| Optical fiber sensors have been developed rapidly in the past few decades with unique characteristics such as passive,anti-electromagnetic interference,high-temperature and high-pressure resistance,et al.,which are different from traditional electronic sensors.Especially in long-distance optical fiber sensing systems,fiber acts as both sensing element and transmission medium,which can detect temperature,strain,vibration and other physical quantities variation along tens or even hundreds of kilometers-long fiber link.It is easy to realize large-scale sensing and networking of optical fiber sensors,resulting in irreplaceable advantages in application fields of seismic wave monitoring,oil/gas exploration and development,borderline security,underwater acoustic detection,structural health monitoring,electricity and other energy security and livelihood security.At present,armored cables with low sensitivity are mostly used as sensing elements in optical fiber sensing systems,and existing distributed Raman amplification(DRA)pump sources suffer from large intensity fluctuation,which make it difficult to achieve longer distance sensing with better performances.This dissertation aims to meet the state's major demands for smart grid,distributed hydrophone and pipeline safety monitoring,et al.Focusing on two key technologies in long-distance optical fiber sensing systems,which are improvements of sensing element sensitivity and DRA pump performance,new acoustic-sensitive cables,new pump sources and new sensing system are proposed and demonstrated.The results are of leading significance to the development of long-distance optical fiber sensing.The main research contents are as follows:(1)New acoustic-sensitive cables: The researches are supported by the major instrument project of National Natural Science Foundation of China(NSFC)and the UESTC-ZTT Joint Laboratory.In order to solve the problem of the limited sensitivity of cables used for distributed optical fiber sensing systems in oil and gas exploration and pipieline leakage monitoring,the sensitivity-enhanced cable and fully distributed magnetic adsorption sensing cable are proposed,for the first time.The effects of different materials and structures on the acoustic sensitivity of sensing cables are theoretically analyzed.A small diameter,flexible sensing cable with enhanced sensitivity is designed and fabricated with acoustic sensitivity of 10 times of the rigid armored cable,which provides a new scheme for sensing element of distributed acoustic sensing.Furthermore,a fully distributed magnetic absorption multi-parameter sensing cable is proposed,which can be tightly fitted to the wall of metal pipeline without additional fixing devices.It provides an efficient coupling way between sensing cable and pipeline,which can eliminate the noise caused by poor coupling effect of optical fiber sensing systems in practical applications.(2)New pump sources: With the support of key projects of NSFC and Zhejiang Lab,aming at the problem of relatively high intensity noise of existing 2nd-order DRA pump,the innovative method of utilizing cascaded random Raman fiber lasers(CRRFLs)with no longitudinal mode to replace conventional Raman fiber lasers are proposed.By optimizing the pump from narrow band to broad band,the relative intensitiy noise(RIN)of 1.3 ?m CRRFL is significantly reduced,achieving a breakthrough of low-noise DRA pump source.With a discrete wavelength-and bandwidth-tunable ytterbium-doped random fiber laser as pump,a 1.3 ?m 4th-order CRRFL with RIN of-115.19 d B/Hz is realized.The experimental results verify the effectiveness of pump with broader bandwith on improving the spectral purity and suppressing the RIN of CRRFL.On this basis,with the help of a special designed point mirror,a 1.3 ?m 4th-order CRRFL with RIN of-119.17 d B/Hz is realized pumped by a common-cavity ytterbium-doped Raman random fiber laser with a simpler structure.Furthermore,the broadband amplified spontaneous emission in ytterbium-doped fiber with better temporal stability is used as pump,resulting in the realization of a 1.3 ?m4th-order CRRFL with RIN as low as-128.37 d B/Hz,which is about 30 d B lower than the maximum RIN of the commercial Keopsys Raman fiber laser in the same wavelength band.Combining the low-noise CRRFL with 1.45 ?m broadband seed,a low-noise random fiber lasing amplification system is proposed and fabricated.Adopting the low-noise random fiber lasing amplification system in coherent optical time-domain reflectometer,a 100 km quantitative distributed acoustic sensing based on2nd-order distributed amplification was realized,for the first time,with the spatial resolution of 10 m and the minimum measurable strain of 468 p?/?Hz.The experimental results verify the value of the low-noise CRRFL in long-distance distributed sensing systems.(3)New sensing system: Supported by the key project of NSFC and UESTC-ZTT joint laboratory project,a method of using high-order random lasing and new transmission fiber to prolong the cavity length of half-open backward-pumped random fiber laser is proposed,which provides design guidance of ultra-long fiber lasers and sensors.The effects of high-order random fiber lasing,different parameters of transmission fiber and erbium-doped fiber on performance enhancement of ultra-long half-open backward-pumped random fiber laser are theoretically analyzed.The results show that cavity length of high-order random fiber laser with fiber of low transmission loss,low Rayleigh backscattering coefficient and low Raman gain coefficient can be significantly prolonged.A 200 km-long half-open backward-pumped 6th-order CRRFL is demonstrated with optical signal-to-noise ratio reaching 25 d B.A multiplexing fiber Bragg grating sensing system is further realized of 200 km ultra-long distance and >15d B high optical signal-to-noise ratio.The results,which are the longest high-performance single-ended half-open backward-pumped random fiber laser and sensor,verify the correctness and effectiveness of the proposed method and achieve breakthroughs in distance and performance of half-open backward-pumped random fiber lasers and sensors.The new acoustic-sensitive cables proposed in this dissertation are expected to provide key technical support to meet the state's major demand of oil/gas exploratioin and development.The flexible acoustic-sensitive cable has been applied in the field test of BGP Inc.,which has made contributions to the major instrument project of NSFC and the successful cooperation between UESTC and ZTT Group.The proposed new pump source has been successfully applied to the fiber distributed hydrophone research,major project of Zhejiang Lab,and made contributions to build a new generation of distributed optical fiber sonar,which is the key tool of national coastal defense security.In addition,the proposed new pump has great potential to be used in ultra-long optical fiber communications systems as low-noise and high-power DRA pump in the future.The demonstrated new sensing system provides a feasible solution to solve the long-standing problem of real-time ultra-long powerlines safety monitoring,which has great practical significance and commercial value. |
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