| As a new type of optical waveguide in the sub-wavelength scale,the micro/nanofiber not only has the advantages of low cost,anti-electromagnetic interference and corrosion resistance,but also has the unique advantages of strong evanescent field,fast response and good flexibility.The microfiber knot resonator(MKR)has been extensively studied for its structural stability,high quality factor and strong interaction with the external environment Due to the shortcomings of a single MKR in improving optical properties,a variety of composite structures based on MKRs have been proposed,such as coupling multi-knot,cascade multi-knot,and Ushaped nested MKR structures.Compared with other composite structures,the U-shaped nested MKR has the advantages of compact size and ease of preparation,however,most of the research on this structure is focused on silicon-based integrated optics.Therefore,the research on allfiber U-shaped nested MKRs is of practical importance and value in promoting the application of the structures in related fields.In this work,the comprehensive investigation of U-nested MKRs was presented,in which different moisture-sensitive materials were used to achieve high-sensitivity humidity sensing and ultra-fast response for human respiration monitoring,respectively.In addition,three Ushaped nested parallel MKRs were proposed with both a wide free spectral range(FSR)and a deep extinction ratio(ER)response through interference superposition.The main contents of this thesis are as follows.(1)Based on the conventional single MKR model,the optical transmission characteristics of the U-shaped nested MKR were analyzed using the transmission matrix method.The effects of the U-shaped waveguide length,the coupling coefficient and the transmission loss coefficient on the output spectrum were discussed.It was demonstrated that when the U-shaped waveguide length was an integer multiple of the circumference of the knot,a periodic resonant spectrum with wider FSR could be achieved without reducing the MKR diameter.(2)Two sensing applications were achieved by coating two moisture-sensitive materials on the U-shaped waveguide,respectively: 1)a humidity sensor with a maximum sensitivity of2.442 nm/%RH was realized by coating the agarose combined with the periodic envelope spectrum with Vernier effect;2)by coating the gelatin,the U-shaped MKR sensor showed a wide intensity response at different humidity levels,and successfully applied to human respiration monitoring sensing,which was experimentally shown to have a response speed of 84 ms and a recovery speed of 29 ms.(3)Three U-shaped nested parallel MKR composite structures were proposed by progressively introducing parallel inner knots and couplers,and their transmission characteristics were explored in a theoretical study.Three samples were prepared based on the U-shaped nested parallel single-knot MKR structure,all of which obtained high ER responses of about 10 d B with FSR and quality factor of 760 pm,1500 pm,1125 pm and 17158,14498,24599,respectively;a U-shaped nested parallel double-knot MKR sample was prepared with FSR and ER of 986 pm and 5.75 d B,respectively,and quality factor of 14090.Compared with the conventional single MKR structure,all three composite structures could exhibit transmission spectra with wide FSR and high ER by selecting suitable structural parameters,achieving an effective improvement in transmission performance.Based on the optical transmission characteristics of the basic MKR structure,this work proposed and prepared a series of compact MKR structure resonators,and analyzed their transmission characteristics and applications.It was fully demonstrated that the proposed several structures could effectively improve the transmission performance of the conventional MKR and provide new ideas and methods for the optimal design of the MKR structure. |
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