Research On Novel Micro-nano Structured Optical Fiber Sensing Technology

Micro-nano structured optical fibers have attracted extensive attention from scholars both at home and abroad for decades due to their flexible design,multiple transmission mechanisms,and easy integration with other mechanisms.With the rapid development of the drawing technology,processing and post-processing technology of the micro-nano structured optical fibers,the micro-nano structured optical sensors have also been given more feasibility and functionality,making it an important application in the sensing field.Advanced fiber drawing technology endows micro-nano structured fiber with many novel structures,such as multi-core micro-structured fiber,negative curvature fiber,photonic bandgap micro-structured fiber,etc.The developed processing and post-processing technology endow the micro-structured optical fiber with different functions,such as coating metal film,filling metal or temperature-sensitive liquid in the micro-nano structured optical fibers,to improve their temperature or refractive index(RI)sensitivity,or helical twisting the micro-nano structured optical fibers to break the traditional cylindrical symmetry,thus bringing new characteristics to the micro-nano structured optical fibers.In the field of biochemical detection,metal oxides and catalysts can be coated on the surface of micro-nano optical fibers to detect the progress of certain chemical reactions,or modify antibodies to detect specific bacteria and viruses in samples.When the external environment changes,some characteristic loss peaks or interference spectra of the micro-nano structured optical fiber sensors will change.At present,the sensitivity of optical fiber temperature sensors is low and their detection range is narrow.It is difficult for fiber RI sensors to detect analytes in low RI ranges.The detection limit and accuracy of biochemical sensors are insufficient and their detection time is long.To solve the above three problems,this thesis studied and designed three kinds of micro-structured fiber(MSF)temperature/low RI sensors,and uses three different micro-nano fibers(MNFs)to carry out experimental research on biochemical sensing.The specific research content and innovation points are as follows:(1)Two kinds of temperature sensors are researched and designed based on dual-core all-solid cladding MSF and helically twisted(HT-MSF),respectively.The transmission mechanism of the temperature sensor with the dual-core all-solid cladding MSF is the supermode interference effect.The dual-core structure of the optical fiber is formed by filling the two air holes of the optical fiber with high RI temperature-sensitive liquid(toluene),and an ethanol-filled air hole is introduced in the center of the coupling bridge of the two toluene-filled fiber cores to further adjust the coupling between the two fiber cores,thus adjusting the supermode interference effect.By optimizing the fiber structure parameters,the average temperature sensitivity of the proposed fiber temperature sensor can reach-11.64 nm/℃ and-7.41 nm/℃ in the temperature range of 0 to 7℃ and-80 to 0℃,respectively,and is insensitive to hydrostatic pressure.For the helically twisted micro-structured optical fiber temperature sensor,the original cylindrical symmetry is broken by helically twisting the optical fiber,and the traditional linear polarized modes in the optical fiber are converted into circularly polarized modes,thus providing an additional adjustment dimension.By combining the helical twist,surface plasmon resonance(SPR)effect and filling with temperature-sensitive liquid,the average sensitivity of the fiber sensor can reach 14.35 nm/℃ and 17.29 nm/℃ in the range of-20℃ to 20℃ and 20℃ to 70℃,respectively,and it is also insensitive to hydrostatic pressure.(2)A low RI sensor based on negative curvature fiber is researched and designed.For the traditional RI sensor,the detection range of the RI is usually higher than 1.33 due to the limitation of the traditional total internal reflection mechanism.However,for many commonly used analytes,such as sevofluoroether and halogenated ether,their RIs are usually about 1.27.The negative curvature fibers(NCFs)depend on the anti-resonance mechanism,which makes their core mode transmit in the low RI region.Based on this advantage,a low RI sensor by the NCF with high birefringence is researched and designed by combining NCF and the SPR effect.The simulation results show that the average sensitivity and maximum sensitivity of the RI sensing,calculated by using the confinement loss method,are 4667 nm/RIU and 5700 nm/RIU respectively in the RI range of 1.20 to 1.34.By using birefringence,the average sensitivity and maximum sensitivity of the RI are 4833 nm/RIU and 6100 nm/RIU,respectively.(3)Experimental study of biochemical sensor based on U-shaped MNF.First of all,the label-free detection of Staphylococcus aureus(S.aureus)based on the U-shaped "single mode-tapered multimode-single mode" optical fiber structure was studied.The specific detection of S.aureus in phosphate buffered salt(PBS)solution and food samples was carried out by functionalizing the pig IgG antibody on the surface of tapered multimode fiber.The experimental results show that the sensor can stably detect S.aureus at a concentration of 4 cells/mL.When the concentration of pig IgG and S.aureus samples is 200 μg/mL and 4 cells/mL,the average wavelength shift reached 1.552 nm.After 13 consecutive tests,a sample of S.aureus with a concentration of 0.4 cells/mL was successfully detected once.Compared to the results in PBS,the average percentage deviation of wavelength shift in milk and lettuce samples was-21%.Hydrogen concentration detection based on the U-shaped "single mode-tapered four core-single mode" optical fiber structure is studied.The hydrogen-sens i ti ve material(Pt/WO3)is modified on the surface of the tapered four-core optical fiber to react with hydrogen.According to the change in the RI before and after the chemical reaction,the optical fiber interference spectrum will change to detect the chemical reaction’s progress and measure the hydrogen concentration,The experimental results show that the sensitivity of the optical fiber sensor reached-17.706 nm/%in the range of hydrogen concentration from 0%to 1%.It is also insensitive to temperature and relative humidity.(4)Label-free detection of Salmonella Typhimurium(S.Typhimurium)by fiber ring laser biosensor based on "single modetapered-seven core-single mode" structure.Generally,the spectrum of optical fiber interferometer has a large full width at half maximum(FWHM)and a low extinction ratio(ER),which greatly limits the resolution and detection limit.The above optical fiber structure is embedded into the loop containing Erbium-doped Fiber Amplifier(EDFA),and the extremely narrow FWHM and high ER output spectra are obtained.The experimental results show that the biosensor has good stability,high sensitivity and fast response time,and can stably detect S.Typhimurium at 10 cells/mL concentration.S.Typhimurium was also detected in two food samples(chicken and pickled pork).Compared with the results in PBS solution,the average percentage change of wavelength shift in chicken and pickled pork samples was-27%and-23%,respectively.In this thesis,the structural characteristics and light conduction mechanism of different kinds of MSFs are studied,and the performance of three kinds of fiber temperature and fiber RI sensors are designed and analyzed numerically.The high sensitivity temperature and RI sensing are realized.By utilizing the advantages of a high evanescent field and high sensitivity of MNFs,experimental research was conducted on the applications of MNFs in biochemical sensing,achieving high sensitivity detection of bacterial concentration and hydrogen concentration,respectively.The relevant design ideas and experimental schemes in this thesis provide an important reference for the subsequent design of micro-nano structured optical fibers and their applications in biochemical sensing.