In-Fiber Liquid Microsphere Whispering Gallery Mode Resonator Fabricated by Femtosecond Laser

An in-fiber liquid microsphere whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The in-fiber resonator is fabricated by creating an air cavity in tangential contact with the fiber core using femtosecond (fs) laser micromachining and fusion splicing. The air cavity is then filled with high index liquids through a laser drilled channel to form a liquid microsphere. The size and location of the microsphere can be controlled by using different fabricating parameters, while higher Q factors can be achieved with smaller coupling distance. The spectrum of the liquid sphere WGM resonator can be tuned by the index value of the liquid, and the resonant peaks shift toward longer wavelength as the refractive index (RI) of the liquid increases, with a sensitivity of 602.3 nm/RIU in the index range of 1.584∼1.688. On the other hand, the liquid microsphere resonator exhibits high temperature sensitivity due to the high thermo-optic coefficient of the liquid. Temperature sensitivity of −360.9 pm/°C is obtained with a resonator with sphere diameter of 49 μm and liquid RI of 1.692. Such a device can be an attractive candidate for sensing in high index environments such as oil industry.