Gas Sensing in a Microresonator System Using Non-Adiabatic Tapered Fibers*

Adiabatically tapered fibers have commonly been used to excite whispering gallery modes (WGMs).  It has recently been shown that non-adiabatically tapered fibers can enhance refractive index sensing.  The light within a non-adiabatic taper transition excites both fundamental and higher-order fiber modes, whereas an adiabatic taper transition only excites the fundamental fiber mode.  The sensing enhancement comes from the interference between different fiber modes.  We have theoretically shown that enhancement is also possible for absorption sensing.  The enhancement can be predicted by the measured power when the modes are in and out of phase.  The enhancement is proven by sending light through the adiabatic side of an asymmetrically tapered fiber.  We have shown this enhancement using a setup combining the asymmetric tapered fiber and a hollow bottle resonator (HBR) with internal analyte.  The light absorbed by the analyte results in changes in the dip depth of the WGM.  The ratio of changes, non-adiabatic to adiabatic, shows the enhancement factor.  For carbon dioxide an enhancement factor of 150 was found, in agreement with the theoretical model.