Large CP-ϕOTDR for Quasi-Static to Single-Shot Nanostrain Measurements With High Dynamic Range

Chirped pulse phase-sensitive optical time domain reflectometry (CP-ϕOTDR) presents a simple yet effective direct-detection approach for distributed acoustic sensing (DAS), offering distinct advantages and limitations compared to traditional phase-measuring techniques. Although CP-ϕOTDR has been widely adopted in both research and industry - typically in the ≈1 GHz chirp range - the use of large optical chirps has been mostly avoided due to increased cost and complexity. In this work a holistic study of the performance attainable with CP-ϕOTDR using large chirps (8GHz) is presented, allowing for a vast range of operation to be covered, from fast and large strain variations to quasi-static high sensitivity measurements. Under a single optical setup, operating at kHz acoustic sampling and with 10's of pϵ/√Hz ASD noise floor, the dynamic strain range is increased to single-shot variations of ±3μϵ (two orders of magnitude higher than traditional phase-measuring DAS for similar spatial resolutions), whilst readily demonstrating nanostrain measurements into the μHz regime. The long-term CP-ϕOTDR instrumental uncertainty is placed at sub-MHz, and differential measurement between adjacent fiber positions can be used for discounting cross-sensitivities to other effects (such as temperature), reasonably demonstrating the possibility for long-term nanostrain measurements under quiet fiber environments.