Abstract
In this work, we present the experimental results of a direct detection $\varphi $ -OTDR based distributed acoustic sensor system. The system uses two cascaded acousto-optic modulators in order to generate optical pulses with very high extinction ratio and dual photodetector scheme for high dynamic range. The proposed schemes are investigated in detail and their performance enhancement is experimentally verified. Four piezoelectric based fiber stretchers are placed on a ∼104 km single-mode test fiber at the distances of 1 km, 10 km, 87 km and 102.7 km and used for perturbation tests. The stretchers generated vibration signals which are analyzed to quantify the system performance. The signal-to-noise ratio (SNR) of vibration signals at the monitored distances is measured over the 12-hour recorded data within 34-second time windows considering the multi-point random interference of scattered light and fading phenomena. Using the 12-hour data, SNR histograms at four different locations are generated and mean SNR values are obtained. The signals received from 102.7 km has a maximum SNR of 24.7 dB and a mean SNR of 7.3 dB with a spatial resolution of 15 m. To the best of our knowledge, this is the highest-range reported direct detection $\varphi $ -OTDR based distributed acoustic sensor system.
Highlights
The security of long-distance structures such as pipelines and borders is a challenging problem and requires expensive solutions
In this work, we present the experimental results of a direct detection φ-OTDR based distributed acoustic sensor system
We have proposed an ultra-long-distance fiber optic distributed acoustic sensing system which uses dual acousto-optic modulator and dual photodetector techniques
Summary
The security of long-distance structures such as pipelines and borders is a challenging problem and requires expensive solutions. Dual acousto-optic modulators (AOM) are used in order to minimize the coherent noise [9] generated by the leaked continuous light wave from the modulators which makes it challenging to realize long-distance event detection along the sensing fiber. The use of dual AOMs increases the extinction ratio (ER) of the generated optical pulses to higher than 100 dB which is essential for the long-distance measurement of the φ-OTDR system [10], [11] With this level of extinction, the optical noise level caused by the optical fluctuations becomes lower than the other noise sources (amplified spontaneous emission and thermal noise). Our results show that the proposed system has a mean SNR of 7.3 dB at 102.7 km
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