【摘要】：近年來,以光纖中散射光為傳感載體的分布式光纖傳感器以其獨(dú)特的優(yōu)勢,在傳感領(lǐng)域內(nèi)得到眾多研究者的青睞。如光纖傳感和傳輸集于一身的優(yōu)勢,可實(shí)現(xiàn)溫度和應(yīng)力等多參量的同時(shí)監(jiān)測,并能實(shí)現(xiàn)全方位智能監(jiān)測,從而克服傳統(tǒng)點(diǎn)式監(jiān)測漏檢的弊端,提高監(jiān)測的成功率等。基于布里淵光時(shí)域反射技術(shù)(BOTDR)單端測量的特性便于實(shí)際應(yīng)用,是目前國內(nèi)外研究的熱點(diǎn)之一。在BOTDR分布式光纖傳感技術(shù)中,以相干檢測的傳感精度最高,在大型工程如石油天然氣管道泄露、大型混凝土結(jié)構(gòu)(大壩、隧道、建筑物等)等的健康監(jiān)測方面具有廣泛的應(yīng)用前景。由于布里淵散射光光功率十分微弱,容易淹沒在噪聲中,且頻移比較小,信號(hào)檢測非常困難,因此研究信號(hào)解調(diào)方法,提高信號(hào)解調(diào)精度,對系統(tǒng)測量的精準(zhǔn)性和可靠性尤為重要。 本文在深入研究布里淵散射機(jī)理的前提下,提出一套切實(shí)可行的基于自發(fā)布里淵散射的分布式光纖溫度應(yīng)變傳感系統(tǒng)方案,以提高信號(hào)解調(diào)精度為重點(diǎn)進(jìn)行逐步研究,在該過程中進(jìn)行一系列的實(shí)驗(yàn)驗(yàn)證和測試。該系統(tǒng)方案采用外差相干檢測方法,激光器發(fā)出窄線寬的連續(xù)光分成90：10兩部分,90%連續(xù)光經(jīng)光電調(diào)制器調(diào)制成脈沖光之后采用摻餌光纖放大器進(jìn)行放大,然后注入傳感光纖以產(chǎn)生布里淵后向散射脈沖信號(hào)；10%的連續(xù)光經(jīng)電光調(diào)制器產(chǎn)生移頻邊帶,通過加載在電光調(diào)制器的微波掃頻源和直流偏置電壓的調(diào)制,獲取具有布里淵頻移的參考光。背向布里淵散射光經(jīng)光纖光柵濾波后與參考光在光電探測器處相干進(jìn)入信號(hào)處理系統(tǒng),進(jìn)行去噪處理和計(jì)算分析,獲取溫度和應(yīng)力在傳感光纖的分布情況。 本文采取一系列措施提高系統(tǒng)測量精度：(1)在參考路獲取參考光方面,本系統(tǒng)采用電光調(diào)制器產(chǎn)生的兩個(gè)1階邊帶作為布里淵頻移的參考光,獲取具有高1階邊帶與0階邊帶光強(qiáng)差(1-0光強(qiáng)比)的參考光,是系統(tǒng)的信號(hào)解調(diào)的關(guān)鍵。第一,詳細(xì)研究了偏振對電光調(diào)制器產(chǎn)生邊帶的光強(qiáng)的影響,提出了偏振控制器與直流偏置電壓結(jié)合的方式獲取高1-0光強(qiáng)差的參考光。第二,采用步進(jìn)為1MHz的微波掃頻源進(jìn)行自動(dòng)微波掃頻,找到最佳參考光頻率,提高系統(tǒng)信號(hào)解調(diào)精度。(2)在探測路提取布里淵信號(hào)方面,首先,使用本底噪聲較小的放大器,提高信號(hào)信噪比。其次,運(yùn)用濾波精度達(dá)到0.001nm/℃可調(diào)溫控光纖光柵濾波技術(shù)提取斯托克斯信號(hào)光,提高系統(tǒng)信號(hào)解調(diào)精度(3)在信號(hào)相干處理系統(tǒng)方面,先采用自外差相干探測法解調(diào)信號(hào),將太赫茲數(shù)量級的布里淵高頻信號(hào)降至易于探測和處理的百兆赫茲數(shù)量級的中頻信號(hào),提高了系統(tǒng)的探測精度。再運(yùn)用LabVIEW分析處理去除噪聲等措施,提高系統(tǒng)信號(hào)解調(diào)精度等等,增強(qiáng)系統(tǒng)的穩(wěn)定性和可靠性,降低了系統(tǒng)的成本。最后,進(jìn)行了一系列的實(shí)驗(yàn)研究和系統(tǒng)測試及結(jié)果分析。具體地說,全文包括以下幾個(gè)方面： 第一章緒論,簡單介紹了分布式光纖傳感的基本知識(shí),包括分布式光纖傳感技術(shù)的特點(diǎn),分布式光纖技術(shù)的分類,和基于布里淵散射的光纖傳感技術(shù)(BOTDR、BOTDA、BOFDA),然后綜述了本文主要研究的基于自發(fā)布里淵散射光纖傳感技術(shù)的研究現(xiàn)狀和課題意義及研究目標(biāo),最后介紹了本文所做的主要工作。 第二章是研究課題的理論部分,首先介紹了光纖中的散射現(xiàn)象和三種散射譜,闡述了自發(fā)和受激布里淵散射的產(chǎn)生機(jī)理,從理論上分析了光纖應(yīng)變和溫度與布里淵頻移和散射強(qiáng)度的變化關(guān)系,根據(jù)光纖中的自發(fā)布里淵散射的頻移和強(qiáng)度受光纖溫度和應(yīng)變影響的機(jī)理,得到了利用光纖布里淵散射進(jìn)行溫度和應(yīng)變同時(shí)測量的傳感模型,為BOTDR系統(tǒng)的設(shè)計(jì)奠定了理論基礎(chǔ)。 第三章介紹偏振與獲取高光強(qiáng)的參考光,是本文的核心研究內(nèi)容之一。詳細(xì)研究了電光調(diào)制器的移頻特性,從理論和實(shí)驗(yàn)兩方面研究了偏振對EOM調(diào)制的光波邊帶光強(qiáng)的影響。提出邊帶光強(qiáng)隨偏振的改變呈現(xiàn)cos2x的變化趨勢,實(shí)驗(yàn)結(jié)果與理論預(yù)測相符。提出使用偏置電壓和偏振控制相結(jié)合的方法得到穩(wěn)定的,1-0光強(qiáng)差為24dB的參考光,比僅僅調(diào)偏置電壓時(shí)高18dB,使用該參考光明顯提高了系統(tǒng)信噪比,提高系統(tǒng)信號(hào)解調(diào)精度。 第四章系統(tǒng)測試及實(shí)驗(yàn)結(jié)果分析,是本論文另一核心內(nèi)容。包括系統(tǒng)的整體搭建,各子系統(tǒng)中器件和實(shí)驗(yàn)參數(shù)的選取,以及系統(tǒng)溫度傳感實(shí)驗(yàn)結(jié)果討論與LabVIEW分析處理等。選擇本底噪聲低的放大器,運(yùn)用濾波精度達(dá)到0.001nm/℃的可調(diào)溫控光纖光柵濾波技術(shù)提取斯托克斯光,和采用自動(dòng)微波掃頻等方法提高了系統(tǒng)信號(hào)解調(diào)精度。實(shí)驗(yàn)中頻譜分析儀采集到的波峰頻率為10.853GHz,與布里淵頻移的理論值相符。在系統(tǒng)溫度傳感實(shí)驗(yàn)中得到的溫度系數(shù)為1.0843MHz/℃,該實(shí)驗(yàn)數(shù)據(jù)接近于前人報(bào)道的數(shù)據(jù)1.2MHz/℃,它們之間的差值主要是由于儀器誤差。采用LabVIEW軟件分析處理了布里淵散射強(qiáng)度信號(hào),詳細(xì)闡述了累加平均去噪、小波去噪和低通濾波去噪的原理,并對了三種去噪方法的處理效果。 第五章總結(jié)與展望,對本論文進(jìn)行總結(jié),分析該系統(tǒng)存在的不足之處,指出下一步需繼續(xù)進(jìn)行的研究。
[Abstract]:In recent years, distributed optical fiber sensors based on scattered light in optical fibers have been favored by many researchers for their unique advantages. For example, the advantages of optical fiber sensing and transmission are integrated, which can realize simultaneous monitoring of temperature and stress, and realize omnidirectional intelligent monitoring, thus overcoming the traditional points. Based on the characteristics of Brillouin Optical Time Domain Reflectometry (BOTDR), which is convenient for practical application, is one of the research hotspots at home and abroad. The Brillouin scattered light is very weak and easy to be submerged in the noise, and the frequency shift is small, so it is very difficult to detect the signal. Therefore, the signal demodulation method is studied to improve the signal demodulation accuracy and the accuracy of system measurement. And reliability is particularly important.
On the premise of thorough study of Brillouin scattering mechanism, a feasible scheme of distributed optical fiber temperature and strain sensor system based on spontaneous Brillouin scattering is proposed in this paper. The emphasis is to improve the signal demodulation accuracy, and a series of experiments are carried out to verify and test the system. In dry detection, the laser emits a narrow-linewidth continuous light which is divided into two parts: 90:10, 90% of which is modulated into pulse light by a photoelectric modulator, then amplified by a bait-doped fiber amplifier, and then injected into a sensing fiber to generate Brillouin backscattering pulse signal; 10% of the continuous light generates a frequency-shift sideband via an electro-optic modulator and adds a frequency-shift sideband through an electro-optic modulator. The reference light with Brillouin frequency shift is obtained by modulating the microwave sweep source and the DC bias voltage loaded on the electro-optic modulator. The back Brillouin scattered light filtered by the fiber grating is coherent with the reference light at the photoelectric detector to enter the signal processing system for denoising processing and calculation analysis, and the temperature and stress in the sensing fiber are obtained. Cloth situation.
In this paper, a series of measures are taken to improve the measurement accuracy of the system: (1) In the reference path, two first-order sidebands generated by electro-optic modulators are used as Brillouin frequency-shift reference beams, and the reference beams with high first-order sideband and zero-order sideband intensity difference (1-0 intensity ratio) are obtained. This is the key to the signal demodulation of the system. The influence of polarization on the intensity of side-band light produced by electro-optic modulator is studied in detail, and a method combining polarization controller with DC bias voltage is proposed to obtain the reference light with high 1-0 light intensity difference. In the aspect of extracting Brillouin signal, firstly, the amplifier with less background noise is used to improve the signal-to-noise ratio. secondly, the Stokes signal is extracted by using the temperature-adjustable fiber grating filtering technology with filtering accuracy of 0.001 nm / The detection method demodulates the signal, reduces the Brillouin high frequency signal of terahertz order to the intermediate frequency signal of 100 MHz order which is easy to be detected and processed, improves the detection precision of the system, improves the demodulation precision of the system signal and so on, enhances the stability and reliability of the system, reduces the detection precision by using LabVIEW analysis and processing to remove the noise. Finally, a series of experimental studies and system testing and results analysis are carried out. Specifically, the full text includes the following aspects:
In the first chapter, the basic knowledge of distributed optical fiber sensing is briefly introduced, including the characteristics of distributed optical fiber sensing technology, the classification of distributed optical fiber technology, and the Brillouin scattering-based optical fiber sensing technology (BOTDR, BOTDA, BOFDA). Then the research of this paper based on self-issued Brillouin scattering optical fiber sensing technology is summarized. The status quo, the significance of the project and the research objectives are introduced. Finally, the main work of this paper is introduced.
The second chapter is the theoretical part of the research project. Firstly, the scattering phenomena and three kinds of scattering spectra in optical fibers are introduced. The mechanism of spontaneous and stimulated Brillouin scattering is expounded. The relationship between strain and temperature of optical fibers and Brillouin frequency shift and scattering intensity is analyzed theoretically. The sensing model of temperature and strain simultaneous measurement by fiber Brillouin scattering is obtained, which lays a theoretical foundation for the design of BOTDR system.
In the third chapter, polarization and obtaining high-intensity reference light are introduced, which is one of the core research contents of this paper. The frequency-shift characteristics of electro-optic modulator are studied in detail. The influence of polarization on the intensity of EOM-modulated optical sideband is studied theoretically and experimentally. Theoretical predictions are consistent. A stable reference light with a 1-0 intensity difference of 24 dB is obtained by combining bias voltage with polarization control. The reference light is 18 dB higher than that by only adjusting the bias voltage. The signal-to-noise ratio of the system is obviously improved and the demodulation accuracy of the system signal is improved by using the reference light.
Chapter 4: System test and experiment result analysis, which is another core content of this paper, includes the whole structure of the system, the selection of devices and experiment parameters in each subsystem, the discussion of system temperature sensing experiment results and LabVIEW analysis and processing. The demodulation precision of the system signal is improved by using the technique of controlling fiber grating filter to extract Stokes light and the method of automatic microwave sweeping. The peak frequency of the spectrum analyzer is 10.853GHz, which is in agreement with the theoretical value of Brillouin frequency shift. According to the data close to 1.2MHz/C reported by predecessors, the difference between them is mainly due to instrumental error. Brillouin scattering intensity signal is analyzed and processed by LabVIEW software. The principles of cumulative average denoising, wavelet denoising and low-pass filtering denoising are expounded in detail, and the processing effects of three denoising methods are given.
The fifth chapter summarizes and prospects the paper, analyzes the shortcomings of the system, and points out the further research.
【學(xué)位授予單位】：東華大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號(hào)】：TP212;TN911.7

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