Abstract
A hybrid phase-sensitive optical time domain reflectometry (Φ-OTDR) and Brillouin optical time domain reflectometry (B-OTDR) system which can realize simultaneous measurement of both dynamic vibration and static strain is proposed. Because the Rayleigh scattering light and spontaneous Brilliouin scattering light are naturally frequency-multiplexed, the heterodyne asynchronous demodulation of frequency shift keying (FSK) in optical fiber communications is utilized, and the demodulations of the two scattering signals are synchronized. In addition, the forward Raman amplification is introduced to the system, which not only makes up for the deficiency of spontaneous Brilliouin scattering based distributed fiber sensor, but also has the merit of the single end measurement of B-OTDR. The designed Φ/B-OTDR hybrid system has the sensing range of 49 km with 10 m spatial resolution. The vibration and strain experiments show that this hybrid system has great potential for use in long-distance structural health monitoring.
Highlights
IntroductionPhotonic Sensors the structure usually changes
As the most attractive distributed fiber sensor (DFS), phase-sensitive optical time domain reflectometry (Φ-OTDR) is successfully employed in the safety monitoring of petroleum, transportation, and structures where the vibration/sound waves need to be measured [1,2,3,4,5]
After N·T (T is the period of the light pulse, and N is the number of the light), the polarization of the local oscillator (LO) light is switched to polarization orthogonally to eliminate the polarization related gain of Brillouin scattering (BS) light by averaging the BS signal of the two LO polarizations
Summary
Photonic Sensors the structure usually changes . It is difficult to cover the ultra-low frequency for the current Φ-OTDR or distributed acoustic sensor (DAS) [2]. By detecting spontaneous BS in Φ-OTDR, a hybrid Φ/B-OTDR (Brillouin optical time domain reflectometry) is proposed to simultaneously measure vibration (dynamic) and strain (static) in one sensing system. A comprehensive analysis of vibration and strain can be realized by this hybrid system, offering more useful information for accurate estimation of structure health
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