Dual-source distributed optical fiber sensor for simultaneous temperature and strain measurements

Abstract

A dual-source distributed optical fiber sensor system with combined Raman and Brillouin scatterings is designed for simultaneous temperature and strain measurements. The optimal Raman and Brillouin signals can be separately obtained by adjusting the powers of the two sources using an optical switch. The temperature and strain can be determined by processing the optimal Raman and Brillouin signals. The experimental result shows that 1.7• C temperature resolution and 60-µε strain resolution can be achieved at a 24.7-km distance.OCIS Recently, intensive research has been aimed at the development of distributed optical fiber sensors (DOFSs) because of their notable advantages such as continuous strain and temperature measurements, and strong resistance to electromagnetic interference and corrosion. Some primary applications have been made in monitoring large structures such as dams, tunnels, pipelines, oil wells, bridges, landslides, etc [1] . In the DOFS system based on the Brillouin optical time domain reflectometry (BOTDR) technique, temperature and strain variations can be measured using the Brillouin frequency shift combined with a change in Brillouin intensity. However, the accuracy of the intensity measurement limits the performance of long-range combined temperature and strain sensors [2] . The Brillouin frequency shift is dependent on both temperature and strain, whereas the Raman signal is only sensitive to temperature. Moreover, the intensity of the Raman signal exhibits more sensitivity to temperature (∼0.8%/ • C) [3] than the anti-Stokes Brillouin signal (∼0.36%/ • C) [2] . Thus, the Brillouin intensity can be replaced by the Raman intensity. The temperature along the fiber can be determined using the Raman intensity, whereas the strain can be computed using the Brillouin frequency shift. Commercial Raman distributed temperature sensors can now readily achieve 1• C temperature resolution for a 30-km distance [4] . Therefore, the temperature resolution and strain would be greatly improved if the Raman distributed temperature sensor is combined with Brillouin frequency measurement. In this letter, we propose a method for measuring the anti-Stokes Raman intensity and Brillouin frequency shift generated from dual sources with optimal output optical power to obtain the temperature and strain along the fiber.The DOFS system for simultaneous temperature and strain measurements is shown in Fig. 1. The seed laser light radiated from the distributed feedback laser diode (DFB-LD) is split using a 90/10 coupler, with 10% of the power used to produce the local oscillator (LO) for coherent detection and 90% of the power amplified by the erbium-doped fiber amplifiers (EDFAs). The continuous light is converted to pulse light using the acoustooptic modulator (AOM) and amplified using EDFA2. After passing through the circulator (C1), the pulse light is launched into the fiber for measurement. The polarization scrambler (PS) is used to reduce polarization noise [5] . The spontaneous Raman and Brillouin backscattered signa...