| Mid-infrared hollow-core Bragg fiber (HC-BF) is completely different from thetraditional optical fiber on fiber structure, guiding mechanism, material selection andtransmission characteristics. Hence it is possible to break the performance limitation ofthe traditional optical fiber and explore new application areas of optical fibers. Thanksto its ability of mid-infrared light transmission in a hollow-core, mid-infrared HC-BFhas great potential on the application of trace gas sensing, which is very important inenvironment protections and medical applications.Aiming at the application in the trace gas sensing, this dissertation, which issupported by the National Basic Research Program (973Program) project of China andthe Science Foundation of Beijing, has comprehensively investigated the mid-infraredHC-BF’s transmission characteristics, fabrication techniques, gas sensing applicationsand new fiber designs for specific requirement of the gas sensing applicaiton.Theoretical methods of the HC-BF and its one dimensional photonic crystal (1DPC)cladding were established to investigate the bandgap characteristics of its1DPCcladding and the fiber modal characteristics. The impact of the fiber outer diametervariations on the transmission characteristics of the mid-infrared HC-BF was alsoinvestigated theoretically. Based on the analysis of the relation between the maximumeffective fiber length and the fiber outer diameter variation, the tolerance of the fiberouter diameter variation for the mid-infrared HC-BF was found to be6.5%, whichprovides a useful theoretical guidance for the fiber fabrication.The fabrication process of the mid-infrared HC-BF was studied firstly in China.The fabrication platform and related techniques were established. Combining thedesigns of the fiber preform and fiber drawing parameters, mid-infrared HC-BFs withtransmission wavelengths in the whole mid-infrared band could be fabricatedsuccessfully. The mid-infrared HC-BF samples were fabricated at the first time in China.The transmission losses are2.35dB/m for the fiber sample with a transmission bandaround10.6μm and4.55dB/m for the fiber sample with a transmission band around3.3μm.The relation between the performance of the mid-infrared HC-BF and the limit of gas concentration detection has been deduced theoretically. The trace gas sensing ofmethane using the mid-infrared HC-BF was demonstrated experimentally with a fibersample with a transmission band around3.3μm. The limits of gas concentrationdetections under different fiber lengths were measured by the exponential dilutionmethod, which were consistent with the theoretical analysis. The optimized fiber lengthwas found to be0.86m for our experiment setup, with an optimized limit of gasconcentration detection of7.41ppm.The multi-wavelength transmission mid-infrared HC-BF was proposed bymodifying the1DPC structure and extending the characteristics of the higher-orderphtonic band gaps. Its multi-wavelength transmission within one-octave was confirmedtheoretically and experimentally. The trace level multi-gas sensing of methane andcarbon monoxide was performed experimentally with a multi-wavelengthtransmissionHC-BF sample, of which the2ndand3rdtransmission bands are within thewavelength range of3μm~7μm, demonstrating its great potential on the applicationof multi-gas sensing. |
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