Sub-centimeter spatial resolution dynamic strain sensing using time-expanded ΦOTDR

Abstract

Time-expanded phase-sensitive (TE-φ )OTDR is a distributed optical fiber sensing (DOFS) technique that takes advantage of the dual-frequency comb technology to offer distributed, dynamic, and high-spatial resolution measurements. The performance delivered by this recent approach is unmatched by any other DOFS, combining the high resolution of OFDR with the potential for long range and fast sampling of φ OTDR. In this contribution, we present an optimized TE-φ OTDR scheme with important improvements with respect to the traditional one. In particular, the new architecture uses electrooptical phase modulation instead of intensity modulation, increasing the energy-efficiency. Additionally, it employs an optical hybrid to double the spectral efficiency of the system, which in practical terms results in doubling the spatial resolution for the same interrogating comb bandwidth. The proposed architecture has been experimentally validated through a scheme providing 5 mm of spatial resolution, 80 m of range and 70 Hz sampling rate with a simple, compact and low-cost setup using field-programmable gate arrays (FPGA) and relatively low bandwidth photodetection (2 MHz).