Phase-based, high spatial resolution and distributed, static and dynamic strain sensing using Brillouin dynamic gratings in optical fibers

Arik Bergman,Moshe Tur,Tomi Langer

doi:10.1364/oe.25.005376

Abstract

A novel technique combining Brillouin phase-shift measurements with Brillouin dynamic gratings (BDGs) reflectometry in polarization-maintaining fibers is presented here for the first time. While a direct measurement of the optical phase in standard BDG setups is impractical due to non-local phase contributions, their detrimental effect is reduced by ~4 orders of magnitude through the coherent addition of Stokes and anti-Stokes reflections from two counter-propagating BDGs in the fiber. The technique advantageously combines the high-spatial-resolution of BDGs reflectometry with the increased tolerance to optical power fluctuations of phasorial measurements, to enhance the performance of fiber-optic strain sensors. We demonstrate a distributed measurement (20cm spatial-resolution) of both static and dynamic (5kHz of vibrations at a sampling rate of 1MHz) strain fields acting on the fiber, in good agreement with theory and (for the static case) with the results of commercial reflectometers.

Highlights

Brillouin dynamic sensing is of importance in many applications [1]
The SA techniques employ a tunable laser source (TLS) adjusted to the linear region of the slope of either the reflection spectrum of a fiber Bragg grating (FBG) [8] or the intrinsic Brillouin gain spectrum (BGS) [9], such that changes induced by measurand variations are translated to changes in the measured quantity
Problems still remain and new ones are frequently discovered, as evidenced by [13], where it was shown that the BGS linewidth broadens with increasing pump power, which affects the performance of the slope-assisted Brillouin optical time-domain analysis (SA-Brillouin Optical Time Domain Analysis (BOTDA)) techniques, indicating an additional drawback of techniques based on the direct detection of optical power

Summary

Introduction

Recent implementations of the Brillouin Optical Time Domain Analysis (BOTDA) [2] and Brillouin Optical Correlation Domain Analysis (BOCDA) [3] techniques, have demonstrated sampling rates of the order of kilohertz's with a centimetric spatial resolution (10cm over a range of 145m for the fully distributed case of [2] and 3cm over 6m for the random access approach of [3]). Slopeassisted (SA) techniques, using a single (or at most a few) pair(s) of pump and probe frequencies can be much faster As such, they have played a key role in taking the Brillouin distributed fiber optic sensing to the fast dynamic regime [1, 4], including demonstrations of its practical utilization for monitoring the propagation of mechanical waves [5,6] (for the use of slope-assisted interrogation of a fiber-Bragg grating see [7]). Problems still remain and new ones are frequently discovered, as evidenced by [13], where it was shown that the BGS linewidth broadens with increasing pump power (with obvious ramifications on its shape and slopes), which affects the performance of the slope-assisted Brillouin optical time-domain analysis (SA-BOTDA) techniques, indicating an additional drawback of techniques based on the direct detection of optical power

Methods

Results

Conclusion