【作者】 余贶琭；

【导师】 吴重庆；

【作者基本信息】 北京交通大学 ， 光学工程， 2011， 博士

【摘要】 物联网(IOT:Internet of Things)是目前研究的热门领域之一,而光纤传感网络是物联网的一个重要部分。在环境保护和现代化工业生产中,人们都急需一种可监测多点气体浓度信息的传感网络。光纤型气体传感设备和方案因其抗干扰性强、本质安全、基于光通信技术、易于组网、智能化监测等显著特点而成为当今国内外传感领域研究的主要对向。虽然科研人员已经对光纤单点气体传感器研究了多年,取得了一定的成果,但是仍然存在一些问题；另一方面,如何利用单点传感器构成一个光纤传感网络正成为当前热点之一。本文针对吸收光谱法的光纤气体传感器进行了深入的研究。首先是对于单耦合器光纤环结构的气体传感器进行了研究,分析了测量理论并搭建了实验系统；随后对利用增益钳制EDFA(Erbium-Doped Fiber Amplifier)的光纤腔衰荡气体传感器进行了研究,在理论和实验两方面探究增益钳制EDFA性能的基础上,实际搭建了一种基于增益钳制EDFA的光纤腔衰荡气体传感器；第三,对于基于波分复用的多点光纤气体传感网络进行了深入研究,分析了耦合器网络、EDFA补偿型耦合器网络、波分复用器网络、DLOB(Dual-Loop Optical Buffer)网络等不同组网方式下的网络性能,包括损耗及信噪比等问题,理论分析了波分复用型传感网络的测量原理和最大可监测点数,并搭建实验系统进行验证；最后对于DLOB和基于码分多址的光纤气体传感网络的若干关键问题进行了深入研究,对于DLOB的环长与信号包速率长度关系、信号包的编码方案、U波段的光开关等问题进行了研究。本文主要研究内容与所取得的创新性成果为：1.提出了一种1665nm基于单耦合器光纤环的甲烷传感器结构,分析了该传感器的性能：包括1665nm系统中使用1550nm耦合器的最佳分光比与信号光环行圈数的关系；与传统的吸收光谱法相比,该传感器将气室的有效吸收长度增加了4倍。利用该传感器实际测量了两组甲烷气体样品,其测量结果误差小于2.5%。2.光纤腔衰荡型传感器是一种常见的传感器结构,但其损耗较大、信号衰荡次数较少、衰荡时间短；虽然加入光放大器可以补偿衰荡损耗,但光放大器是一种非线性放大器件,具有增益饱和特性,将严重影响检测精度。为解决此问题,提出了一种基于增益钳制EDFA的光纤腔衰荡气体传感器新结构。理论分析获得了增益钳制EDFA的线性放大范围以及对应的输入功率阂值,并进一步导出了消光比与归一化输入光功率的关系。这些理论成果,在实际搭建的基于增益钳制放大器的传感器中得到了验证。实验表明,该增益钳制结构能够将EDFA归一化增益的线性范围从0.02扩大到约0.55、传感器衰荡时间从63.82 ns增加为2.90 us,测量误差从±0.70降为±0.0054 dB。利用该传感器实际测定了乙炔在1534.100nm的吸收峰,与光谱仪测量得到的结果吻合较好；对1.03%的乙炔样品的测量结果表明检测误差小于3.5%。3.对四种不同组网方式的光纤传感网络进行了分析比较,包括系统损耗、信噪比等性能。提出了一种波分复用型光纤乙炔传感网络,该网络利用密集波分复用器将宽谱光源分束为不同波长的窄带信号光,并利用不同的波长来区分不同传感点的位置,其系统损耗、串扰、信噪比恶化等都远远小于其他三种组网方式。在深入分析这种网络测量原理的基础上,得到了适用于该网络的谱线展宽形式的Beer-Lambert定律,分析得知该方案最大可监测18个传感点。最后实际搭建了一个3点的波分复用型光纤乙炔传感网络,各点的测量误差均小于1.8%。4.提出了一种适用于波分复用型乙炔传感网络的半导体光放大器SOA(Semiconductor Optical Amplifier)级联EDFA的新型宽谱光源,可有效地覆盖乙炔气体在1510-1540nm内的吸收峰。分析得到了该宽谱光源的理论输出光谱,与光谱仪的实际测量结果吻合较好。该宽谱光源兼具SOA的谱宽宽、平坦度好和EDFA的功率高等优点。5.提出了一种基于全光缓存器(DLOB)和码分多址(CDMA: Code Division Multiple Access)的光纤气体传感网络。该传感网络的优点在于：利用DLOB的双波长多圈缓存特性,将脉冲光信号在DLOB中多次循环通过气室,极大地增加了有效气体吸收长度；同时利用不同编码来区分传感点的位置。对这种传感网络的若干关键技术问题进行了研究：分析提出了一种适合该网络的编码方案；推导了码字长度(一个用户码字的比特数)、信号速率与DLOB光纤环长度的关系；对半导体光放大器(SOA)和光子晶体光纤(PCF:Photonic Crystal Fiber)在U波段的增益和相位调制特性进行了分析,并实验得到了PCF损耗值、非线性系数等参数。这些关键技术的解决,对本方案的进一步研究有重要的指导意义。更多还原

【Abstract】 The Internet of Things (IOT) is one of the most popular research areas, and the fiber optics sensing network is one key aspect of the IOT. Intrinsic safe, reliable, and real-time gas sensing networks are badly needed both in environment protections and industrial production processes. Fiber optics gas sensing methods and devoices are the main research interests because of their incomparable advantages, such as, anti-interference and intrinsic safe, easy to make use of the state-of-art optical communication technologies, easy to build up networks, smart and multi-parameters monitoring. On one hand, the fiber optics gas sensors are maturing and commercializing through all these years’intensive research and they however could be improved. On the other hand, the fiber optics sensing networks are under mass investigations and attracting more interests of scientists within labs, institutes and research centers.In this doctoral thesis, fiber optics gas sensors and sensing networks were studies thoroughly. First of all, a single coupler fiber optics methane sensor was theoretically analyzed and then actually built up. After that an Erbium-Doped Fiber Amplifier (EDFA) gain-clamped Fiber Loop Ring-down Spectroscopy (FLRDS) sensor was put forward after theoretically and experimentally researches on the gain-clamped EDFA performances. Thirdly, a fiber optics gas sensing network based on Dense Wavelength Division Multiplexing (DWDM) was proposed and investigated. The network losses as well as the Signal Noise Ratios (SNR) of four different networks were studied, the maximum sensing points of the DWDM network and the detecting theory was discussed, and a DWDM network was actually established. Finally, a sensing network based on Code Division Multiplexing Access (CDMA) and Dual-loop Optical Buffers (DLOB) was proposed. Several key problems of that network, the signal packet coding method, the restrict conditions of the DLOB loops’lengths, U-band optical Cross-phase Modulation (XPM) and optical switches were deliberated respectively.The main research achievements and creative outcomes are presented as following,1. A fiber loop gas sensor configuration with a single coupler was proposed and used for methane detection at 1665 nm. The sensor performances were studied, the measurement theory as well as the relationship between the best split ratio of the coupler and the signal travelled loops were obtained. Two gas samples were tested by the sensor, and the gas gap was lengthened for 4 times while the measurement errors were within 2.5%. 2. Considering that the traditional FLRDS approaches have drawbacks like big round-trip losses because of couplers used, short ring-down times with small numbers of signal round-trips, and also the shortcoming of reduced sensitivity in case of adding amplifiers directly into the fiber loop, a new type of configuration with gain-clamped EDFA FLRDS gas sensor was proposed. The detailed EDFA gain-clamp theory was analyzed; the gain and the extinction ratio of the gain-clamped EDFA with respect to normalized input power were given. A FLRDS sensor based on a gain-clamped EDFA was practically built up. The round-trip loss was largely cut down from 6.836 to 0.394 dB, while the ring-down time lengthened from 63.82 ns to 2.90μs and the measurement errors from±0.70 to±0.0054 dB comparing to the former sensor setup. Finally, the acetylene absorption line at 1534.100 nm was obtained with the sensor and has a good agreement with the spectrum from the Optical Spectrum Analyzer (OSA), and A 1.03% acetylene gas sample was measured with errors less than 3.5%.3. After comparing the system attenuations and SNRs of four different optical fiber sensing networks, a novel acetylene gas sensing network which has much smaller system attenuation and cross-talk based on wavelength division multiplexers was brought forward. The system take advantages of DWDMs to part broad-band light into different narrow-band signal lights and assign them to corresponding sensors while utilize narrow-band signal lights’ wavelengths to mark different probes. The measuring method and theory was obtained and given, the maximal probe numbers of the network was also calculated as 18 for acetylene. A three-probe sensing network was set up and its measurement errors are no more than 1.8%.4. A new broad band light source using cascaded SOA and EDFA for the DWDM gas sensing network was suggested, which is broad enough to cover the 1510-1540 nm absorption branches of acetylene. The formulary results of the output spectrum have good agreements with the experiment outcomes. The superiority of this light source is that it has a very good flatness as the SOAs while also a very broad Half Maximum Full Width (HMFW) and large output optical powers as EDFAs.5. A proposal of fiber optics gas sensing network based on DLOBs and CDMA was put forward. This kind of fiber sensing networks has excellence basically for two reasons. One is that the network takes advantages of the multi-wavelength multi-loop buffering capabilities of the DLOBs, the optical signal or packages could be "trapped" in and travel through the DLOBs which contain gas cells for dozens of times, hence the gas gaps were largely extended. The other is that the sensors were marked by codes and the probe number of the network can be much bigger than other methods by a proper coding method. Several key problems on this network were under investigated:1st a suitable coded method for this network configuration was given, and 2nd the five restrictive equations of the signal package length, the signal rate, and the loop lengths were obtained. Moreover, the gain and phase modulation properties at U-band of the SOA and Photonic Crystal Fiber (PCF) were both theoretically and experimentally discussed, their possibilities as U-band XPM devoices were also investigated. All these works on the key problems laid a solid basis for the future research and analysis.更多还原