Refractive-index Sensor Of Microcapillary Based On Fano Resonance

In the field of optical microcavity,the small volume and high quality factor of whispering-gallery-mode(WGM)microcavity make it useful in the fields of laser,filter,sensor and so on.Especially for the microcapillary cavity,the micro fluid can be transmitted through the tube channel,making it easy to achieve microfluidic integration,so this kind of cavity has a significant advantage in the field of biochemical sensing.In recent years,compared with the traditional WGM spectrum in microcavity,Fano resonance with asymmetric lineshape has begun to attract people's attention and research.In this paper,the refractive index sensor of microcapillary based on Fano resonance is studied.Firstly,several basic parameters of WGM microcavity are introduced,including resonance wavelength,quality factor,free spectral range,fineness and mode volume.From the Maxwell equation,the electromagnetic field fluctuation equation in cylindrical coordinate system is obtained.Then we derive and obtain the intrinsic equation of WGM radial component in the ring microcavity,and a simple analysis is made to observe the influence of the parameters' change on resonant wavelength.Based on the electromagnetic field fluctuation equation in the cylindrical coordinate system,the relationship between the propagation constant of taper fundamental mode and the diameter is obtained.The phase matching condition of the microcavity coupled fiber taper is briefly discussed.Then we talk about the coupling model between microcavity and waveguide in ideal conditions,and analyze the quality factor of the liquid core microcapillary.We introduce the performance parameters of microcapillary sensor,and analyze the influence of various factors on the sensitivity,including wavelength,polarization,outer diameter,wall thickness and radial modulus.The wavelength shift coming from temperature disturbance is also discussed.Fano resonance mechanism based on the coupling between tapered fiber and single microcavity is first discovered.When adding loss and phase shift in the ideal coupling model,we obtain the Fano resonance transmission formula of the microcavity coupled fiber taper,in which we introduce the parameter N and ? to characterize Fano resonance.In the microcapillary coupling experiment,we discover the Fano resonance phenomenon and evolution process obviously.The experiment results can agree well with the theoretical simulation results.Using high-order Fano resonance modes for sensing experiment,both high sensitivity(?800nm/RIU)and high figure of merit(FoM,?4100)can be achieved,which will improve the overall performance of biochemical sensing.