Ultra-high resolution strain sensor network assisted with an LS-SVM based hysteresis model

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 op-tical time domain reflectometer(φ-OTDR)technology and special packaged fiber with scatter enhanced points(SEPs)ar-ray.Specifically,an extra identical fiber with SEPs array which is free of strain is used as the reference fiber,for com-pensating the ultra-low frequency noise in theφ-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 in-troduced 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 hyster-esis 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 pεat 0.001 Hz.It is the first reported a large sensing capacity strain sensor network with sub-nεsensing resolution in mHz frequency range,to the best of our knowledge.

300 km ultralong fiber optic DAS system based on optimally designed bidirectional EDFA relays

Optical fiber distributed acoustic sensing(DAS)based on phase-sensitive optical time domain reflectometry(φ-OTDR)is in great demand in many long-distance application fields,such as railway and pipeline safety monitoring.However,the DAS measurement distance is limited by the transmission loss of optical fiber and ultralow backscattering power.In this paper,a DAS system based on multispan relay amplification is proposed,where the bidirectional erbium-doped fiber amplifier(EDFA)is designed as a relay module to amplify both the probe light and the backscattering light.In the theoretical noise model,the parameters of our system are carefully analyzed and optimized for a longer sensing distance,including the extinction ratio(ER),span number,span length,and gain of erbium-doped fiber amplifiers.The numerical simulation shows that a bidirectional EDFA relay DAS system can detect signals over 2500 km,as long as the span number is set to be more than 100.To verify the effectiveness of the scheme,a six-span coherent-detection-based DAS system with an optimal design was established,where the cascaded acoustic-optic modulators(AOMs)were used for a high ER of 104 dB.The results demonstrate that the signal at the far end of 300.2 km can be detected and recovered,achieving a high signal-to-noise ratio of 59.6 dB and a high strain resolution of 51.8■at 50 Hz with a 20 m spatial resolution.This is,to the best of our knowledge,a superior DAS sensing distance with such a high strain resolution.