Distributed Acoustic Sensor for Liquid Detection Based on Optically Heated CO 2+ -Doped Fibers

In this work, we propose the use of Coherent Optical Time-Domain Reflectometry (C-OTDR) in order to discriminate the medium surrounding one or more Co2+-doped fiber sections. The proposed approach can be used to perform vibration measurements along the whole fiber by means of C-OTDR measurements at 850 nm, as well as liquid detection in selected positions by active thermometry at 1550 nm. By pumping the fiber with a modulated optical pump, the induced fiber temperature modulation is monitored through C-OTDR, as the latter is sensitive to optical phase changes occurring along the sensing fiber. As the induced phase modulation (i.e. the C-OTDR signal) is affected by the thermal conductivity of the surrounding medium, the latter can be retrieved from the phase signal. In our tests, a 5 Hz modulation frequency was chosen to discriminate between air, water, and acetone.