Microwave-Frequency OTDR for Distributed Sensing of Large Strain-Rate Perturbations

We present a microwave-frequency optical time-domain reflectometry (MF-OTDR) scheme for distributed acoustic sensing (DAS) of large strain-rate mechanical perturbations, supported by both theoretical analysis and experimental demonstration. The system employs microwave-modulated optical probe pulses injected into the fiber, where large external mechanical perturbations induce localized phase changes on the backscattered signal at the microwave frequency, and the resulting beat encodes group delay variations from which strain signals are efficiently recovered through phase demodulation, even under large strain-rate conditions.
The performance is validated by both theoretical analysis and proof-of-concept experiments, demonstrating high-fidelity recovery of strain amplitudes up to the microstrain level, reaching 1.8 microstrains, and dynamic frequencies up to the kilohertz range over multi-kilometer distances, currently demonstrated over 4 km. Compared with conventional phase-demodulation DAS, the proposed scheme extends the measurable strain range by up to four orders of magnitude at the same spatial resolution. In particular, direct detection enables linear strain quantification up to 1000-fold the saturation limit of conventional DAS under identical performance conditions. Meanwhile, the proposed scheme has a system architecture and hardware requirements highly similar to existing phase-sensitive OTDR configurations, making the two approaches complementary for constructing a sensing system that simultaneously offers high sensitivity and high dynamic range.
Such an experimentally verified increase in saturation level is especially important for earthquake early warning (EEW), where near-field strong motions may generate extremely large strain-rate signals. Peak ground strain rate is known to scale approximately exponentially with earthquake magnitude, implying that a three- to four-order-of-magnitude increase in measurable strain-rate amplitude can correspond to an increase of several magnitude units. In practical terms, while conventional DAS systems may saturate for events on the order of M3 at distances of ~10 km, the enhanced dynamic range demonstrated here could in principle extend measurable conditions toward much larger-magnitude events, thereby substantially reducing signal saturation in near-field strong-motion scenarios. The proposed MF-OTDR scheme is therefore a promising solution for distributed sensing of large strain-rate dynamic events, including strong ground motions in EEW scenarios.
To enable such performance under large strain-rate conditions, a key challenge must also be addressed: direct detection of the microwave beat introduces phase distortion within the perturbed region, as well as amplitude fluctuations and phase deviations after the perturbation, including irregular π phase jumps. The underlying mechanism of this issue is theoretically analyzed, and a corresponding data processing strategy is developed, which is experimentally implemented and addressed by a dedicated processing procedure involving phase-jump correction and smoothing to suppress amplitude anomalies. This ensures accurate phase demodulation and strain reconstruction.