Abstract
We have demonstrated a distributed vibration sensor based on phase-sensitive optical time-domain reflectometer (φ-OTDR) system exhibiting immunity to the laser phase noise. Two laser sources with different linewidth and phase noise levels are used in the φ-OTDR system, respectively. Based on the phase noise power spectrum density of both lasers, the laser phase is almost unchanged during an extremely short period of time, hence, the impact of phase noise can be suppressed effectively through phase difference between the Rayleigh scattered light from two adjacent sections of the fiber which define the gauge length. Based on the phase difference method, the external vibration can be located accurately at 41.01 km by the φ-OTDR system incorporating these two lasers. Meanwhile, the average signal-to-noise ratio (SNR) of the retrieved vibration signal by using Laser I is found to be ~37.7 dB, which is comparable to that of ~37.5 dB by using Laser II although the linewidth and the phase noise level of the two lasers are distinct. The obtained results indicate that the phase difference method can enhance the performance of φ-OTDR system with laser phase-noise immunity for distributed vibration sensing, showing potential application in oil-gas pipeline monitoring, perimeter security, and other fields.
Highlights
As one of the typical optical fiber sensors for distributed vibration sensing, a phase-sensitive optical time-domain reflectometer (φ-OTDR) has been demonstrated as a promising technique due to its high sensitivity, wide monitoring range, and accurate locating capacity, and it is widely applied in fields of oil-gas pipeline monitoring, perimeter security, and so on [1,2,3,4]
Optical phase suffers from phase noise originating from the finite coherence length of laser source even if a narrow linewidth laser is used in the φ-OTDR system [13, 14]
In order to prove the enhancement of laser phase-noise immunity by the phase difference method, we have investigated the φ-OTDR system incorporating two different lasers by extending the gauge length to 80 m that is a few times longer than the distance occupied by the probe pulse and the vibration event
Summary
As one of the typical optical fiber sensors for distributed vibration sensing, a phase-sensitive optical time-domain reflectometer (φ-OTDR) has been demonstrated as a promising technique due to its high sensitivity, wide monitoring range, and accurate locating capacity, and it is widely applied in fields of oil-gas pipeline monitoring, perimeter security, and so on [1,2,3,4]. Alekseev et al have experimentally verified that the presence of intensity noise caused by laser phase noise in φ-OTDR fundamentally limits the sensitivity of the φ-OTDR system to external phase vibration [12]. Since phase noise can be accumulated over the entire round-trip, the associated growth of phase noise leads to a sharp decrease in signal-to-noise ratio (SNR), and as a result, the measurement range is limited [15, 16]. A narrow linewidth laser with low frequency drift and low phase noise is believed to be required for long-distance distributed sensing in the φ-OTDR system. Such critical requirement of laser source increases the total cost of φ-OTDR system
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