Abstract
A high-accuracy Brillouin frequency shift (BFS) measurement system for vector Brillouin optical time-domain analysis-based temperature sensor is proposed, in which double sideband modulation is used and the stimulated Brillouin scattering (SBS) gain and loss processes work together to generate a superimposed SBS phase-shift spectrum. The measurement principle is analyzed by mathematical modeling and the proof-of-concept experiment is performed by using a 100-m long standard single-mode fiber. The theoretical and experimental results reveal that the temperature sensitivity of BFS obtained from the measured SBS phase-shift spectrum is 1.059 MHz / ° C , and the measurement error of temperature is only half that in traditional single sideband-based system, which indicates that the proposed technique can realize high-accuracy temperature measurement and have huge potential in the field of long-distance and high-accuracy sensing.
Highlights
Stimulated Brillouin scattering (SBS) in silica fiber is described as a nonlinear interaction between two counterpropagating optical waves with a frequency difference of Brillouin frequency shift (BFS) mediated through an acoustic wave, which transfers energy between these two optical waves and changes their phases.[1]
The lower branch is used as a probe wave and is input to a Mach–Zehnder interferometer (MZI) in which the lower arm is used to generate the local wave through a 200-MHz down-shifted acousto-optic frequency shifter (AOFS), and the upper arm is used to adjust the polarization of the probe wave through a polarization controller (PC) to ensure the maximum visibility of the beat signal of the probe wave and local wave
A 100-m long standard single-mode fiber (SMF) is used as the test fiber and immersed loosely in a temperature-controlled water tank with the water temperature increasing from 10°C to 80°C by a step of 10°C, and the frequency of the microwave generator is changed from 10.752 to 10.952 GHz with a step of 4 MHz
Summary
Stimulated Brillouin scattering (SBS) in silica fiber is described as a nonlinear interaction between two counterpropagating optical waves with a frequency difference of Brillouin frequency shift (BFS) mediated through an acoustic wave, which transfers energy between these two optical waves and changes their phases.[1]. The measurement accuracy in the amplitude spectrum-based system is limited by pump depletion and nonlocal effect.[8,9,10,11] In recent years, the vector BOTDA (VBOTDA) system capable of measuring both the amplitude and phase-shift spectra of SBS response is introduced by Dossou et al.,[12] and is used to feature the high-order acoustic resonances that are not clear on the SBS amplitude spectrum, by the SBS phase-shift spectrum. Since the temperature- or strain-induced BFS change results in a frequency shift of the SBS phase-shift spectrum, and the BFS can be measured by scanning the SBS phase-shift spectrum and locating its zero point, the SBS phase shift is applied in BOTDA sensor to measure temperature or strain. A BOTDA sensor employing optical self-heterodyne detection and synchronous demodulation achieved a distributed phase-shift measurement over a 25-km long fiber and
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