Extended range of repeaterless distributed acoustic sensing with coherent OTDR interrogators utilising optical amplification

Abstract

We demonstrate Distributed Acoustic Sensing (DAS) with extended distance range utilising repeaterless all-optical amplification and single-side interrogation schemes. This work addresses the need for over 200 km range of distributed fibre-optic sensing in applications where very long assets need to be monitored, e.g., subsea power cable monito ring. A commercially available DAS interrogator and an advanced prototype interrogator setup with increased system performance are used. Both are based on coherent OTDR to detect Rayleigh backscattering. Range extension is achieved by compensating optical losses with amplifying the pulse traveling down the fibre as well as the Rayleigh backscattered signal coming back to the interrogator. We do this by launching CW pump light into the sensing fibre to create a combination of distributed Raman amplification and a remote optically pumped amplifier in an erbium doped fibre. We analyse the DAS interrogator’s ability to detect acoustic events at distances between 170 km and 200 km of ultra-low loss telecom fibres in a quantitative and linear way. To this end, events are simulated by periodically modulating the length of a short fibre at those distances with a piezo fibre stretcher. Results show that the thereby created optical phase shift is correctly measured and that amplitude and frequency of the applied signal are successfully reconstructed. It is thereby proven that singleended DAS is possible with the presented interrogators and all optical amplification schemes to achieve at least 200 km range.