Research On Optical Fiber Humidity Sensors And Their Test Systems

Many industries such as aviation,food,chemicals and medicine are in need of using the humidity measurement at present.Every walk of life also puts forward higher and higher requirements to sensors while measuring the humidity.The performance of humidity sensors(namely mechanical structure,anti-interference ability,response time,humidity sensitivity and humidity range)will directly affect the applications of humidity sensors.Fiber optic sensors have many advantages,such as smaller size,stronger anti-interference ability,faster response and higher sensitivity.Therefore,it is of great significance to study the fiber humidity sensors.The purpose of this paper was about optical fiber sensors.The main content of this thesis included the research background and the common types of optical fiber humidity sensors.In this paper,we studied fiber bragg grating(FBG)humidity sensors fabricated using different methods of coating with different thickness and the influence of the thickness and the coating method on humidity sensors characteristics.Moreover,the paper studied humidity sensors made of long-period fiber gratings and their humidity sensitivity and stability.A Fabry-Perot Interferometer based optical fiber sensor was also fabricated and its humidity sensitivity and hysteresis are tested.Here are the details of the work:1)The research reviewed the background of optical fiber humidity sensors and thedevelopment process at home and abroad.The definition of humidity,the methodsof measuring humidity and some common types of fiber optic sensors wereintroduced;2)By directly coating polyimide on fiber grating surface and glass substrates withdifferent thickness,we made the different FBG humidity sensors.We studied theeffect of coating methods and thickness on humidity sensitivity,measurement error,response time and hysteresis.Moreover,we tested the stability of the sensors.Theresults were as follows.The thickness of polyimide coated on the fiber were 0.203,0.327,0.435,0.672 and 0.964 mm.The corresponding sensitivity of five humiditysensors were 9.26,13.58,16.38,23.86 and 28.62pm/%RH.The response timewere 15.1,18.5,19.7,23.6 and 26.5min,respectively.The maximum hysteresiswere 1.6,1.8,2.1,3.2 and 4.4%RH,respectively.The thickness of polyimidecoated on the glass were 0.230,0.324,0.443,0.531 and 0.653 mm.Thecorresponding sensitivity were 25.14,30.05,33.52,37.17 and 41.63pm/%RH,respectively.The response time were 17.4,19.1,20.5,25.2 and 28.9min,respectively.The maximum humidity were 1.4,1.6,1.7,2.6 and 3.4%RH,respectively.We found that the humidity sensitivity,the measurement error,theresponse time and the hysteresis increased with the coating thickness increased.The sensors in the case of coating polyimide on the glass have higher humiditysensitivity,smaller hysteresis,less measurement error and slower response timethan that directly coating on the surface of fiber grating.By contrast,we chose thefiber grating that coated with polyimide for three times to fabricate humiditysensors.The humidity characteristics,sensitivity,response time and hysteresiswere superior.And it will have some potantial applications.3)With the method of coating polyimide on a piece of glass for three times,we madea long-period fiber grating humidity sensor.And the humidity sensitivity of thelong period fiber grating we measured was 48.9pm/%RH.The sensitivity wasgood.Then we tested the stability in seven days,the deviation was 0.25%RH.Therefore,the sensor has good sensitivity and stability.4)We fabricated a fiber optic humidity sensor based on FPI.The structure was asingle-mode fiber connected with a section of photonic crystal fiber.Thesingle-mode fiber was used as the transmission optical fiber and the photoniccrystal fiber was used as the sensing fiber.After fabricating the sensor,theinterference spectrum was demodulated and the interference was obvious,whichshowed that the sensor was great.The humidity sensitivity of F-P optical fiberhumidity sensor was 101.95pm/%RH.Experiments showed that the maximum ofhumidity hysteresis was 2.6%RH.The hysteresis of the sensor was good.