Optical fiber sensor network has attracted considerable research interests for geoscience applications. However, the sensor capacity and ultra-low frequency noise limits the sensing performance for geoscience data acquisition. To achieve a high-resolution and lager sensing capacity, a strain sensor network is proposed based on phase-sensitive optical time domain reflectometer (φ-OTDR) technology and special packaged fiber with scatter enhanced points (SEPs) array. Specifically, an extra identical fiber with SEPs array which is free of strain is used as the reference fiber, for compensating the ultra-low frequency noise in the φ-OTDR system induced by laser source frequency shift and environment temperature change. Moreover, a hysteresis operator based least square support vector machine (LS-SVM) model is introduced to reduce the compensation residual error generated from the thermal hysteresis nonlinearity between the sensing fiber and reference fiber. In the experiment, the strain sensor network possesses a sensing capacity with 55 sensor elements. The phase bias drift with frequency below 0.1 Hz is effectively compensated by LS-SVM based hysteresis model, and the signal to noise ratio (SNR) of a strain vibration at 0.01 Hz greatly increases by 24 dB compared to that of the sensing fiber for direct compensation. The proposed strain sensor network proves a high dynamic resolution of 10.5 pε·Hz-1/2 above 10 Hz, and ultra-low frequency sensing resolution of 166 pε at 0.001 Hz. It is the first reported a large sensing capacity strain sensor network with sub-nε sensing resolution in mHz frequency range, to the best of our knowledge.
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Schematic diagram of the proposed high-resolution strain sensor network.
(a) Phase change induced by laser frequency shift. (b) Relationship between the reference channel and sensing channel for laser frequency shift.
(a) Phase change induced by temperature fluctuation. (b, c) Enlarged images for phase change. (d) Relationship between the reference channel and sensing channel for temperature fluctuation.
Hysteresis operator. (a) Relay operator. (b) Play operator.
Block diagram of the LS-SVM based hysteresis model.
(a) Received beat frequency signal. (b) Beat frequency signal of sensor element 55 and 54.
The relationship between phase change and strain.
(a) Temperature change waveform and corresponding phase change for model train. (b) Hysteresis loops for temperature change and the regression result. (c) Compensation error.
(a) Original phase signal for temperature change and strain signal. (b) Compensation results for two methods. (c) PSD of the original result and compensation results.
(a) Original phase signal in a quiet environment. (b) Compensation results for two methods. (c) Noise floor PSD of the original result and compensation results.