| The gas sensing technology is widely used in industrial and agricultural production, environmental monitoring, health and military fields.Non-optical gas detector can achieve lower detection limits. However, it is vulnerable to be influenced by other gas components via cross-sensitivity. Besides, it has other disadvantages including relatively slow response, low reusable rate, short life span and difficulty to achieve simultaneously monitoring. Each gas molecule species is characterized by its unique spectrum absorption pattern. Based on this principle, the optical gas sensor, with high sensitivity, gas molecular selectivity and non-destructive testing, has gained millions of attention in the research filed worldwide. In recent years, with the fast development and maturation of optical waveguide, the hollow waveguide (HWG) has become a hot spot as absorption cell. Compared to traditional stainless steel collimating gas chamber, multipath reflection-type gas chamber, the hollow optical fiber and other waveguide absorption cells such as the photonic crystal fiber (PBGF) have many advantages such as low optical transmission loss, stable coupling with other system components, resistance to external electromagnetic interference, etc. HWG is an inner-coated capillary glass tube, thus needs small volume of detection sample and has good flexibility. However, the small inner diameter capillary chamber structure becomes the bottleneck of the system’s response time. When we use long fiber to gain long light path length and improve the measurement accuracy, the response time may up to several hours and becomes a limited factor to the system applications. In addition, the high pressure gradient during the inflation process of long fiber can easily lead to fiber jitter and uneven distribution of internal gas and finally causes measurement error.This paper aims at achieving high efficient sample gas acquisition and replacement in hollow waveguide absorption cell. Theoretical simulation model is established to analyze the two gas-exchanging mechanisms of the hollow-core waveguide system under the diffusion and fluid dynamics theories. Two measuring systems corresponding to the diffusion and hydrodynamic applications were established by using Fourier transform infrared spectroscopy (FTIR) equipment, ultraviolet and visible (UV-VIS) spectroscopy, combined with specially fabricated low-loss hollow fiber as the absorption cell. Optimization of the response time of the measuring systems contains two aspects:design and changing the waveguide chamber structure and adding external pressure with pump. Low concentration CO2, CO, CH4gas detection experiments were carried out in FTIR system focusing on their fundamental mid-infrared frequency absorption. The experimental results showed good performance of the FTIR system-very low detection limit of CO2and CO (1ppm and20ppm) and high correlation coefficients of experimental theoretical in range10～70ppm (0.99and0.97). After structural optimization of waveguide chamber, the system’s response time reduced from3hours to22min in natural environment while the detection limit of CH4gas reached40ppm. On the other hand, the UV-VIS system’s response time can reach several seconds after optimization with pump power pressure. And the system showed high sensitivity to benzene organics.All gas experiment results showed good agreement with the theoretical analysis, providing that response time can be dramatically shortened by optimizing the structure of the waveguide cell while maintaining high sensitivity in gas sensing. The system can achieve excellent detection performance in many applications such as the long-term distributed monitoring of the environment and fast response real-time detection. The characterization results provide useful guidance for the design and application for gas sensor using hollow waveguides as absorption cells. |
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