Study On In-fiber Michelson Interferometric Sensing Technology

In-fiber Michelson interferometric sensors have attracted increasing attention inthe field of fiber sensing due to their advantages, including small size, low cost andhigh sensitivity, et al. Compared with the traditional all-fiber Michelsoninterferometric sensors, all-fiber Michelson interferometric sensors not only resolve theproblem that the sensing fiber is different from the reference fiber, but also have theunique characteristics of simple fabrication and flexible structure. In the thesis, weproposed two different In-fiber Michelson interferometric sensors. Combined withMagnetic functional nano-material and photonic crystal fiber, magnetic field sensorand refractive sensor are designed respectively.The contents of this thesis are asfollowings:①The basic principle and the sensing principle of the fiber Michelsoninterferometric sensors have been theoretically studied. In order to acquire highersensitivity, two different in-fiber Michelson interferometric sensors have beenproposed. Then the Michelson sensors with single-side geometry and molten photoniccrystal fiber are simulated by the beam propagation method, verifying that these twostructures have the function of splitting and combining light. From the simulationresults, it can be found from the first structure that the fundamental core mode excitesasymmetric cladding modes when it comes to the single-side geometry. Meanwhile,the second structure shows that the molten photonic crystal fiber (PCF) excites highercladding modes and keeps the optical field stable when the length of the molten regionis170μm, which provides the basis for sensors fabrications and experiments.②Based on the stimulation results, a single-side geometric deformation is formedby melting the single mod fiber with high frequency CO2laser. The effects of sensingfiber length on the interference spectrum have been analyzed. The magnetic fieldsensor is fabricated and studied with the magnetic fluid whose refractive changes withmagnetic field intensity. The experimental results show that the sensitivity of the fiberMichelson sensor magnetic field sensor is~3.2pm/mT in the range from0mT to110mT. According to the analysis of the experimental results, the sensing fiber isetched with hydrofluoric acid to enhance the evanescent-field of the cladding mode inthe external environment, optimize the structure and improve the sensitivity. Themagnetic field intensity is then measured by the optimized sensor. The results show that the highest sensitivity is obtained when the diameter of the cladding is etched to50μm. The sensitivity of the optimized structure is~74.9pm/mT, which is almost23times as that of original structure.③Based on the stimulation results of the refractive index sensor, single modefiber and photonic crystal fiber are fused by the fusion splicer with optimizedparameters and the length of the molten region between the two fibers is170μm. TheMichelson interferometric sensor is finally formed by spherical reflector fabricated bythe splicer discharge on the other end of the photonic crystal fiber. Then the refractiveindex of the liquid is measured by the sensor. The experimental results show that thefiber Michelson sensor has good linear refractive index response within1.325-1.375,and the sensitivity is~199nm/RIU with linearity of~0.993. The effect of the moltenregion on cladding modes excited by the fundamental core mode has been studied andoptimized. At last, the temperature characteristic of the sensor is measured. Thesensitivity is~12pm/℃within20-150℃.Compared to the refractive index, theexperiment results show that the temperature has little influence on the inferencespectrum, which can be affirmed that the fiber Michelson interferometric refractivesensor based on photonic crystal fiber is temperature-independent, avoidingtemperature caused cross effect.