Fig. 1. Schematic diagram of the multidimensional tuning method. The waist of the MF is about 1 μm and the minimum outer diameter of the silica microcapillary is about 53 μm with a wall thickness of about 2.5 μm.

Fig. 2. (a) Coupling efficiency as a function of the fiber taper diameter. (b) Transmission spectra of the silica MCR at different coupling spots along the gradient profile of the MF (moving distance S is 0, 10, 30, 50, 70, 90, 120, 150, and 170 μm, respectively, from high to low ER). (c) Transmission spectra of the silica MCR at around 1550 nm. (d) ER tuning as a function of the moving distance S.

Fig. 3. (a) Transmission spectra of the silica MCR at different coupling spots along the gradient profile of the silica microcapillary (the moving distance L is 0, 400, 500, and 560 μm, respectively). (b) Resonance tuning as a function of the moving distance L.

Fig. 4. Electric field in the MCR with wall thickness of (a), (b) 20 μm and (c), (d) about 2.5 μm. (e) Energy percentage of the WGM in the core as a function of the microcapillary wall thickness for the fundamental mode.

Fig. 5. Resonance tuning of the silica MCR with wall thickness of (a) about 2.5 μm and (b) about 2 μm.

Fig. 6. (a) Optical microscopic image of the microbubble resonator. (b) Transmission spectrum of the microbubble resonator. (c) ER tuning of a resonance around 1560.5 nm (the moving distance S is 0, 40, 90, 120, 180, 210, 270, 330, and 400 μm, respectively, from high to low ER). (d) ER tuning as a function of the moving distance S.