| Whispering gallery mode(WGM)resonators show great potential in a series of sensing and technology applications due to their super high quality factor,extremely small mode volume,very high power density and very narrow spectral linewidth,and have gradually attracted the attention of researchers.Meanwhile,the advances in material science,fabrication techniques,and photoelectric sensing strategies have endowed the WGM resonators with new functionalities,unique sensing mechanisms and unparalleled detection sensitivities.Various geometric resonators have been developed to meet the requirements of different sensor applications,among which fiber-coupled microsphere resonators are the simplest threedimensional WGM resonators with low transmission loss and high quality factor.In addition,microcapillary resonators have natural fluid transmission channels,which provide the possibility of highly sensitive optical fluid sensing in the fiber.In this paper,some theoretical and experimental studies were carried out on the fiber-coupled microsphere resonators and microcapillary resonators,along with their resonant properties,fabrication technologies and sensing applications.The main research work includes the following contents:(1)The coupling models of WGM resonator were established,and the resonance characteristics were simulated and analyzed.By using the transfer matrix analysis method,the coupling model of resonator was analyzed and the expression of transmission characteristics was obtained.Then,by using the numerical analysis software FDTD Solution based on finite time domain difference algorithm,the influences of the refractive index(RI)of the environmental medium and structural parameters on the resonance characteristics of the microsphere resonator and the microcapillary resonator were simulated and analyzed.The simulation results showed that when the RI of the environmental medium increases,the resonance dip will move to the long wavelength direction,and the RI sensitivity of the microsphere resonator is inversely proportional to the diameter of microsphere.On the other hand,the RI sensitivity of the microcapillary resonator is directly proportional to the diameter of microcapillary.In addition,reducing the wall thickness of the microcapillary can effectively increase the RI sensitivity of the microcapillary resonator.These simulation results will provide some theoretical guidance for the fabrications of resonators and the studies of sensing characteristics.(2)The fabrication and packaging technologies of the WGM resonators were studied.By using hot stretching method,fusion machine electrode discharge method and hydrofluoric acid etching method,the tapered coupling fibers,microsphere and microcapillary with different structural parameters were fabricated,respectively.Besides,the coupling system could produce an optimal resonant spectrum by controlling the structural parameters of the microsphere and microcapillary and using a three-dimensional adjusting mount to adjust the coupling state between the resonator and the tapered fiber.Finally,in order to improve the stability and practicability of the WGM resonators,special packaging and fixation schemes were proposed for microsphere and microcapillary resonator respectively.The experimental results proved the effectiveness of the packaging scheme,which can built an experimental foundation for the sensing application of the WGM resonator.(3)The experimental platforms of the fiber-coupled microsphere and microcapillary resonator sensors were built and their sensing characteristics were tested and analyzed.By analyzing the movement of the WGM spectrum under different RI ethanol solutions,it was found that the RI sensitivity and the detection limit of the microsphere resonator sensor were about 20.49nm/RIU and 4.3×10-4RIU.The temperature characteristics of the microsphere sensor were measured by a water bath heating method,and the sensitivity was about 7.38pm/?with the detection limit of about 1.26?.In addition,this paper first proposed an acoustic intensity sensing method based on the microsphere resonator,and the sensitivity was about-1.9pm/dB.The RI sensitivity and the detection limit of the microcapillary resonator sensor were about 153.7nm/RIU and 2.8 ×10-3RIU,by analyzing of the spectrum changes of the resonator when ethanol solutions with different RI were infiltrated into the microcapillary.The temperature sensitivity of the microcapillary resonator sensor was measured to be about 9pm/?by the incubator heating method.Finally,a microcapillary resonator magnetic field sensor was fabricated by filling magnetic fluid into the microcapillary,and the magnetic field sensitivity of the sensor was about 0.54 pm/Gs when the wall thickness was 4 ?m.In this paper,the sensing characteristics of the fiber-coupled microsphere and microcapillary resonators were systematically studied both in theory and experiment.The encapsulation and fixing schemes of microsphere and microcapillary resonator sensors were proposed,which showed good stability.In addition,the microsphere resonator sensor was used for acoustic intensity sensing for the first time,which could promote the application of the WGM sensor in the field of acoustics. |
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