| Distributed optical fiber sensing technology based on stimulated Brillouin scattering(SBS)has been widely applied in the large-infrastructure structure health,military or national defense,abysmal-sea or deep-space exploration and other monitoring fields due to the capability of temperature,strain and vibration detecting.The application demands of long-reach accurate positioning and high-resolution real-time monitoring is becoming increasingly impending.However,the pulse laser is served as sensing signal in traditional Brillouin Optical Time Domain Analysis(BOTDA)and the sensing positions of physical quantity to be measured are addressed by time-of-flight method,which lead to that the spatial resolution is difficult to exceed 1 m due to the limitation of pulse width being not less than Brillouin phonon lifetime.Moreover,the sinusoidal frequency-modulation laser is served as sensing signal in traditional Brillouin Optical Correlation Domain Analysis(BOCDA)and the sensing positions are addressed by optical filed correlation method,which result in that the sensing distance is only several hundred meters owing to the periodic coherent optical field.Therefore,the traditional SBS distributed optical fiber sensing technology has met a trade-off problem between the spatial resolution and sensing distance in principle.In addition,the dynamic response ability encounters a great challenge due to that the distributed temperature or strain information are demodulated by frequency-sweep Brillouin gain spectrum(BGS).In a word,the practical applications of sensing technologies are severely restricted.Focusing on the above problems,a chaotic BOCDA is proposed in this thesis.The chaotic laser of noise-like,wideband RF spectrum and broadband optical spectrum is served as the sensing signal.The sensing distance is improved by using the chaos autocorrelation characteristic,the spatial resolution is promoted by using the broadband property,and the chaotic BGS is broadened by using the Gaussian optical spectrum.A theoretical model,which is concerning the influence of time delay signature(TDS),bandwidth enhancement and spectral linewidth controlling on the system performance,is established.Finally,a fast chaotic BOCDA with long sensing distance and high spatial resolution is achieved by using temporal gating,lock-in detection or slope assistant methods.The main research contents are as follows:(1)The sensing mechanism of BOCDA is deeply analyzed,and the research progresses and challenges of that in sensing distance,spatial resolution and measurement speed are expounded,which provide a theoretical guidance for future development of chaotic BOCDA.(2)The sensing model of chaotic BOCDA is established and the characteristics of chaos SBS are deeply explored.Firstly,the experimental setup of chaotic laser generation is introduced,and the properties of broadband spectrum in frequency domain and autocorrelation in time-domain are expounded.Then,the physical mechanism of chaotic BOCDA is elaborated and the broadband chaotic pump-Stokes beat spectrum with Gaussian shape at the extremely correlated position is simulated.The coupling mechanisms between the spatial position of chaotic Brillouin coherent optical field and chaos autocorrelation property,or between the spatial range of that and chaos broadband characteristic,are revealed.The distributed strain measurement along the fiber is achieve by simulation.Experimentally reveal the nonlinear gain characteristic of Stokes light in SBS process.According to the property,the linewidth variation equation of Stokes light is established,and the spectral width regulation or control of chaotic BGS is realized.(3)A time-gated long-reach BOCDA based on TDS-suppressed chaotic laser is proposed to achieve a 10-km long distance with centimeter-level spatial resolution for temperature measurement.Firstly,according to the deterioration degree of the probe's gain and that of the signal-to-noise ratio(SNR)of acoustic field caused by the chaos TDS,a theoretical model of TDS limiting the sensing distance is established.Then,the simulations and experiments verify that the time-gated scheme based on pulse modulation,whose duration is approximately equal to the external cavity feedback time of chaotic laser,could significantly suppress the noise SBS acoustic field excited by the TDS and non-zero background of autocorrelation curve.Finally,by using the time-gated configuration with extinction ratio of 24.30 d B and pulse duration of 120 ns,the SNR of SBS acoustic field is improved by about 5.62 d B,and the signal-to-background-noise ratio of chaotic BGS is improved by more than 1.48 d B.The effective sensing distance is increased to 10.2 km,breaking through the limitation of several hundred meters sensing distance in traditional BOCDA technology,and the spatial resolution is still maintained at 9.0 cm.(4)A millimeter-level-spatial-resolution BOCDA based on bandwidth-enhancement chaotic laser is proposed to achieve the static strain measurement with spatial resolution being better than 3.5 mm and the effective sensing points(the ratio between sensing distance and spatial resolution)being more than 47000.Firstly,according to the width variation trend of central correlation peak in the adjusting process of chaos bandwidth,a theoretical model of bandwidth-enhancement chaos improving the spatial resolution is established.Then,the measurement SNR would be greatly promoted by using lock-in detection,where the SNR is improved by about 0.86 d B when the chaos bandwidth being 10 GHz.Finally,the spatial resolution is promoted to 3.5 mm at a sensing distance of 165 m,and the highest spatial resolution can reach 3.1 mm.The effective sensing points are more than 47000.(5)A single-slope-assisted large-dynamic-range BOCDA based on broadband chaotic BGS is proposed to achieve the dynamic strain measurement with a range of 1200 ??,which is twice as large as the traditional scheme.Firstly,the principle of slope assistant and the limitation of spectral width of traditional BGS on dynamic range are expounded by simulation.The linear negative relationship between chaos TDS and main-to-sub peak ratio of BGS is confirmed and the theoretical model of slope-assisted broadband chaotic BGS is established.Then,a single-slope-assisted chaotic BOCDA is proposed,where the-3 d B linewidth of chaos optical spectrum is 5.60 GHz and the full width at half maximum of BGS can reach 55 MHz.Experimentally,the range of dynamic strain is increased to 1200 ??,which is more than twice than that of traditional single-slope scheme,and the spatial resolution is maintained at 3.5 cm.(6)A dual-slope-assisted high-precision BOCDA based on broadband chaotic BGS is proposed to achieve dynamic strain measurement with the accuracy of ± 8.1 ?? and resolution of 20 ?? at a range of 800 ??.Firstly,the impact of inherent power stochastic fluctuations on measurement accuracy in single-slope-assisted system is theoretically analyzed and a theoretical model of dual-slope assistant improving dynamic strain measurement accuracy is established.Then,in the proposed dual-slope-assisted chaotic BOCDA,the experimental results show that the accuracy of dynamic strain is promoted to ±8.1 ??,the relative error being only ±1%,and the dynamic range is 800 ??.Finally,the resolution of static strain and that of dynamic strain can reach 10 ?? or 20 ??,respectively. |
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