Microbubble resonators combine the unique properties of whispering gallery mode resonators with the capability of integrated microfluidics. The microbubble resonator is fabricated by heating the tapered tip of a pressurized glass capillary with oxyhydrogen flame. Firstly, a microtube with a diameter of 250um is stretched under heating of oxyhydrogen flame, the heating zone length is set to be 20mm and the length of stretch is set to be 7000um.Then nitrogen will be pumped in to the tapered microtube with the pressure of 0.1Mpa, the tapered tip will be heated by the oxyhydrogen flame continuously until a microbubble forms. An optical fiber taper with a diameter of 2 um, fabricated by stretching a single-mode optical fiber under flame was brought in contact with the microbubble to couple the light from a 1550nm tunable diode laser into the whispering gallery mode. The microbubble resonator has a Q factors up to 1.5 × 107 around 1550nm. Different concentrations of ethanol solution (from 5% to 30%) are filled into it in order to test the refractive index sensing capabilities of such resonator, which shows a sensitivity of 82nm/RIU.
Wavelength division multiplex technology can enhance the sensing capacity by detecting various samples simultaneously. Whisper-Gallery-Mode (WGM) can be selected simulated in the micro-bubble by a directional coupler made by Si3N4 grating. Some grating parameters, including period, width, and refractive index modulation are numerically simulated by FDTD solution software to find their impacts on the WGM selected process. Grating with a particular period can simulated a WGM in micro-resonator on purpose. The interference of different bubble resonators is also discussed in this paper.