Large CP-ϕOTDR Assisted by Raman Amplification for Long-Range Millikelvin Fiber Thermometry

The use of CP-ϕOTDR with large (8 GHz) optical chirps is researched for long-range long-term distributed fiber millikelvin thermometry. Copropagating and bidirectional first order Raman distributed amplification is tested, demonstrating operation in up to 75km of SMF with 8 m spatial resolution. For varying input optical powers, the impact of nonlinear effects is analyzed. Using the increased measurand dynamic range and long-term fiber reference stability, the system is able to resume coherent temperature tracking, without discontinuities, after multiple hour-long measurement interruptions. A 10 Hz fiber sampling ensures compatibility of operation in dynamic environments. Instrumental error is placed at sub-MHz, (equivalent to sub-millikelvin errors for SMF), considering the tracking of daily variations of several degrees. As an application showcase, positions separated by ∼500 m (in fiber length), but placed in equivalent physical locations in a fiber spool, recorded similar weeklong temperature cycles, with only a few mK of average deviation between them (while intermediate fiber positions recorded deviations of 100's of mK), reinforcing the notion of long term millikelvin thermometry. Spatial symmetries associated with different fiber spools were readily mapped by tracking the distributed temperature variation patterns.