The Speciality Of D-type Few-mode Fiber Bragg Grating And Its Applications In The Refractive Index Measurement

Based on presenting the characteristics, sensor principle and transmission speciality of Fiber Grating in brief, this thesis summarize the study actuality of refractive index sensor using Long Period Fiber Grating (LPG) and put forward our new refractive index measurement using Few-mode Fiber Bragg Grating (FBG). LPGs have severe cross-sensitivity, unstable transmission spectrum and lower precision; normal FBGs are insensitive to the Surrounding Refractive Index(SRI) and their mechanism intensity will get weak when corrupted very thin. The measurement using D-type Few-mode FBG brought forward will lift the sensitivity depend on deeper etching into core with stronger toughness.This thesis validate the feasibility of the scheme through a lot of simulation and experiments, and bring forward solution to the problems maybe occurred in the actual application.The tasks of analog computation include : i) Simulating the coupling coefficients of D-type Few-mode FBG transmission modes and LP02 mode changing relation with work wavelength. ii)Simulating relations of the Bragg wavelengths changing with envelope thickness and SRI of normal, D-type and D-type with deep etching FBGs respectively and make contrastive analyses. iii)Simulating the wave-guide bi-refraction effect of D-type FBG with deep etching changing relation with several parameters and the splice loss issue between fiber and D-fiber.In the experiment, we observe the section thickness of fiber and D-fiber under different etching times using Scan Electron Microscope(SEM) and get the etching velocity under HF with 20% concentration; measure Bragg wavelength of normal and D-type FBGs changing relations with SRI and temperature respectively; analyze the effect of the stress and temperature on the refractive index measurement of FBGs.