Fano Resonance Of Whispering Gallery Modes In Conical Microresonator

Due to the high quality factor and small mode volume,whispering gallery mode(WGM)microresonators are able to greatly enhance the interaction between the optical field and matter in the microresonator,thus have been attracted much attention from researchers.They have been widely applied in high-sensitive sensing,low-threshold laser,nonlinear optics and so on.Recently,with the research development of theory and practical application of WGM microresonators,researchers are combining the new technology and traditional physical phenomenon with WGM microresonators.They are having a more in-depth study on all kinds of WGM microresonators both theoretically and in practical application.Among these research work,Fano spectra with asymmetric lineshapes in microresonators are of great interest due to their asymmetric lineshapes different from symmetric Lorentz lineshapes of conventional WGM spectra.In this paper,we study the Fano resonance based on conical WGM microresonators.The details are summarized as follows:(1)For the first time,we theoretically and experimentally study the oscillating Fano spectra on fiber-taper-coupled conical WGM microresonators.At theory,a theoretical model of the coupling between the fiber taper and the conical microresonator is established,and the oscillating transmission spectrum is established based on the coupling model.The oscillating transmission spectrum comes from the interference between the discrete optical field coupling from the microresonator to the fiber taper(coupling field)and the continuous optical field directly passing through the fiber taper as the background(transmitted field)in the coupling junction.We introduce a Fano parameter to characterize their complex amplitude ratio.Via changing the Fano parameter,asymmetric Fano spectra are generated.In the experiment,by continuously changing the size of the fiber taper in the coupling junction,dynamic Fano spectra are generated because of the variation of Fano parameter.The dynamic Fano spectra agree well with the theoretical shapes.(2)For the first time,we explore the practical application of the oscillating Fano spectra in the conical microresonator.In the experiment,by adjusting the size of the conical microresonator in the coupling junction,we demonstrate the wavelength tunability of the resonance spectra.In addition,when the diameter of the coupling thin fiber taper is about 1 ?m,the bandpass resonance modes exhibiting peak-like shapes are generated.These wavelength-tunable bandpass spectra can be used for tunable bandpass filtering for lasing.Based on the wavelength-tunable bandpass spectra,we apply the thin-fiber-taper-coupled conical microresonator as a tunable bandpass optical filter,and install it in the ring fiber laser.The laser achieves a continuously and linearly wavelength-tunable emission.