Joint phase and birefringence estimation in MIMO-DAS for dynamic strain anisotropy discrimination and static fiber characterization

Optical fiber sensing has become increasingly important to provide continuous monitoring of optical fiber cables as well as their surrounding environment. In this context, Distributed Acoustic Sensing (DAS), based on Rayleigh scattering, has gained momentum and is now widely used to detect vibrational events for infrastructure monitoring and security, characterization of geophysical phenomena, or industrial process monitoring in energy, oil and gas, and smart city environments…

Our work focuses on multi-parameter estimation to detect and characterize a wider range of perturbations and gain knowledge about their nature. Using a coded-interrogation Multiple-Input-Multiple-Output Distributed Acoustic Sensing (MIMO-DAS) architecture, we estimate the Jones matrices describing the round-trip propagation along the optical fiber. From these matrices, we extract through post-processing two parameters of interest for each fiber section of 1.3m on average, independently of adjacent sections. These quantities are the common or polarization-averaged differential phase, representing the phase delay introduced by the fiber section and common to both polarization tributaries, and the retardance, corresponding to the phase shift between the two eigenpolarizations introduced by the fiber section, proportional to the effective birefringence magnitude.

Compared to conventional differential phase Optical Time-Domain Reflectometry (ΔΦ-OTDR) that focuses solely on differential phase estimation or Polarization Optical Time-Domain Reflectometry (P-OTDR) that considers polarization-related properties only, our approach aims at providing further information on environmental events by allowing a joint estimation of phase and polarization effects. Moreover, the coded-interrogation scheme eliminates the need to send several input State of Polarization (SOPs) and enables the coexistence of sensing and data transmission.

First, we demonstrate the detection and localization of dynamic strain events of frequency up to 3 kHz on standard single-mode fibers with a mean spatial resolution of 1.3m, and show the ability to discriminate between purely axisymmetric strains and anisotropic strains. Indeed, while the differential phase is sensitive to both kinds of events, the retardance is only responsive to perturbations that break cylindrical symmetry. Second, we validate our model through experiments in two scenarios: in the presence of longitudinal strain and anisotropic transverse strain. In addition, we use our system to estimate the effective birefringence magnitude along the fiber length in static conditions, providing insights into the fiber characteristics and its surrounding environment.  In the future, we foresee exploring the advantages of our technique over various field-deployed fiber cable configurations.