Fully Distributed Multi-parameter Sensors Based on Acoustic Fiber Bragg Gratings

A fully distributed multi-parameter acoustic sensing technology is proposed. Current fully distributed sensing techniques are exclusively based on intrinsic scatterings in optical fibers. They demonstrate long sensing span, but their limited applicable parameters (temperature and strain) and costly interrogation systems have prevented their widespread applications.

A novel concept of acoustic fiber Bragg grating (AFBG) is conceived with inspiration from optical fiber Bragg grating (FBG). This AFBG structure exploits periodic spatial perturbations on an elongated waveguide to sense variations in the spectrum of an acoustic wave. It achieves ten times higher sensitivity than the traditional time-of-flight measurement system using acoustic pulses. A fast interrogation method is developed to avoid frequency scan, reducing both the system response time (from 3min to <1ms) and total cost.

Since acoustic wave propagates with low attenuation along varieties of solid materials (metal, silica, sapphire, etc.), AFBG can be fabricated on a number of waveguides and to sense multiple parameters. Sub-millimeter metal wire and optical fiber based AFBGs have been demonstrated experimentally for effective temperature (25~700 degC) and corrosion sensing. A hollow borosilicate tube is demonstrated for simultaneous temperature (25~200 degC) and pressure (15~75 psi) sensing using two types of acoustic modes. Furthermore, a continuous 0.6 m AFBG is employed for distributed temperature sensing up to 500 degC and to accurately locate the 0.18 m long heated section.

Sensing parameters, sensitivity and range of an AFBG can be tuned to fit a specific application by selecting acoustic waveguides with different materials and/or geometries. Therefore, AFBG is a fully distributed sensing technology with tremendous potentiality.