Whispering gallery mode excitation in borosilicate glass microspheres by K+ion-exchanged channel waveguide coupler

Abstract

Optical microsphere resonators, with their exceptionally low optical losses and high Q-factors, are attracting a lot of interest in integrated optics and related fields. Not being accessible by free-space beams, whispering gallery modes (WGM) of a microsphere resonator require near-field coupler devices. Efficient evanescent coupling has been demonstrated previously by using thin tapered fibres, fibre half-block couplers, angle-polished fibres and bulk prisms. In this work, we demonstrate WGM excitation in microspheres, from 8 to 15 μm in diameter, by using an integrated optics channel waveguide. Light from a tunable laser was coupled into a single mode K⁺ ion-exchanged channel waveguide formed in BK7 glass substrate. Dry borosilicate glass microspheres were dispersed on the substrate surface. Polystyrene microspheres were suspended in electrolyte water solution and confined in a closed cell on top of the waveguide. The light was coupled to the particles sitting on the waveguide surface. The scattered light was observed through the microscope. As the laser wavelength was tuned, the observed images were recorded with a CCD camera. WGM excitation was observed through the increased scattered light intensity at certain wavelengths. In the case of glass microspheres and a Ti:Sapphire tunable laser, the obtained resonance quality (Q-) factors were about 400. The resonances observed in polystyrene microspheres using a tunable diode laser had lower Q-factors and were deteriorating with decreasing particle size.