Multi-Parameter Detection Of Long-Period Fiber Gratings Inscribed In The Few-Mode Fibers With Coating Enhancement And Composite Structure

In recent years,long-period fiber grating（LPFG）with high sensitivity,easy fabrication,and long-distance real-time sensing,has become one of the research hotspots in the field of optical fiber sensing.However,due to the structural limitation of the single-mode fiber,it is difficult for the traditional LPFG to achieve a higher sensitivity.The coupling between the higher-order core mode and the cladding modes makes the LPFG inscribed in a few-mode fiber have different sensing characteristics from the LPFG inscribed in a single-mode fiber.Aiming to realize the muti-papameter detection,we will focus our work on the LPFG inscribed in a few-mode fiber,which is formed by the coupling between the higher-order core mode and the cladding modes.The refractive index sensitivity of the LPFG inscribed in a two-mode fiber is improved by coating enhanced film.Two kinds of the optical fiber sensors with composite structures,including a cascaded LPFG and a Mach-Zehnder interferometer（MZI）,are designed,to measure torsion,temperature,and refractive index.The major work of this thesis as follows:1.According to the coupled mode theory,we have analyzed the sensing characteristics of the LPFG inscribed in a few-mode fiber.The influences of the refractive index of film,thickness of the film,and surrounding refractive index on the transition region are simulated and analyzed.It is found that the high sensitivity can be achieved by selecting the appropriate film and thickness.The refractive index sensing mechanism of the bent LPFG has been described and analyzed theoretically.The results show that the bending LPFG is beneficial to improve the sensitivity of the sensor,and the refractive index sensitivity will decrease with the increase of the bent radius.2.The refractive index response characteristics of the enhanced LPFGs inscribed in a two-mode fiber coated with zinc oxide and graphene oxide films are studied.Firstly,a LPFG inscribed in a two-mode fiber is successfully fabricated by CO₂ laser,and then nano-material films are coated on the surface of the LPFG to enhance the refractive index sensing performance.The experimental results show that the highest refractive index sensitivity of the graphene oxide-coated LPFG inscribed in a two-mode fiber can reach 11605.79 nm/RIU through optimization.3.A novel biosensor based on a U-bent graphene oxide-coated LPFG inscribed in a two-mode fiber is proposed,and it can be used to detect the concentration of human Immunoglobulin（IgG）.The relationship between the bent radius and the refractive index response of the sensor is studied.The experimental results show that along with the decrease of the bent radius,the refractive index sensitivity of the sensor increases.To optimize the immune-sensing performance,the goat anti-human IgG is immobilized on the surface of the sensor with a bent radius of 15 mm,the lowest detection limit of human IgG can reach 23 ng/ml.4.A twist sensor based on the cascaded LPFG inscribed in a four-mode fiber is designed.Due to the unilateral exposure characteristics of CO₂ laser and the coupling between the high-order core mode and the core fundamental mode,the sensor has the advantage of high twist sensitivity.Besides,it is insensitive to temperature and refractive index.The sensitivity is greatly improved than that of the ordinary LPFG,and the maximum twist sensitivity can reach-0.519 nm/（rad/m）.5.A temperature sensor based on the composite structure of MZI and a LPFG inscribed in a two-mode fiber is designed.The temperature sensing performance of MZI sensor based on a symmetrical double-grooved is studied.The MZI sensor composed of two spherical structures is combined with a LPFG inscribed in a two-mode fiber.Beacuase of the interference peak of MZI and the resonance peak of LPFG have different sensitivities of temperature and refractive index,the temperature and refractive index are measured simultaneously,and the temperature and refractive index sensitivities are 118.23 pm/℃ and 828.09 pm/℃,-46.23nm/RIU and-36.76 nm/RIU,respectively.