Large chirped-pulse phi-OTDR for single-shot to day-long nano-strain measurements with high dynamic range

Chirped pulse phase sensitive optical domain reflectometry (CP-ΦOTDR) provides a simple but efficient direct-detection alternative to traditional phase-measuring DAS, with fundamentally different trade-offs and limitations. While the technique has been extensively used both academically and commercially in the ≈1GHz regime, so far, the use of large chirps has been largely avoided due to the cost/complexity increase. In this work, the performance limits of CP-ΦOTDR are investigated when operating with large optical chirps (8GHz). Preliminary results display a supra-linear increase of the dynamic strain range with the optical chirp content, as well as disruptive increase of the long-term performance, with a dramatic decrease of instrumental 1/f noise which is essentially eliminated down to mHz, whilst measuring perturbations equivalent to 100’s of MHz of frequency shift. Single-shot strain variations of ±3με at 1kHz trigger, <100 pe/√Hz ASD noise floor and an estimated instrumental nanostrain uncertainty over hour-long measurements were readily obtained.