Research On Extension Of Sensing Distance In Distributed Optical Fiber Vibration Sensing System

Vibration is one of the most common phenomena. Vibration sensors are of high importance in the industrial world as they have the ability of securing numerous places such as bridges, dams, civil construction, and so on. They can also be used as a breakage alarm for oil pipelines. Distributed optical fiber sensors are interesting candidates as they have the advantages of providing distributed measurements, of being insensitive to EM disturbances, resistant to corrosion and usable in harsh environments.The distributed vibration sensors we discussed in this paper include phase-sensitive Optical Time Domain Reflectometry (Φ-OTDR) and polarization sensitive optical time-domain (POTDR). Both of them are on the basis of OTDR, using the phase information or polarization changes of the backward Rayleigh scattering, respectively, to detect the external vibration along the fiber.Sensing distance is an essential technical indicator of system. In this paper, the extension of optical fiber sensing distance to make a detailed discussion of distributed vibration monitoring system, its main content is as follows:(1) At frist, we carefully examine the gated Raman amplification which utilizes a pulsed source as the Raman pump. The front edges of the probe pulse and gated Raman pump pulse are aligned, and the optimized pulse width of the pump is determined by the fiber length. In this way, compared with the continuous wave (CW) Raman pump, the ASE noise caused by the Raman pump has been reduced; whereas the amplification effect would not be weaken because both the probe pulse and the Rayleigh scattering (RS) signal can experience the same amplification process in the case of CW Raman pump. A 50 km Φ-OTDR based on the gated Raman amplification has been demonstrated experimentally, and the influence of the delay time between the pump and probe pulse on the system’s performance has been discussed. As a result, the noise level of the RS trace of Φ-OTDR has been reduced, especially at the front part of the fiber. The results also indicate that the delay between the pump and probe pulse would weaken the amplification effect and cause the distortion of the transmitted signal’s pulse shape.(2) However, as the detection is based on phase measurement, a highly coherent laser with a low frequency drift is needed. These two features make such systems very complex to implement and greatly increases their cost. So the POTDR system is discussed, the 2nd Raman amplification based on random fiber laser (RFL) is used to enhance the sensing range of POTDR to 86km. A 1365 nm Raman pump and a half-open cavity is used to generate the 1 st random lasing. The distributed Rayleigh backscattering and a fiber loop mirror feedback constitute a half-open cavity. Then the generated 1 st order random lasing acts as the Raman pump of 1550nm probe light. This technique can extend the sensing distance and improve the sensitivity. Also, we demonstrated a 125 km POTDR by using 2nd Raman amplification based on RFL and 1 st Raman amplification.(3) The distributed vibration system set based on the principle of POTDR assisted by the amplification we mentioned in this paper is made up, and authenticated by China metrology institute of physics.