Investigation Of A Novel Optical Fiber Humidity Sensor Based On Singlemode-multimode-singlemode Fiber Structure

The singlemode-multimode-singlemode(SMS)fiber structure has been widely investigated and developed into a wide range of optical fiber devices in fields of optical fiber telecommunication and optical fiber sensing,such as filters,optical splitters,and optical fiber sensors,due to its advantages of low cost,easy fabrication,and high sensitivity.Especially for optical fiber sensing applications,the SMS fiber structure based on multimode interference presents its excellent performance.Therefore,this thesis presents a comprehensive investigation on an SMS fiber structure,and furthermore,reports and investigates a humidity sensor based on a gelatin coated with a singlemode-side polished multimode-singlemode(SSPMS)fiber structure.In the first chapter of this thesis,we introduce the research background and motivation of both SMS fiber structures and SMS fiber structures based optical fiber sensors.In the second chapter of this thesis,we theoretically analyze the multimode interference and self-imaging phenomenon when the light is launched from a singlemode waveguide and then propagates along the multimode waveguides and multimode fibers.In the third chapter,the multimode interference within a practical SMS fiber structure is simulated by employing the mode propagation analysis(MPA)and the beam propagation method(BPM),respectively,and the two different self-imaging positions are predicted.The experimental results have verified that the predicted self-imaging position based on the MPA is closed to the measured result rather than that of the BPM.Next,we investigate both theoretically and experimentally the optical modal distributions at the self-imaging positon as a function of the surrounding temperature.A novel method for investigating temperature dependence of SMS fiber structure is proposed finally in this chapter.In the fourth chapter,an optical humidity sensor based on a gelatin coated SSPMS fiber structure is proposed and investigated,the SSPMS fiber structure is fabricated by a short piece of side polished multimode fiber sandwiched between two singlemode fibers.The influences induced by both side polished depth and the variations of surrounding refractive index are theoretically predicted by employing a BeamPROP software.Then the relationship between the gelatin coating thickness and the humidity sensing performance is investigated experimentally to verify the theoretical models.In addition,the temperature dependence of the SSPMS fiber structure is also investigated in this thesis.The experimental results demonstrate that this proposed SSPMS fiber structure possesses a high humidity sensitivity and a fast response time,as well as a low temperature dependence over a temperature range from 25 ? to 55 ?.In order to overcome the limitation caused by the coated gelatin on the proposed SSPMS fiber structure based humidity sensor,a humidity sensor based on an SSPMS fiber structure without extra coating is proposed in the fifth chapter of this thesis.The influence of the surface roughness of the bare side polished fiber on humidity sensing performance over a relative high temperature range(70 ? to 90 ?)is investigated.The surface roughness of the bare SSPMS fiber structure can be determined by simply adopting different grades of abrasive papers during the polishing process.The obtained experimental results demonstrate that the humidity sensitivity of the bare SSPMS fiber structure increases when the surface roughness increases,furthermore we found that the bare SSPMS fiber structure can maintain a stable humidity sensing performance over a relative high temperature range from 70 ? to 90 ?.Overall,the work presented in this thesis demonstrate the design,fabrication and humidity sensing applications of SSPMS devices that are based on multimode interference within different side-polished multimode fibers.This fiber sensing device can have high humidity sensitivity and it has a low temperature dependence.These properties make the SSPMS a promising sensing device for multimode interference devices and sensors,where low cost and ease of fabrication are important.