Research On Fiber Optic Refractive Index Sensors Based On Bent Structure And Demodulation Methods

Bending will result in bending loss of the fiber.By detecting changes in the bending fiber loss,sensing of the parameter causing the change can be achieved.In the thesis,optical fiber sensors based on bent single mode fiber structure were studied.In order to achieve refractive index measurements,overcome temperature crosstalk and light source power fluctuations in demodulations,two new demodulation methods are proposed.The main research studied is as follows:1.The theoretical research on the bent single-mode fiber were conducted.Two models describing the bending loss of the fiber were given.And the phenomenon of fiber bending loss fluctuation with wavelength and bending radius was explained.The refractive index and temperature sensing mechanisms of bent single-mode fibers were qualitatively analyzed from the perspectives of geometric optics and mode theory.The mode field distribution in bent single-mode fiber was simulated.2.A droplet-like fiber interferometer(DLFI)based on bent single mode fibers was proposed.The production process of DLFI was illustrated.And the refractive index and temperature sensing characteristics of DLFI were experimentally studied,which verified the sensing theory and laid the foundation for demodulation method research.3.An edge filtering demodulation method based on DLFI cascaded with fiber Bragg gratings(FBGs)structure was designed.The refractive index sensing and the automatic temperature compensation principles based on the demodulation method were expounded in theory.Experimental studies found that this demodulation method has a linear sensitivity of 193.18 d B/RIU in the refractive index range of 1.3378-1.3969,and the temperature crosstalk is only2.07?10^-4 RIU/?.4.An intensity differential demodulation method based on DLFI cascaded with FBGs structure was designed.The refractive index sensing principle based the demodulation method was expounded in theory.The method can not only realize automatic temperature compensation,but also eliminate the influence of source power fluctuations on demodulations.Experimental studies found that this demodulation method has a linear sensitivity of 385.25 d B/RIU in the refractive index range of1.3388-1.3998.And the temperature crosstalk is only 6.05×10^-5 RIU/?.In addition,the result of intensity difference is affected by the fluctuation of the light source,which is only 1.6%of the direct intensity demodulation.