Research On Characteristics Of LPG Temperature Suppression And Refractive Index Sensor Based On Plastic Optical Fiber

The underwater missile needs to have good water tightness to prevent sea water from being immersed in the process of sailing or getting out of water.Therefore,the water leakage test must be carried out for the underwater missile during the sailing or getting out of water experiment.The internal environment of the underwater missile is complex,the liquid leakage sensor must be immune to electromagnetic interference and its structure is simple and easy to be arranged.Under this condition,the traditional electric measurement method almost fails.Besides,the state of liquid leakage is complex,and the humidity inside the equipment and the change of liquid refractive index(RI)is closely related to the liquid leakage.Therefore,in order to ensure the real-time performance and accuracy of leakage event measurement,the multi-parameter sensitive unit is developed with practical significance and value,which is specially used for underwater missile leakage state monitoring.Under the background of water leakage detection of the underwater missile,a plastic optical fiber(POF)refractive index sensitive unit based on long period grating(LPG)was proposed by taking advantage of the advantages of optical fiber autoimmune electromagnetic interference and easy installation.The step-change of RI of leakage liquid can guarantee real-time measurement.The LPG is a kind of optical device which is very sensitive to the change of refractive index of the external environment.At present,most optical fiber LPG refractive index sensitive units are completed on quartz optical fiber,but quartz material has the disadvantages of hard texture,poor toughness,difficult structure modification,and temperature cross-sensitivity.The plastic optical fiber(POF)has the advantages of large bending,easy processing,and modification.In addition,the large size of the POF core makes it easy to align with the light source and detector,so it is an ideal material to realize the refractive index sensitive unit with a simple structure and low price.In this paper,the sensitive structure is modified on the surface of the POF,and the RI sensitive unit based on the LPG structure and the humidity(gas)sensitive unit based on the side throw structure is proposed.The sensing characteristics of the two types of refractive index are studied and integrated.The main contents include:(1)Design of LPG-POF RI sensitive unitThe LPG is analyzed theoretically from the Angle of geometry optics.According to the LPG sensitivity principle,the POF RI sensitive unit based on LPG structure and the sensitized type RI sensitive unit based on side-polished LPG structure was designed.The influence of the LPG processing cycle on temperature dependence was analyzed.The effects of laser processing parameters on sensitive unit structure,LPG processing width/depth,and RI output characteristics were studied.(2)Design of LPG-POF RI sensitive unit modified with gold filmThe LPG-POF RI sensitive unit based on gold film modification is proposed.The RI test platform was built,and the intensity interrogation was used to test the proposed sensitive unit.The sensitive unit has a linear output in the range of 1.340-1.356 RIU and has good repeatability.Compared with the LPG sensitive unit without gold film,the power variation of the proposed sensitive unit is reduced tens of times,indicating that the sensitive unit modified with gold film can suppress the temperature dependence.Compared with other metal materials,it is further proved that the gold film material has high efficiency in temperature compensation of POF.(3)Research and preparation of the POF relative humidity(gas)sensitive unitWhen a leakage event occurs near the measuring point,the concentration of surrounding relative humidity(RH)(gas)changes.To solve the problem that the water leakage at the measuring point is insufficient or the RI sensitive unit cannot respond to the water leakage quickly,the humidity(gas)sensitive unit is introduced to provide a double guarantee for water leakage detection.To realize the non-contact refractive index detection of liquid leakage events,a humidity-sensitive material(agarose)and a gas-sensitive material(polypyrrole)were prepared and modified on the surface of the side-polished POF to form an RH(gas)sensitive unit.In this paper,the output response characteristics of the proposed sensitive unit with or without sensitive material modification were compared,and the effects of sensitive material thickness,POF surface roughness,and bending radius on the sensitive unit sensitivity were analyzed.The experimental results show that the humidity sensing unit detection range is20-90%.When the RH is 20-80%,the sensing unit sensitivity is 0.595 W/%RH.The sensitivity is 0.305 ?W /% RH when the humidity changes from 80% to 90%.The response time and recovery time of the RH sensing unit were 800 ms and 4s,respectively.The limit of ethanol concentration was 140 ppm,and the response time and recovery time were 5s and 8s,respectively.The gas sensitive unit expands the application background of liquid leakage sensitive units,and the two sensitive units provide a foundation for the research of non-contact liquid leakage sensitive units.(4)Aiming at the complex liquid leakage state,this paper studies and integrates a double-parameter measurement system based on plastic optical fiber.The refractive index sensitive unit and humidity sensitive unit were connected in serial or parallel mode,and the output responses of the sensitive unit under different liquid leakage conditions were simulated and measured.The experimental results show that the integrated double-parameter sensitive unit can monitor the leakage status in real-time,respond quickly to the leakage,and improve the accuracy of leakage detection.