Repeatable sensitivity of OTDR-based reentrant loop strain measurement using single-mode and graded-index fibers

Abstract

The authors constructed and tested optical time-domain reflectometry-based reentrant loop strain gauges using single-mode fibers and graded-index multimode fibers. With the single-mode fiber, using 1300 nm excitation and an amplified avalanche photodiode detector, they measured the time of flight (TOF) for 42 transits around the loop. With the graded-index fiber, usign 850 nm excitation and a photon-counting detector, they measured the TOF for 50 transits around the loop. Time delay between the first useful pulse and the last useful pulse versus known applied strain was observed. In both configurations, repeatable strain sensitivity was approximately 30 microstrains. Pulse dispersion was not appreciable in both configurations. The number of pulses whose time delays could be repeatably measured was ultimately limited by detector dynamic range in both configurations.