High Temperature Tolerant Fiber Optical Micro Interferometer Sensor

Optical fiber sensing technology is one of the very promising techniques in sensingfields because it can be used in many harsh environments, such as high temperature,high humidity, and strong electro-magnetic interference. Moreover, optical fiber sensorshave the advantages of small size, lightweight, high sensitivity, all fiber and so on. So ithas attracted many researchers’ attention around the world. Among the fiber sensors, thehigh temperature sensor based on optical fiber is one of the important. Severalfabrication methods to produce the high temperature fiber sensors have been proposedin recently, for example, femtosecond laser machine method,157nm laser machinemethod and ion beams method. In this dissertation, a low cost and simple method byemploying a commercial electric arc fusion splicer to produce a kind of hightemperature fiber sensors is presented after the analysis and simulation works about theoptical fiber lateral offset structure were finished. The fabricated sensors including thehigh temperature sensor, the high temperature refractive index sensor and the hightemperature strain sensor. Also the sensing characteristics of the temperature, refractiveindex and strain are studied in details. The main contents are as follows.①We studied the optical mode stimulation and coupling of the optical fiberlateral offset structures, analyzed the optical field distributions before and after thelateral offset structure with different offset values were spliced under the differentelectronic arc power levels.②Based on the analysis results of the optical fiber lateral offset structure withsmall offset value, we designed and fabricated two kinds of interferometric sensors; oneis the tip FPI sensor using the solid-core photonic crystal fiber (PCF) and the hollowcore optical fiber (HOF), and the other one is the MZI sensor using the HOF. And theirhigh temperature responses are carried out. A good linarility and high sensitivity wereobtained. For example, the tip FPI based on PCF having a sensitivity of14.5pm/℃withgood linearity and good repeatablilty in temperature range from room temperature to800℃.③We proposed and demonstrated a MZI and a FPI sensor with the light insensing arm totally exposed to the outside environment based on the optical fiber lateraloffset structure with the large offset value. Because the materials such as gas or liquidcan easily interact with the optical field in the sensing arm, the refractive index sensitivity for this kind of interferometric sensors is very high. Experiments resultsshow, that the MZI have a refractive index sensitivity of~-3400nm/RIU in refractiveindex range of1.0~1.0022and temperature cross sensitivity of～36pm/℃, the FPIhave a refractive index sensitivity of over1500nm/RIU in refractive index range of1.0~1.0022and a temperature cross sensitivity of lower than0.25pm/℃.④In order to overcome the disadvantages such as the large stress concentrationsat the geometric discontinuities and the nonlinear relationship in strain response of theoffset sensing structures, we fabricated the micro-bubble-based FPI sensors through seala little air in the fiber joint during splicing. Two methods were employed to achieve this,one is by splicing two sections of SMFs with a flat tip and a hemispherical tip,respectively, and the other one is by splicing SMFs whose ends have been etched inhydrofluoric acid for several minutes. Experiments results show that this sensor have ahigh strain sensitivity of~4pm/με and low temperature sensitivity of less than0.9pm/℃.