Study Of Fiber Sensor Based On Multimode Interference Coupler And Few-Mode Fiber Laser

Fiber sensor based on multimode interference coupler has an important meaning in intelligence,automation and internet of things for its concise structure,low cost and excellent sensing performance with an extensive research in recent years.Furthermore,the rapid growth in fiber manufacture progresses with the maturity of the handling technology,and the designed novel comb fiber filter based on modal interference is used in fiber laser with a nonnegligible potential value in multi-wavelength,tunable fiber laser.Under the supports of National Nature Science Foundation of China and Fundamental Research Funds for the Central Universities,the main work in this paper is exhibited as follows:1.The scalar simulation model of cascaded core-offset fiber structure based on SMF and NCF-CoreOffset fiber structure are established,and characteristics of the filter based on few-mode/two-mode fiber(FMF/TMF)are analysized and optimized.A stable fiber laser with single-,dual-and triple-wavelength lasing emission is realized at room temperature.Combing with core-offset fiber splicing structure,the comb filter based on Er-doped few-mode fiber is established and optimized with gain medium and filter simultaneously.Based on unidirectional/bidirectional pump and nolinear polarization rotation(NPR)effect to suppress mode competition,different multi-wavlength lasing characteristics are researched.2.A fiber laser sensing system with single-wavelength emission is proposed and realized based on singlemode-hollowcore-singlemode(SHS)fiber structure by fused splicing with band-pass filter,and the transfer matrix simulation model is established based on the slowly-varying fiber coupling structure.The response characteristic between resonant wavelength of filter and surrounding RI is optimized with a maximum linear RI sensitivity of 316.2 nm/RIU.Meanwhile,the fiber ring cavity laser based on the filter is established,improving the slow monitoring reaction speed from the wide 3 dB line-width,and the relationship between the lasing wavelength and surrounding RI is studied.3.A high curvature sensitivity sensing structure is proposed and optimized based on multi-cladding fiber sandwiched dual no-core fibers structure.The dependence of resonant wavelength on curvature is increased with a linear curvature sensitivity about-39.02 nm/m-1 from 0.3 to 2.14 m-1,and the local sensitivity as high as-43 nm/m-1.4.The method based on tapering a SMF with hydrogen flame overcomes the difficults of weak tapered fiber structure based on fiber splicing machine and controlling parameters to grasp.Cascaded dual long-taper fiber structure with comb filter is proposed based on slowly varying bending method of the long taper with SMF,and a brief analysis are applied to a fiber slowly varying coupling structure based on the output beam profiles with different bending states under the injecting wavelength of 1550 nm.After optimized,a good stability of the filter is achieved in the experiment and a fiber laser with stable dual-and triple-wavelength emission is realized at room temperature.