Optical Fiber Mach-Zehnder Interferometer Sensor Based On Core-offset Structure

As a category of novel sensors, optical fiber sensors have been widely applied tomonitor quantities such as temperature, strain, gas phase concentration, vibration,acceleration, or refractive index (RI) change in solutions, etc, thanks to the drasticdevelopment in the past few decades. Compared with the other sensors, fiber sensors havemany distinct advantages, such as low loss, compact and small size, low cost, immunity tomagnetic interference, and the capability to work in harsh environment. Thus, fiber sensorsare good candidates for earthquake precursor monitoring. Researched core-offset structurebased on multimode interference, a single-mode optical fiber sensor based on core-offsetstructure with high extinction ratio (ER) is proposed and demonstrated.The main work points of this thesis are as follows:(1) Multimode interference theory of optical fiber sensors based on core-offset structureis analyzed, and the implicit dispersion equation of cladding modes can be derived based onvector Helmholtz equation and electromagnetic field boundary conditions. A graphic methodis adopted to solve the effective refractive index of cladding modes. Theoretical analysis andnumerical simulation of the effective refractive index of cladding modes in the differenttemperature, RI, and strain are completed.(2) Transverse field profile of sensing SMF in different distances and ER in differentcore-offset direction between two misaligned spiced joint are simulated by Rsoft. Theimproved sensor is immune to the core-offset direction, which takes advantages of its easyfabrication and the high extinction ratio. And the simulated result shows that the improvedsensor has a four times higher ER than the previous sensor with two offset structures in4μmcore offset.(3) A fusion splicer with a built-in attenuator program was used to fabricate the twosensors. Experimentally, the transmission spectrums are analyzed. The result indicated thatthe improved sensor has a1.5times higher ER than the previous sensor with two offsetstructures both in1.7μm offset. And two fiber sensors with different structures areinvestigated experimentally in sensing applications of temperature, RI, strain. For theimproved sensor the sensitivities of73pm/°C,-64.889nm/RIU, and-1.9pm/με, respectively can be achieved. Moreover the sensors with two core-offset sections have sensitivities of50pm/°C,-38.692nm/RIU, and-1.3pm/με, respectively. The experimental result shows thatthe sensitivities of the former are higher than the latter.