Hybrid BOTDA/FBG sensor for discrete dynamic and distributed static strain/temperature measurements

Abstract

ABSTRACT We propose and experimentally demonstrate a hybrid fiber optic sensing technique that effectively combines Brillouin optical time-domain analysis and a time -domain multiplexing interrogation technique for Fiber Bragg Gratings (FBGs). The highly-integrated proposed scheme employs broadband apodized low-reflectivity FBGs with a single optical source and a shared receiver block, allowing for simultaneous measurements of distributed static and discrete dynamic temperature and strain, over the same sensing fiber. Keywords: Brillouin scattering, Fiber Bragg Gratings, fiber optic sensors, temperature sensing, strain sensing. 1. INTRODUCTION During the last years, distributed optical fiber sensing based on stimulated Brillouin scattering (SBS) has become an interesting approach for temperature and strain sensing in many industrial applications. In particular, distributed sensing with meter and sub-meter spatial resolution along several tens of km has been demonstrated using Brillouin optical time-domain analysis (BOTDA) technique [1]. Even though the sensing capabilities provided by BOTDA systems are attractive in many applications, the relatively long measurement time (of the order of a few minutes) makes this technology mainly suitable for static measurements, limiting its potential range of applications. Although the distributed information provided by BOTDA sensing is essential for static strain and temperature monitoring, there are several applications, such as for example in industrial Oil&Gas production plants monitoring, which also require dynamic strain measurements at specific critical points, ad ding in this way crucial information on the structure integrity (e.g. vibrations). Distributed sensing based on either spontaneous Brillouin scattering (SpBS) or SBS has been recently combined with point based Fiber Bragg Grating (FBG) sensors [3,4]. In particular, BOTDA technique has been combined with wavelength-division-multiplexed FBG point sensors, using different demodulation techni ques for distribute d and discrete sensing, as well as distributed Raman amplification, increasing cost and complexity of the whole sensor system [4]. In this paper we propose a highly-integrated hybrid sensing system that effectively combines the advantages of both standard BOTDA and in-line time-division-multiplexed (TDM) FBG-based sensing, allowing for simultaneous dynamic discrete strain measurements and distributed (static) strain/temperature sensing using a single optical fiber, a common pulsed narrowband optical source and a shared receiver unit.