Fiber Optics Gas Sensors And Gas Sensing Networks Based On The Absorption Spectrum Method

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.