Improvement of spatial resolution for strain measurements by analyzing Brillouin gain spectrum

Abstract

A Brillouin optical time domain reflectometer (BOTDR) is a fiber optic strain distributed sensing system which can measure strain along arbitrary regions of an optical fiber. We have used this system for structural health monitoring of composite structures, such as racing yachts. Although the BOTDR is useful to measure strain distributions of a large area efficiently, it is difficult to accurately measure strain changing sharply within the length of the spatial resolution. The resolution depends on the width of the pulse light emitted from the BOTDR and it is 2 m for the pulse width of 20 ns. In this paper, we propose a new technique to improve the spatial resolution of a commercially available BOTDR in strain measurements. This technique is based on analysis of the Brillouin gain spectrum along a sensing fiber. We confirmed experimentally that the BOTDR using 20 ns pulse could measure strain which has a rectangular distribution of 0.3 m in the sensing fiber.