Abstract
With phase-sensitive optical time-domain reflectometry (Φ-OTDR), an optic fiber can be used as a distributed vibration sensor to perceive the external disturbance along the fiber through the phase variation of the Rayleigh backscatter light. However, due to the inevitable interference fading phenomenon, the signal-to-noise ratio (SNR) of Rayleigh backscatter light at certain points is insufficient, thus affecting the fidelity of vibration sensing. In this paper, we propose the spectrum extraction and the rotated-vector-sum (SERVS) method to suppress the interference fading in the pulse-intensity-coding Φ-OTDR, which applies Golay complementary sequence through acousto-optic modulator (AOM). Benefited from the easier implementation, the experimental results demonstrate that the percentage of fading points can be reduced from 2.98% to 0.33% significantly and the standard deviation of differential phase is also decreased from 0.6556 rad to 0.2901 rad. Also the minimum level of averaged power spectral density (PSD) of the differential phase at whole locations along the fiber is reduced about 6 dB/Hz effectively. For the demodulated disturbance signal, no influence on the SNR is observed thus it maintains a SNR improvement of 8.6 dB compared to the traditional single pulse Φ-OTDR system, achieving a noise floor of 2.38 pϵ/√Hz.
Highlights
D ISTRIBUTED optical fiber sensor (DOFS) can realize distributed measurement of physical parameter variation by collecting and analyzing the specific scattered light at consecutive position along the optical fiber, such as temperatureManuscript received September 26, 2021; accepted October 13, 2021
We set up an experimental pulse-intensity-coding Φ–OTDR system as shown in Fig. 3 to evaluate the performance of the proposed interference fading suppression method in pulseintensity-coding Φ-OTDR
The total length of fiber under test (FUT) is 2 km and a piezoelectric transducer (PZT) wrapped with 10 m fiber is placed at the end of fiber, i.e., at 1.98 km
Summary
D ISTRIBUTED optical fiber sensor (DOFS) can realize distributed measurement of physical parameter variation by collecting and analyzing the specific scattered light at consecutive position along the optical fiber, such as temperature. There are two common methods to improve the amplitude of RBS signal: one is to increase the peak power of the input pulse, which is limited by nonlinear impairments [5]; the other is to increase the pulse width, which will reduce the spatial resolution To overcome these issues, pulse coding scheme is proposed to send the pulse sequence to the sensing fiber. What’s more, it achieves a SNR of 43.96 dB for the demodulated vibration signal applied through a piezoelectric transducer (PZT) It is at the same level as the case without interference fading suppression, and 8.6 dB higher than the single pulse Φ-OTDR due to the correlation characteristics of Golay complementary sequence [20], [21]. The method here provides a feasible scheme to deal with the interference fading as well as maintain a high SNR of the demodulated phase in pulse-coding Φ-OTDR, which is promising to improve the vibration distribution detectability [22]
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