Study On Optical Fiber Interferometer Humidity Sensors Based On Offset Splicing

Sensors have been becoming a dispensable measurement tool with theconstructions of the system network and intelligent city. And the fiber optical sensorshave raised much more interest due to compactness, light weight, corrosion resistance,electromagnetic immunity and wide use. Based on the studies of phase modulatedinterferometric fiber optical sensors and the sensors with lateral offset structure, threekinds of fiber optical interferometer with different lateral offsets is fabricated usingcommercially available fusion splicer and used for humidity sensing. The structures andoptical characteristics of the three interferometers are simulated by use of the beampropagation method (BPM) and the humidity and temperature responses are studiedtheoretically and experimentally. The contents of this thesis are as followings:1.Three kinds of all-fiber interferometric sensors including Mach-Zehnder interferometers with small or large lateral offset and large laterall offset based FP interferometers are fabricated by using commercially available fusion-splicer based on the study of thecategories of fiber sensors and phase-demodulated fiber interferometers. And the structure and optics characterisitcs are analyzed theoretically by use of beam propagation method and the advantages and disadvantages of them are also sumarized, providing theoritical guidance for measuring the relative humidity.2. The chemical material PAM that is sensitive to humidity is coated on aMach-Zehnder interferometer fabricated by fusion splicing a piece of single-mode fiberwith small lateral offset to two single-mode fibers, creating a novel all-fiberinterferometric humidity sensor. The experimental results show that the sensor can sensethe humidity ranged within38%-98%, and the humidity sensitivity is proportional to thelength of the interferometer beam and the thickness of the coated PAM film. Thewavelength shows linear shift with respect to the humidity within38-78%RH, agreeingwell with theoretical analysis. However the change in temperature has cross effect onthe humidity sensor, which should be compensated in practical applications.Aninterferometric humidity sensor based on the large lateral offset formed Mach-Zehnderinterferometer is proposed in this thesis. The humidity response of the sensor is thenstudied experimentally, showing that the compact and easy fabricated Mach-Zehnderinterferometric humidity has higher humidity sensitivity since that the sensing beampropagates along the humidity sensitive material directly. The fringes become a single line within the range of78-98%RH, therefore such a sensor can be used only formeasuring the humidity of38-78%RH. The temperature cross effect can not be ignoredthough the sensitivity is higher than that of the small lateral offset formedMach-Zehnder interferometric humidity sensor, which means that temperaturecompensation should be made such a humidity sensor.3. A novel high sensitive interferometric humidity sensor is fabricated by filling thehumidity sensitive material PAM to the cavity of an all-fiber F-P sensor formed by largelateral offset. The experimental results shows that the maximum wavelength shift is~4nm within38-78%RH, and the sensitivity of the fringe is~0.1nm/(1%RH). Theexperimental results shows that the maximum wavelength shift is~59nm within38-78%RH, and the sensitivity of the fringe is~5.868nm/(1%RH), agreeing well withthe theoretical analysis and showing well linearity. The temperature experimental resultsshow that the F-P humidity sensor with large lateral offset is insensitive to temperature.The temperature sensitivity is~0.0006nm/℃, which is less than the humidity sensitivityby several orders, meaning that temperature compensation is not needed for such ahumidity sensor. So the F-P humidity sensor has wide potential application in humiditymeasurement.