Research On The Sensing Characterristics Of Tapered Multimode Fiber Bragg Grating

Compared with the interferometric sensor,the multimode fiber Bragg grating sensors are more resistant to interference,because the realization of fiber grating sensing is mainly based on wavelength detection.It's not only easy to couple with various optical devices owing to its large core size,but also the high order mode can be used in the core.The respective advantages of multimode fiber and Bragg gratings could be developed when the latter written in the former.There is brilliant prospects in the field of fiber communication and sensing.In this thesis,the modes characteristics of the step and graded fiber were analyzed by using multimode fiber theory,and the mode field of the low order linear polarization mode was studied with numerical simulation.By means of coupled-mode theory,the mode coupling condition and the influence of different parameters on the reflection spectrum of were demonstrated.A tapered multimode fiber Bragg gratings sensor was proposed,which is theoretically analyzed and simulated.The relationship between the modes and the sensitivity were established according to the three layer optical fiber model and the coupled-mode theory.The effects of thickness of the cladding,the radius of the core,the mode order,the refractive index distributed number and the taper on the sensing properties were discussed.The sensing characteristics of step-fiber Bragg gratings is superior to graded-fiber gratings due to the strong evanescent wave effect of the previous one,which is obtained through comparative analysis between them.Different performances of tapered and untapered graded-fiber gratings in temperature and strain sensing,which provides a sufficient theoretical basis for temperature and pressure dual parameter sensing.