Extending DAS operation bandwidth to LonG rAnge Millikelvin distriButed fIbre Thermometry

The interest in distributed acoustic sensing (DAS), using different implementations of Phase-Sensitive Optical Time-Domain Reflectometry (ΦOTDR), has dramatically increased in the last decade, particularly in the context of seismic signals, due to the possibility of high spatial coverages over >100km with a single fibre interrogator. However, while the possibility for high sensitivity measurements up to the vicinity of 1Hz regime have been extensively researched, potential of this technique for long-term (>24h) measurements has been so far largely unexplored.

In this paper Large Chirped-Pulse ΦOTDR (LCP-ΦOTDR) employing large chirps (8GHz) and assisted by distributed Raman amplification was used to extend traditional DAS bandwidth to day long high-sensitivity (mK) measurements over 50km of fibre.

With the use of large chirps in Chirped-Pulse ΦOTDR (extended almost one order of magnitude from the previously researched ≈1GHz to 8GHz, while retaining 10m spatial resolution), the stability of a fibre reference is greatly increased, allowing for long-term measurements without 1/f noise accumulation. Therefore, the intrinsic DAS sensitivity (sub-mK, sub-nε) observed over high frequencies (>1Hz) is maintained over much longer periods (>24h) even for several temperature variations of several ºC in the fibre.

Long range operation was achieved via distributed Raman amplification, which ensured high optical SNR over 50km of a standard single-mode fibre. The system’s nonlinearities were characterized both in the optical domain and in the dynamic strain sensing results thus ensuring an operation regime with sensor linearity and distortion free DAS measurements.

At the end of the 50km fibre (the worst point of optical SNR), calibrated dynamic strain signals (0.5Hz, 3000nε peak-to-peak) were applied to verify the system's response for traditional DAS operation.  Then, temperature variations of several ºC were tracked over 72h, with an upper bound error of a few millikelvin. Coherency was maintained even after several hour-long measurement interruptions.

Finally, it should be noted that the LCP-ΦOTDR upper bound error of a few millikelvin over 72h is a conservative estimation, since an accurate assessment of the system’s noise floor was not experimentally possible (as the LCP-ΦOTDR sensitivity exceed our own experimental capability of applying or measuring mK temperature variations in the fibre).