Brillouin Distributed Optical Fiber Sensors Based On Ultra-long Fiber Ring Laser Pumping

Brillouin optical time-domain analysis(BO TDA) has evolved into a consolidated fiber sensing technology that is now widely used in different application domains(civil engineering, pipelines, fire detection, etc.).The spatial resolution, sensing distance, measurement accuracy of BOTDA is the main performance index. There exists constraint relationship among them. O n the one hand, the fiber loss is limiting the sensing distance extension. On the other hand, the spatial resolution is proportional to the brillouin pump pulse width. High spatial resolution needs more narrow pulses, reducing the signal-to-noise ratio(SNR). In order to ensure the end measurement accuracy of the sensing optical fiber, we need to appropriately increase brillouin pump peak power and continuous wave power. However this method has limitation to improve measurement precision. On the one hand, the high pump peak power of brillouin could cause modulation instability(MI) and self-phase modulation(SPM), eventually leading to the brillouin gain spectrum(BGS) broadened and measurement accuracy declined. On the other hand, the stronger of the detecting light easyly ca uses pump depletion and non- local effect(namely the sensing information crosstalk from the front to the end. the BGS have deformation and multi-peak structure). The long the distance, the more non- local effect.Therefore, puting forward an effective solution to improve the SNR along distance of BO TDA, for the further development of the high performance distributed optical fiber sensing technology has important practical significance.In order to extend the sensing distance, while keeping the spatial resolut ion and the sensor precision.Firstly, compared with lumped optical amplification scheme, we can use the light amplification scheme. Distributed Raman amplifier(DRA) has higher SNR and lower nonlinear damage. We need to suppress relative intensity noise(RIN) of pump-detection transfer at the same time.Secondly, using optical encoding technology can improve SNR of the system, with the more bits, the greater the SN R gain.This paper studies the DRA characteristics of ultra- long fiber laser(UL-FL) with a ring cavity. For the first time, the amplification scheme combined with 127-bit coding technology application have used in long-distance BO TDA. After optimizing the structure and parameters, we demonstrated distributed sensing with 3m spatial resolution and strain/temperature accuracy of 28με/1.4 ℃ covering 94.2km fiber, using Brillouin optical time domain analyzer pumped by ultra- long fiber laser with a ring cavity. Non-local effect and BGS broden have been discussed. In order to improve SNR of the system, a BOTDA and Φ-OTDR separation scheme is proposed.To solve practical problems for application of BOTDA, sensing scheme is put forward, and gives a practical solution for specific problems in debugging. Through these improvements, we had done independent research and developed the BOTDA long-distance to monitor submarine cable. Due to the outstanding performance, our system has successfully bidden project which is national 863 key projects undertaken by southern power grid company of C hina, and now is accepted and being breakthrough application demonstration.