| With the huge drive of industrialization process and rapid increase of vehicle amount in our country,the air pollution caused by the exhaust fumes from heavy industries(such as metallurgy and petrochemical)and the exhaust gases from vehicles has posed severe threat to health and living environment of people.As monitoring is the prerequisite to governance,it is of great significance in reality to realize the rapid,real time,high-precision,and high-sensitive gas concentration detection technology.Besides,in order to widen the detection range and reduce the power and cost of system,there is also a great need for developing miniaturized and portable detection equipment.On the other hand,optical gas sensor has caught widespread attention of domestic and international scholars in recent years due to its peculiar advantages of intrinsically safe,resistance to electromagnetic interference,wide measuring range,fast response speed and the potential to realize remote online monitoring.However,the conflict between miniaturization and high sensitivity has always restricted the further development of the sensing technology.As a new type of optical device,photonic crystal(PC)brings new vitality to the study of miniaturized and high-sensitive optical sensor by virtue of its compact size,flexibility in structure design and unique optical properties behind different structures.Therefore,PC based sensor has rapidly become a hotspot concerned by both domestic and overseas scholars since the concept of PC was put forward.In this thesis,the theory and key technology of gas sensor based on PC were researched,in which the main research subject was PC,the mainline was to explore miniaturized and high-sensitive optical gas sensing method,and the core goal was to promote the further development of optical gas sensor.The detailed research contents are as follows:(1)Research on the theory and key technology of gas sensor based on photonic crystal waveguide(PCW).By analyzing the generation mechanism and the corresponding properties of slow light in PCW,the feasibility of enhancing the measurement sensitivity of gas sensor by slow light in slotted PCW(SPCW)was theoretical demonstrated.Based on this,a novel gas sensing technology based on slow light in SPCW was proposed.Then,considering the requirements of this sensing technology in actual applications,the key problems were discussed from respects such as how to resolve the conflict between group index and bandwidth of slow light,how to overcome the influences of fabrication errors and surrounding environment,how to improve the coupling efficiency and how to design the sensing structure.To solve the above problems,some work was carried out as follows:Firstly,the energy band structure,dispersion curve and guided mode property of SPCW were analyzed by using plane wave expansion(PWE)method and finite-difference time-domain(FDTD)method.Based on these,some methods were innovatively proposed to optimize the slow light properties of SPCW,which included changing the positions of air holes,changing the shapes of air holes and infiltrating liquids into air holes.Simulation results demonstrated that the slow light with high group index of 176,wide bandwidth of 1.15 nm and low group velocity dispersion of 5×106 ps2/km could be obtained in SPCW.And the final optimized SPCW had the merits of easy to fabricate,immune to fabrication errors and surrounding temperature,and the dynamically tuned working wavelength according to practical requirements,which was the best slow light property of the published literatures.Besides,by using the tapered coupling technology and resonant interference technology,the coupling efficiency between SPCW and conventional fiber could be up to 90%.Secondly,combined with the correlation spectroscopy and differential detection technology,a miniaturized and high-sensitive gas sensing system based on slow light in SPCW was designed.Simulation results demonstrated that the measurement sensitivity could be enhanced by 176 times,thanks to the introduction of slow light technology.Finally,the distribution gas device,mixing gas device and the sensing device were built to conduct a preliminary experimental test and performance analysis of the proposed sensing system.The obtained experimental results had confirmed the correctness of theoretical analysis.(2)Research on the theory and key technology of gas sensor based on photonic crystal cavity(PCC).By analyzing the resonant property and sensing mechanism of PCC,a novel gas sensing technology based on PCC infiltrated with selective gas absorption molecule was proposed.Then,we investigated the controlling light properties,optical field distribution and transmission property of PCC in detail by using the FDTD method.Based on these,we first put forward the idea of integrating microfluidic infiltration technology and slow light technology in PCC.Simulation results demonstrated that the refractive index sensitivity of 450 nm/RIU and quality factor of 1105 could be obtained in the optimized PCC.According to the output property of PCC,a novel wavelength demodulation technology based on fiber loop ring down was first proposed,which could not only distinguish the resonant wavelength shift of PCC with a high precision,but also further improve the measurement sensitivity of gas sensor.Simulation results demonstrated that the minimum detectable concentration can be as accurate as 2.37 ppm,which was equivalent to the best resolution of the published gas sensor.Besides,the sensing properties of the propsed system could be further improved by optimizing the resonant properties of PCC.This research first realized a gas sensor that own compact size,high sensitivity and excellent discriminative characteristic,which had great academic value and broad application prospect.(3)Research on the theory and key technology of gas sensor based on photonic crystal fiber(PCF).According to the transmission property and gas sensing theory of hollow-core photonic crystal fiber(HC-PCF),a novel reflective gas sensing technology based on HC-PCF was designed and realized.Firstly,a HC-PCF with rounded hexagon and silica core ring structure was proposed to increase the duty cycle.And then the impacts of the number of cladding,the radius of central core and the thickness of silica core ring on the transmission property of HC-PCF were analyzed by using the finite element method(FEM).Based on these,the optimized HC-PCF was obtained with high optical power ratio of fiber core,low confinement loss and large effective mode field area.Secondly,a reflective HC-PCF chamber was designed and fabricated,which possesses the special features of simple structure,good stability,fast gas infiltration and high coupling efficiency.Finally,a signal modulation method was first proposed by using fiber Bragg grating that pasted on a triangle cantilever beam to transfer broad-band signal into narrow-band sine signal.Combined with second harmonic and differential detection technology,a reflective gas sensing system based on HC-PCF was designed and built.Experiment results demonstrated that the minimum detectable concentration can be as accurate as 200 ppm.Compared to other gas sensors based on PCF,the proposed sensor has the fairly high sensitivity and fairly good accuracy.In addition,the reflective sensing head will further promote the development of PCF in the practical application of gas sensor. |
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