Tunable Fano-Like Lineshape in an Adiabatic Tapered Fiber Coupled to a Hollow Bottle Microresonator

Fano-like resonances and electromagnetic induced transparency (EIT)-like resonances in whispering gallery-mode (WGM) microcavities have important applications in high-sensitivity biosensing, optical switches, and slow light. Hollow microbottle resonators (HMBRs) provide special opportunities for optofluidic devices. Here, we theoretically and experimentally demonstrate a simple system to produce Fano resonance lineshapes in which a HMBR is coupled to the transition region of an adiabatic tapered optical fiber. The Fano resonance results from the interference between the fundamental mode and higher order modes of the taper that are coupled to the WGM of the HMBR. The degree of asymmetry can be tuned by scanning the coupling position along the taper and controlling the input light polarization. It is shown that by tuning polarization, the Fano resonance line converts from a Lorentzian shape to an EIT-like line. This system potentially can be used as an optical filter with the advantage of easy alignment.