System for measuring temperature and strain separately by BOTDR and OTDR

Abstract

Brillouin optical time domain reflectometer (BOTDR) using microwave heterodyne detection is able to measure the longitudinal strain distribution along an optical fiber with high accuracy and high stability, and is thus regarded as an effective tool for structural monitoring. However, the frequency shift of Brillouin scattered light varies in proportion to the fiber's temperature as well as to the strain applied to it, and thus the measured Brillouin frequency shift simultaneously includes strain and temperature information. By combining BOTDR with OTDR, we propose a method whereby it is possible to make precise separate measurements of the temperature distribution and strain distribution along an optical fiber. This method involves making simultaneous measurements of an optical fiber's Brillouin scattering distribution and Rayleigh scattering distribution (loss distribution). The net change in the Brillouin scattering light power is then determined using the Rayleigh scattered light, which does not depend on temperature or strain. In this way, it is possible to accurately separate the temperature and strain effects by solving a simultaneous equation related to Brillouin frequency shift and Brillouin scattering light power. Since the measurement ofthe loss distribution by OTDR is affected little by polarization noise and fading noise, the net fluctuation ofthe Brillouin scattered light power can be determined with greater accuracy. We have used this method to measure temperature and strain distributions with a spatial resolution of 1 m. The strain measurements have an accuracy of±50 ??, and the temperature measurements have an accuracy of ±50C