Phase-sensitive Optical Time Domain Reflectometry & Applications

Distributed fiber sensor(DFS) is a revolutionary technology which has shown excellent capability for multiple parameters(temperature, strain, vibration, sound, et al.) measurements over >100 km sensing range with high density sensing points. The fiber is used for both sensing and transmission. Due to these unique characteristics, DFS is extensively studied and widely applied to various areas.As an emerging representative of DFS, phase-sensitive optical time domain reflectometry(Φ-OTDR) is successfully employed in the safety monitoring of petroleum, transportation and structures where the vibration/sound waves need to be measured. Especially, based on Φ-OTDR, distributed acoustic sensor(DAS) offers the capability for long distance acoustic measurement and is one of the most prosperous optical fiber sensors. But, to realize high sensitivity sensing with ultra-long sensing range and high spatial resolution, the current Φ-OTDR still faces many challenges like the performance degradation lead by the attenuation of fiber, the limitation of launching power by nonlinear effects, the detection of very weak sensing signal, and the dynamic signal processing. In addition, taking advantages of Φ-OTDR to solve engineering problems in exploring new applications of Φ-OTDR is another goal of this project.In this doctor dissertation, under the support of the major program of National Nature Science Foundation of China(NSFC): Key Devices and Techniques for Long-Distance Distributed Fiber-Optic Sensing Networks, a novel long distance(>100km), high spatial resolution(SR, <10m) and high sensitivity(SNR>20d B) Φ-OTDR is proposed and demonstrated. Here, Φ-OTDR is first applied to the field of domestic transportation, oil industry and other important areas. The DAS technology is first extended from sensing to communication area and a novel concept of Sound-over-Fiber is proposed for the first time. This research has made great contributions to the development of key technologies of Φ-OTDR and the extension of its applications.The main contents and results of this dissertation are summarized as follows:(1) A novel long distance, high spatial resolution and high sensitivity Φ-OTDR is proposed and its feasibility for long-distance pipeline safety monitoring is demonstrated.The combination of bidirectional Raman amplification, heterodyne detection and wavelet detrending/denoising signal processing ensures high optical SNR(>20d B) over whole 131.5km fiber with 8m SR, leading to the longest Φ-OTDR system with sub-10 m SR for pipeline intrusion detections reported so far. Based on the power optimization of probe pulse and Raman pump, the power depletion of probe pulse and Raman pump due to stimulated Brilliouin scattering(SBS) is avoided, acquiring high SNR and even distribution of Rayleigh scattering signal. By using heterodyne detection, the relative weak Rayleigh scattering signal can be detected with high SNR and the RIN transfer from Raman pump is suppressed. By using the wavelet detrending/denoising, the intrusion signal with high SNR is extracted. The relevant work was oral-presented on OFC2014.(2) The special optical fiber has been used to improve the performance of Φ-OTDR for the first time. Due to much larger Rayleigh scattering coefficient and Raman gain efficiency when compared with conventional single-mode fiber, high performance Φ-OTDR with 10.6km sensing range and 4m spatial resolution is realized with low power probe pulse and Raman pump.(3) A hybrid Φ-OTDR and B-OTDR system is proposed, which can realize simultaneous measurement of dynamic strain(vibration) and static strain. Because the Rayleigh scattering and spontaneous Brilliouin scattering light is naturally FDM, the heterodyne asynchronous demodulation of FSK in optical fiber communication is utilized and the demodulation of the two scattering signals are synchronized. In addition, the forward Raman amplification not only makes up for the deficiency of spontaneous Brilliouin scattering based DFS(compared with SBS-based DFS), but also has the merit of the single end measurement of BOTDR. The designed Φ/B-OTDR hybrid system has the sensing range of 49 km and spatial resolution of 10 m. The related vibration and strain experiments show that this hybrid system has great potential for use in long-distance structural health monitoring.(4) A novel method for the location and velocity measurement of high-speed trains based on Φ-OTDR is proposed. Utilizing the fast response of the vibration along the fiber, the position of the train can be located in real time and the location accuracy is further improved by wavelet denosing. When the motion curve is acquired, the velocity of the train can be calculated by the derivation of the motion curve. Due to this system is completely passive, anti-electromagnetic interference and all-weather, it can make upthe deficiency of the current location and velocity monitoring system, which makes the system great practical value. This work was oral-presented on the 23 rd International Conference on Optical Fiber Sensors(OFS-23).(5) A new concept of distributed acoustic communication- Sound over Fiber is first proposed. The So F based on DAS is proposed and validated: first, the ability of demodulating the amplitude, frequency of sound is verified by the analog acoustic communication and the enhancement of the acoustic signal by using space diversity was achieved experimentally. Based on the analog acoustic communication, the simplex digital communication is then verified in the air and water, respectively. The Baud rate in water is 2.6k and the transmission length is 550 m, while the Baud rate underwater is 1k and the transmission length is 5.4km. Finally, the digital image data is transmitted underwater. These experiments demonstrate that the So F can realize long distance distributed sound communication, and can play an important role where the wireless and optical fiber communication cannot work properly. Hence, the So F would have a good application prospect in the future. This work was oral-presented on the OFC2015.(6) The first seismic underground exploration experiment in domestic based on the DAS developed is carried out and relatively good VSP image is acquired. Cooperate with BGP- the biggest petroleum exploration company in China. This experiment shows that the DAS is a landmark technology in the petroleum exploration area, which has great potential in the future.