Open-cavity Fiber Fabry-Pérot Interferometers For Gas Pressure Sensing

Gas pressure measurement plays an important role in industrial process,automob ile manufactur ing,chemical industry,spaceflight and other fields.Optical fiber pressure sensors have attracted attentions due to the advantages of compactness,immunity to electromagnetic interference and capability of high resolution detection.In this thesis,three types of open-cavity fiber Fabry-Pérot interferometers(FPIs)are proposed and demonstrated for gas pressure sensing,and the main contents include:1.An optica l fiber FPI based on dual capillar ies is proposed and demonstrated.It is fabr icated by splic ing a t iny segme nt of a ma in-capillary wit h a feeding-capillary on one end,and a single mode fiber on the other,to allow gas enters the main-capillar y via the feeding-capillary.The reflect ion spectrum of t he inter ferometer device shifts with t he variat io n of gas pressure due to the dependence of gas refractive index on t he pressure applied.The experime nta l results obtained show t hat t he proposed device exhib its a high gas pressure sensit ivit y of 4147 pm/MPa,a low temperature cross-sensit ivity of less than 0.3KPa/°C at atmospheric pressure.The robust tip structure,ultra-compact device size and ease of fabricat ion make the device an attractive cand idate for re liable and highly sens it ive gas pressure measurement in a precise location.2.A fiber in-line FPI based on a capillary embedded a glass microsphere is proposed.The interferometer is fabricated by fusion splicing a short segment of capillary tube to a standard single mode fiber,followed by inserting a glass microsphere into the tube,the air gap between the SMF end face and the glass microsphere then forms a fiber in-line FPI.Due to the additional reflection occurred on the rear surfaces of the microsphere,the reflection spectrum of the device is a three-beam interference pattern.Owing to the dependence of refractive index of gas on the pressure applied and the thermal-optical effect of glass microsphere,different dips in the interference fringe pattern exhibit different responses to the changes in gas pressure and temperature,which enables a simultaneous gas pressure and temperature sensing by Matrix method and Fourier bandpass filtering method.3.A fiber in-line FPI based on an inner bubble is demonstrated for gas pressure sensing.The inner air-cavity is formed by drilling a micro-hole at the cleaved single-mode fiber end facet,followed by fusion splicing.A micro-channel is then drilled cross the inner air-cavity to allow gas to flow in.Due to the dependence of RI of air filled in the cavity on its pressure,the resonate dips of the FPI shift with the surrounding gas pressure.Experimental results show that a gas pressure sensitivity of 4131pm/MPa is obtained.Compared to other FPIs,the proposed FPI consists of only SMF instead of any specialty fibers.In summary,three types of open-cavity fiber FPIs are proposed and demonstrated for gas pressure sensing.They have the advantages of simple configuration,easy of fabrication and high gas pressure sensitivity,which make the devices attractive in practice.