Study On Optical Fiber Gas Sensing System With Spectral Absorption Ultraviolet

With the fast development of modern industry, a growing number of toxic and harmful gases may do great harm to human beings, and gradually affect people’s daily lives. PM2.5has attracted great public concern recently, and its major sources are the gases from automobile exhaust, various volatile organic compounds, etc. Using fiber optic technology, to develop gas detection techniques and instruments that are small, portable, accurate and reliable with online real-time continuous monitoring ability, can fill the gap of domestic all-fiber sensing system and of great value in application.Hollow-core optical fiber was initially presented as infrared power-transmitting waveguide in1970s. For the recent decades, more and more scholars have conducted in-depth research into this field, and gradually expanded its application ranges. Hollow fiber has good prospects especially in laser power delivery and sensing application, due to its high damage threshold, little end loss and good flexibility, etc. The air-core feature implies that it can be used as a gas-cell directly integrated to all-fiber gas sensor system. Meanwhile, the metallic-film hollow fiber with low loss in ultraviolet-visible (UV-VIS) wavelength region has been successfully fabricated by inner coating aluminum film, due to metallic aluminum’s high reflectivity in corresponding wavelengths. Thus, it is possible to achieve ultraviolet gas sensing using aluminum-coated hollow fiber.In this paper, a gas sensing structure in ultraviolet wavelength bend based on spectrum absorption theory is introduced. The system is composed of UV-VIS light source, micro spectrometer, hollow fiber gas cell, self-made low-loss connector and computer. A theoretical model of gas absorption in hollow fiber is established, and accordingly the influence of system parameters on the sensing performances are analyzed.Transmission characteristics of hollow fiber in UV-VIS region is studied by theoretical analysis and experimental measurements. Angular power distribution inside hollow fiber is analyzed considering a Gaussian incident beam. The influence of incident divergence angle is discussed, and characteristics of different connectors are compared and experimentally investigated. The results show that compensation is necessary to obtain accurate transmission in broadband system because of the dispersion characteristic of the incident beam. The results are of great importance for system parameter choice, system error compensation, and improvement of SNR and detection sensitivity.Absorption of toluene gas with various concentrations is measured at the wavelength band of261nm, and the absorption shows good linear relationship with toluene concentrations, with accuracy of ppm level. Results show that this system can fully satisfy gas detection requirements under certain environment.