Studies On The Enhancement Of Dynamic Range And Sensitivity For Fiber-optic Biosensors

In order to meet the requirements of small size, high integration, low cost, highsensitivity, and broad dynamic range for micro biochemical analysis systems,particularly lab on a chip, a biconical fiber-optic refractometer (BFOR) is studied. Ithas a simple structure, low cost, strong immunity to electromagnetic interference, andthe feasibility for integration. The schemes to improve the sensitivity have beeninvestigated and the application to label-free biological detections has been studied.The accomplished work and main points in this thesis are as follows:(1) Based on the properties of bound modes of a uniform optical fiber, the modetransition in a tapered fiber was illustrated by resort to local modes and the formulaefor calculating the coupling efficiencies were derived.(2) Silica nanospheres were used to modify the BFOR, combination of the Miescattering of nanospheres and multimode propagation in the tapered optical fiber(TOF) contributed to an enhanced sensitivity and a broadened detection range forrefractive index (RI) measurement. The theoretical model of the modified TOF(MTOF) was provided and analyzed, and the theory fitted well with experiments.(3) The effect of filtering higher-order optical fiber modes was discovered andproofed. The underlying mechanism was theoretically investigated and severalexperiments were designed for support. It was also experimentally-revealed that theMTOF was much less sensitive to methylene blue due to the higher-order mode filtering. Thus, while silica nanospheres improved the sensitivity for RI detection,they reduced the absorbancy sensitivity, which suggested the MOTF was suitable forextracting the RI information from an absorbing medium.(4) A simple bent TOF was proposed for highly-sensitive RI measurements. Theprinciple was illustrated. The theoretical models were deduced and the contribution oftapered cladding was shown through theory. A high sensitivity of4000nm/RIU wasexperimentally-obtained.(5) Biconically-tapered optical micro/nano fibers were used to realize label-freebiological recognition of several kinds of antigen dissolved in different solutions. Thelight guiding and mechanical properties of micro/nano fibers were investigated, andthe reasons for high sensitivities were revealed. A detection limit of10pg/mL forabrin in deionized water was obtained. In the end, the release experiments indicatedthe excellent regeneration ability.