Raman-assisted large chirped-pulse phi-OTDR for long range millikelvin distributed fibre thermometry

Chirped-pulse phase-sensitive optical time domain reflectometry (CP-φOTDR) has become of standard use in a number of distributed fiber sensing applications, with particularly promising results in long-term highly sensitive measurements. However, so far, the use of large chirps has been largely avoided due to the cost/complexity increase, with the majority of academically and commercially studies operating in the ≈1GHz regime. In this paper Large Chirped-Pulse φOTDR (LCP-φOTDR) employing large chirps (8GHz) and assisted by optimized distributed Raman amplification was used to extend traditional DAS bandwidth to day long high-sensitivity temperature measurements in the order of mK over 50 km of fiber.