Design And Research On Optical Fiber Sensor For Airflow Velocity Measurement

Measurement of airflow velocity has an important significance in industry and national economic field and so on.With the development of the science and technology,modern airflow measurement technology puts forward higher requirements for sensors in terms of sensitivity,reliability and environmental adaptability.The optical fiber sensor has characteristics of compact volume,high sensitivity,resistant to high temperature and anti-electromagnetic interference,which can be used in harsh environment such as high temperature and pressure,strong electric field and magnetic field,and it can realize signal transmission for distance.Recently scholars lie more and more attention on the optical fiber airflow velocity sensor.After referring a mass of both domestic and foreign correlative literatures,this thesis proposes two optical fiber airflow velocity sensors:thermal type velocity sensor based on inter-mode interference principle and stress type sensor based on cascaded fabry-perot(FP)interferometer,both of sensors can be used to measure tiny airflow velocity with a high sensitivity.The main contents in this thesis are listed as follows:Firstly,different types of airflow velocity meters are introduced,as well as their measurement principles.The research status of the traditional velocity meters and the all fiber airflow velocity sensors is reviewed,especially the advantages and disadvantages of all fiber stress type airflow velocity sensors and all fiber thermal airflow velocity sensors.And two novel sensors based on thermal type sensing principle and stress type principle are proposed in this thesis.Secondly,the sensing principle of the thermal type airflow velocity sensor is introduced,as well as its measurement characteristics.The basic principle of the inter-mode interference fiber structure is analysed in theory,and the temperature effect on the effective refractive index of transmission modes.The structural parameters of the inter-mode interference are simulated and optimized by Rsoft.The optical self-heating principle of the sensor is analysed and the theory model of the thermal type optical fiber airflow velocity sensor is established.Thirdly,the inter-mode interference optical fiber airflow velocity sensor is designed and fabricated,and the airflow velocity experimental platform is set up.The characteristics of the up-taper inter-mode interference airflow velocity sensor such as temperature,laser heating efficiency and airflow velocity detection are experimental analysed.The sensor is fabricated according to the optimized parameters through simulation,and the multimode fiber is selected as the sensing element of the sensor through experimental contrast.Experimental results indicate that the temperature sensitivity of the sensor is 89.42pm/?,which is almost the 8 times that of fiber Bragg grating(FBG),and the laser heating sensitivity of the silver coated sensor is 16pm/mW.The shift amount of interference spectrum is about 4.95nm in the airflow velocity range of 0-8m/s.The velocity sensitivity of the sensor can reach 2.574nm/(m/s)and the sensor resolution reaches 0.0008m/s in the low velocity range of 0?1m/s.Fourthly,the stress type optical fiber airflow velocity sensor based on cascaded FP interferometer is proposed.The stress measurement principle of the sensor is researched,and the cascaded FP fiber structure is fabricated,which could introduce clear optical vernier effect,and the amplification factor of the Vernier effect is analysed.Experimental results show that the envelope of the interference spectrum shifts abut 8nm in the velocity range from Om/s to 7m/s,and the sensitivity of the sensor can reach 1.541nm/(m/s)in the airflow velocity range of 3?7m/s..Besides,the temperature sensitivity of the sensor is relatively low,and the ambient temperature has little influence on airflow velocity measurement.Two novel optical fiber airflow velocity sensors are proposed in the thesis,and both have the characteristics of simple structure,high sensitivity and precise measurement compared with current optical fiber airflow velocity sensors.The sensors are intrinsically safe because of light signal measurement,and they can be used for flammable and explosive gas detection with high sensitivity.However,package technology of the sensor is worth further research because optical fiber is fragile and the silver film is oxidizable.