Fabrication Of Novel Microstructured Fiber Optic Sensors And Study On Their Characteristics

Fiber optic sensors technology has a rapid development in recent years. Varietiesof new fiber sensor structures have been explored and applied. They not only enrichthe kinds of fiber optic sensors, but also deepened our understanding of the fibersensing mechanism, promoting forward of the optical fiber sensing technology. Inorder to simplify the fabrication process, reduce device cost, improve the detectionsensitivity and increase the sensing parameters, many efforts have been made toinvent new microstructure fiber sensors, which has potential applications and beenhighlighted in harsh environment sensing, chemical substances detection,biomedicine monitoring and so on.In this paper, based on the research of new microstructure fiber sensors, wepropose and fabricate two novel microstructure fiber sensor devices, namelymicrohole array fiber Bragg grating (FBG) and S fiber taper (SFT) Mach-Zehnder(MZ) interferometer. Their sensing characteristics have been studied in detail. Theformer has excellent temperature stability and can work at high temperature up to1000℃. It is also able to simultaneous measurement of refractive index (RI) andtemperature, which means that the sensor could be used for detecting RI in a hightemperature environment. The latter has high RI and axial strain sensitivities, whichis1~2orders of magnitude larger than conventional FBG and fiber-opticinterferometer sensors. Based on the SFT, we also fabricate another twohigh-sensitivity fiber sensors for measurement of temperature and transverse load byusing liquid packaging and polymer encapsulation technologies. Their sensingmechanisms have been deeply analysed. The main contents of this paper are asfollows:1. Femtosecond (fs) laser and phase mask method has been used to fabricate the Type II FBG on common single-mode fiber. The grating region extends from thefiber core to cladding outer edge due to fs laser self-focusing effect. Hightemperature annealing and stress sensing experiments have been carried out on thefs-FBG. Its corresponding sensitivites are14.15pm/℃and1.465nm/N. Accordingto the characteristics of HF acid selective chemiacal etching of fs laser modifiedregion, the Type II FBG has been chemical etching to form microholes, microgroove,and microchannel. Through controlling of the etching process, we have fabricatedmicrohole array FBG with good transmission spectrum, and studied its sensingcharacteristics, including RI, temperature, and strain. The obtained correspondingsensitivities are17nm/RIU (RI unit),14pm/℃and1.71nm/N, respectively.Simultaneous measurement of RI and temperature is realized through monitoring theresonance wavelength and transmission power. The corresponding sensitivity matrixis also obtained. The microhole array FBG inherits high temperature stability of TypeII FBG and can be used for RI sensing in high temperature and harsh environment.Since the fiber cladding only has been partially etched to form the microholestructure, this sensor has much better mechanical strength and stability compared tothe FBG RI sensor with the whole cladding etched.2. A fiber fusion splicer has been used to fabricate a novel SFT by off-axistapering method. The SFT works similar to the MZ interferometer. Changing of thefabrication conditions, we get SFTs with different structure parameters and spectralcharacteristics. The fabrication repeatability of the SFT has also been studied. Modecoupling process in the SFT is analysed by beam propagation method and the MZinterference principle of the SFT is introduced. We detailedly studied the sensingcharacteristics of SFT, including temperature, RI, and axial strain. It is insensitive totemperature around room temperature and the maximum RI and strain sensitivitiesare2124nm/RIU and-183pm/με, respectively. In addition, the SFT has beenimproved to a reflective sensing probe by silvering its end face. We have studied thesensing characteristics of the SFT probe, including RI and temperature. The SFTprobe can be used for temperature-insensitive RI sensing. As a novel MZ interferometer, the SFT has many obvious advantages compared with the traditionaldual-coupler MZ interferometer, including:1. Simple structure, the SFT is an inlinefiber structure and fabricated on a single-mode fiber, single input and single output,without multiple fiber connections, it has better stability;2. Easy of fabrictaion, justneed once off-axis fiber tapering, greatly reduces the production cost;3. Small size,the SFT has a length of less than1mm, available in narrow space for specialoccasions, and is conducive to integrated applications;4. High sensitivity, its strainsensitivity is two orders of magnitude higher than that of normal FBG (1pm/με), itsRI sensitivity is in the same order of magnitude with LPFG, and1~2orders ofmagnitude higher than the normal optical fiber interferometric sensors.3. Based on the liquid sealing technology, we have fabricated a liquid-sealedSFT by AB glue enclosing a capillary with the SFT and glycerol/water mixedsolution inside. We have analyzed the temperature sensing mechanism of theliquid-sealed SFT, and pointed out the temperature sensitivity derived from threeaspects, namely the thermal expansion effect of AB glue, the thermo-optic effect ofRI liquid and the hydraulic pressure from RI liquid thermal expansion. We havestudied the temperature sensing properties of the liquid-sealed SFT and obtained itstemperature sensitivity up to-1.403nm/℃, which is two orders of magnitude higherthan the common FBG. The sensor has a simple fabrication process and compactstructure. Its overall size may be less than10mm. It can be used to detecttemperature in the range of20~50℃, suitable for biological, medical sensing area,such as cell culture, temperature monitoring in operation, etc.4. Based on the polymer encapsulation technology, we have fabricated a fibertransverse load/temperature sensor by coating the SFT with PDMS. The SFT positionin the PDMS coating has strong impact on the transverse load sensitivity, andprestress in the SFT when it is packaged affects the linearity of sensor response.Transverse load and temperature sensing mechanisms of the PDMS-coated SFT havebeen analysed in detail. The formulas of transverse load and temperature sensitivityare deduced and their sensitivity values are estimated. Transverse load and temperature sensing characteristics have been studied on two sensors (sensor1:S-bending plane is perpendicular to the top surface of encapsulation layer, withoutaxial prestress; sensor2: S-bending plane has45degrees angle to the top surface ofthe encapsulation layer, with0.2N prestress in the SFT when it is packaged). Themaximum transverse load sensitivity is-29.0nm/N, the maximum temperaturesensitivity is-2.17nm/℃. Based on the dual-wavelength demodulation method, thetransverse load/temperature dual parameters sensing process of sensor2has beenanalyzed.