Design And Performance Of Gas Sensor Based On Special Fiber Coated With Film

In the modern society,going with the development of science,the position of sensors in dailylife has become more and more significant.Especially,the optical fiber sensors are widely used in temperature,refractive index and stress sensing fields because of their real-time monitoring characteristics.However,there are few researches on it in the field of gas detection,like the monitoring of toxic gases.In everyday life,toxic gases can cause various harm to human health.For exemple,CO is a common toxic gas in the workplace,making the monitor of it being increasingly important.In order to avoid CO poisoning,effective measures must be taken.Therefore,this paper designs three kinds of gas sensors coated with special optical fibers to monitor CO in real time,and conducts a series of tests on the these optical fiber sensors.The main research works are as follows:(1)A Sagnac carbon monoxide gas sensor based on ferric oxide/magnesium oxide coated photonic crystal fiber is constructed.Using the collapse fusion method,a section of four-core fiber(FCF)is embedded into two sections of endlessly single-mode photonic crystal fiber(ESPCF)coated with iron trioxide/magnesium oxide,forming a sensitive element of the ESPCF-FCF-ESPCF fiber structure.Finally,the Sagnac fiber sensing structure is obtained by embedding the sensing unit in the Sagnac ring.Besides,the crystal structure,composition and morphology of the sensitive film are analyzed respectively.The experimental results show that the ferric oxide/magnesium oxide composite sensitive material is successfully and uniformly coated on the surface of the optical fiber with a thickness of about 1?m.Gas sensing experiments show that the highest sensitivity of the sensor for CO detection can reach 99.42 pm/ppm,and the concentration of carbon monoxide is in the range of 0?60 ppm.As the concentration of the monitored gas increases,its interference trough shows blue shifts,indicating its good linear relationship.And the response and recovery time of the sensor are 90 s and 100 s respectively.(2)A Mach-Zehnder interference(MZI)carbon monoxide gas sensor based on tin dioxide-coated photonic crystal fiber is constructed.With the method of collapse welding,a section of tin dioxide-coated ESPCF is embedded in the middle of two sections of seven-core fiber(SCF)to form a sensitive element of the SCF-ESPCF-SCF fiber structure.The MZI fiber is used as sensing unit,and a layer of tin dioxide sensitive film with the thickness of about 1.5?m is uniformly plated on the surface of the optical fiber.The gas sensitivity experiment shows that the sensor has a maximum sensitivity of 0.01696d B/ppm for CO detection,and the concentration of carbon monoxide is in the range of0?90 ppm.With the continuous increase of the concentration of the monitored gas,the intensity of the interference wave trough decreases,showing a good linear relationship,the response and recovery time of the sensor are 60 s and 72 s respectively.The sensor has high sensitivity,strong selectivity and good stability which can effectively realize the environmental monitoring of CO.(3)A carbon monoxide gas sensor based on a thin-core fiber(TCF)coated with cerium dioxide is constructed.The single-mode fiber(SMF),no-core fiber(NCF)and the TCF are fusion spliced to obtain SMF-NCF-TCF-NCF-SMF Mach-Zehnder(MZI)optical fiber sensing structure.A uniform Ce O₂film is plated on the surface of the optical fiber.When the sensitive film adsorbs carbon monoxide(CO)gas,the effective refractive index of the fiber changes,the difference between the effective refractive index of the cladding and the core will change,causing the move of the intensity of monitored trough,thereby achieving the purpose of detecting carbon monoxide.The sensor shows a good linear relationship(R²=0.99127)when the carbon monoxide concentration is in the range of 0?70ppm,the response and recovery time are about 40 s and 47 s,respectively with the sensitivity of 0.03224 d Bm/ppm and the detection limit of 0.07630 ppm,having high carbon monoxide selectivity and temperature insensitivity.