longer than the initial rise time.