Fiber pigtailed thin wall capillary coupler for excitation of microsphere WGM resonator in chemical sensing

Abstract

Optical microresonators have been proven as an effective means for sensitive chemical sensors development. The changes in refractive index near the resonator surface lead to the effective refractive index change and thus a shift at certain resonance wavelength. The high quality (Q) whispering gallery modes (WGMs) contributed by the rotationally symmetric structures will interact with the local circumstances through the evanescent field. The high sensitivity in detection was achieved by the long photon lifetime of the high-Q resonator (thus the long light-environment interaction path). In this paper, we present our recent research on using fiber pigtailed capillary coupler for WGM resonator excitation and its sensing applications. Capillary tube with wall thickness of several microns was used as the waveguide. The PMMA microsphere and porous glass microsphere (PGM) were integrated with the etched capillary tube for different sensing purposes. The Q-factors and free spectrum ranges (FSR) of different types of microspheres were measured by coupling light into the microsphere using novel fiber pigtailed capillary coupler. Chemical vapor at different concentrations were tested using PGM microresonator. This alignment free structure provides a new sensing probe based on WGM resonator concept.