Long-range Phase-sensitive OTDR With Hybrid Distributed Amplification

Phase-sensitive optical time domain reflectometry(Φ-OTDR) has been proved to be a powerful tool for real-time distributed sensing of various events, such as borderline intrusion monitoring, seismic waves detection, pipeline safety monitoring, etc., based on the fact that perturbations to the fiber at any location can be detected by measuring the variation of coherent superposition of Rayleigh backscattering light. Sensing distance is an important technical specification for distributed optical fiber sensing system(DOFS). It should be noted that, as the passive fiber-loss increases exponentially with fiber length, an addition of 1km in sensing range results in ~0.4dB increment in round-trip loss, making the range-extension of DOFSs challenging. To extend the sensing distance of Φ-OTDR as well as other DOFSs, valuable research works have been carried out by investigating different optical amplification schemes.In this thesis, a novel hybrid distributed amplification(HDA) scheme is proposed, we cooperatively utilize three different pumping schemes to push the sensing range of Φ-OTDR to 175km(~70dB roundtrip fiber-loss): the co-pumping second-order Raman amplification based on random fiber lasing provides gain mainly in the first half of the whole span, then the counter-propagating Brillouin pump(BP) starts to dominate the amplification in the next 50 km segment; at last, the first-order Raman amplification takes over the signal boosting in the last 37 km segment. It should be noted that the other function of the first-order FRA is to push the BP from the far end of the whole sensing range toward the probe side. Both the numerical study and the experimental demonstration are presented. We build the numerical model to analyze the performance of the hybrid distributed amplification(HDA) scheme for a 175 km OTDR. As a verification of HDA scheme, we build a Φ-OTDR experimental system utilizing HDA and heterodyne detection, whose sensing range is 175km(the longest sensing system reported so far) and spatial resolution is 25 m.What DOFS like Φ-OTDR needs for acousto-optic modulator(AOM) driver is digital pulse signal. However, there is no specific design for DOFS in most commercial signal generator. A majority of commercial generators have a variety of functions, but they are not capable of generating quite narrow pulse. What’s more, commercial generators are usually complex for further development and expensive. In this thesis, a digital signal generator is designed according to the special requirement of DOFS. The hardware circuit solution is given along with a software basing on MFC to control the parameters like pulse width and period of the digital pulse. The narrowest width of output pulse can be 20 ns. The proposed solution is very cost-effective.