Optical fiber distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) are considerably desirable for many important applications including oil and gas industry. Simultaneous measurements of vibration and temperature will exclude the need for two separate DAS and DTS systems, reduce overall cost, and ensure continuous real-time monitoring of these two important sensing parameters. We here devise a hybrid DAS-DTS system using a few-mode fiber (FMF). Although the system requirements for DAS and DTS are quite different, FMF is considered an ideal compromise to satisfy the requirements of the two systems.
We experimentally report a proof-of-concept demonstration of a few-mode fiber (FMF) based distributed acoustic sensor (DAS) design, aiming at upgrading the capabilities of the typical DAS that employs the standard single mode fiber (SMF). We only excite the fundamental mode at the input port of the FMF, and further, we minimize the impact of intermodal coupling within it such that the FMF operates in a quasi-single mode (QSM) state. The QSM operated FMF keeps the basic operation principle of the DAS valid and, in comparison with the standard SMF, it allows injection of higher pump peak-power before reaching the threshold power of nonlinearity. We validate our design by sensing vibration events produced by a piezoelectric transducer (PZT) cylinder. The FMF based DAS successfully figures out the locations and frequencies of these events. This reported design would enable the realization of a DAS design with longer sensing range and higher spatial resolution, in comparison to the standard SMF based DAS.
KEYWORDS: Signal to noise ratio, Acoustics, Sensors, Signal processing, Single mode fibers, Optical fibers, Interference (communication), Fiber optics sensors, Transducers
We experimentally report a normalized differential signal processing technique to improve the signal-to-noise ratio (SNR) of a fiber optic distributed acoustic sensor (DAS), in the time-domain. The introduced method is calibrated through comparing it with the typical differential method when using a noisy DAS system that includes a relatively wide linewidth laser. For this system, the normalized differential method allows measuring the vibration locations, produced by a piezoelectric transducer (PZT) cylinder, with enhanced SNR.
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