| Distributed fiber sensing technology based on Brillouin scattering is one of the most representative sensing technologies,and its sensing performance has been greatly improved with decades of development.Recently,this kind of sensing technology has been preliminarily applied for health monitoring of the oil and gas pipeline,railway traffic,and submarine optical fiber cable,where longer sensing distance with high spatial resolution and high measurement accuracy is usually preferred.However,the sensing distance of traditional Brillouin based distributed sensing system is generally limited to several tens of kilometers,due to the onset of nonlinear effect and nonlocal effects in the sensing fiber.Therefore,optical pulse coding technique is introduced into the Brillouin based distributed sensing system,which is capable of remarkably enhancing the system signal-to-noise ratio(SNR)without increasing the input power of the pump signal.Owing to the outstanding technical superiority in the field of long-range distributed fiber sensing with high spatial resolution,Brillouin sensors employing pulse coding technique have attracted a lot of attention in recent years.In this dissertation,the trade-off relationships among the spatial resolution,measurement accuracy,sensing distance and measurement time of BOTDA sensor are analyzed thoroughly,and the prerequisite for the application of pulse coding technique in BOTDA sensor is discussed.BOTDA experiments employing optical coding technique are carried out,fully demonstrating the improvement of the sensing performance provided by optical pulse coding.In order to further improve the sensing performance,long-range distributed sensing with high spatial resolution is realized by the employment of the newly proposed pre-pumped pulse coding technique in BOTDA sensor.Finally,a BOTDA experimental setup based on the combination of the pre-pumped coding technique and bi-directional distributed Raman amplification is established,enabling research work of ultra-long distributed sensing.The main results and innovations of this paper are as follows:1.The trade-off relationships among the spatial resolution,measurement accuracy,sensing distance and measurement time of BOTDA sensor are analyzed thoroughly.The impact of the nonlocal effects on long distance sensing system is studied,which imposes restrictions both on the maximum input power of the probe wave and the minimum extinction-ratio of the pulsed pump wave.The generations of single-sideband probe with high suppression ratio and pulsed pump with high extinction-ratio are investigated,respectively.2.According to the basic principle of pulse coding technique,the prerequisite for the employment of pulse coding technique in BOTDA sensor are deduced.The sensor SNR enhancement resulting from Simplex coding is theoretically analyzed and experimentally demonstrated.Experimental results also show that coding format of return-to-zero(RZ)with low duty cycle is more suitable for coded BOTDA systems.And on this basis,the pre-pumped Simplex coding technique is proposed,which is experimentally proved to be capable of further improving the sensing performance of BOTDA system.Compared with the conventional Simplex coding method,pre-pumped coding scheme not only maintains the original coding gain,but also compresses the Brillouin gain spectrum(BGS)through the pre-activation of the acoustic field,which further improves the measurement accuracy of the long-distance BOTDA system.With the proposed technique,distributed sensing over ?50 km sensing fiber is achieved with a spatial resolution of 1 m and a temperature/strain resolution of ?0.23 oC/?5 ??.3.The BOTDA system employing Golay complementary coding is studied detailedly.Both the theoretical analysis and experimental results show that,Golay-coded BOTDA system can achieve comparable sensor performance to that of Simplex-coded systems,while shortening the required measurement time at the same time.And on this basis,the pre-pumped Golay coding technique is proposed for long-distance distributed temperature sensing with high spatial resolution.Experimental results show the pre-pumped Golay coded BOTDA sensor is able to achieve 70 cm spatial resolution with a temperature resolution of ?1.5 oC along ~50 km sensing fiber.4.The finite time-domain differential(FDTD)algorithm is used to simulate and analyze the Brillouin signal enhancement provided by the pre-pumped pulse coding in bidirectional Raman-assisted BOTDA system.Ultra-long distance distributed sensing experiments based on BOTDA system employing first-order bi-directional Raman amplification and pre-pumped Golay code are carried out,realizing distributed sensing along ~100 km single mode fiber with 1 m spatial resolution of and a temperature/strain resolution of ?1.40 oC/?28 ??.Compared to the conventional ultra-long distance BOTDA sensing systems,the pre-pumped Golay coding not only achieves similar sensing performance,but also significantly reduces the required averages from thousands of times to 200 times,providing a new method for improving the efficiency of BOTDA sensors. |
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