Fig. 1. Experimental setup. ISO, isolator; PC, polarization controller; OSC, oscilloscope; OSA, optical spectrum analyzer; SG, signal generator; OC, optical coupler; PD, photodetector. The inset shows the microscope of an MBR.

Fig. 2. Transmission spectrum of the Er³⁺ ion-doped MBR measured in (a) 1480 nm and (b) 1550 nm bands, respectively. (c) Typical spectrum of laser emission at 1553.21 nm as the pump wavelength is located at 1464.19 nm. The inset shows the energy level diagram of Er³⁺ ions. (d) Transmission of the pump light (blue curve) and laser light (red curve) as the pump wavelength is finely scanned near the resonant mode at 1470.68 nm.

Fig. 3. (a) Evolution of the spectra at z = 0 µm in the range of 1552 to 1555 nm measured at different pump powers; (b) relationship between the pump power and the emitted laser power at different axial positions of the MBR, z = 0 µm, 15 µm, 30 µm, 45 µm, respectively.

Fig. 4. (a)–(d) WGM field distributions with axial order q = 0, 2, 18, and 49, corresponding to contacted positions of z = 0, 10, 30, and 50 µm, respectively; (e)–(h) evolution of emitted laser spectra at different axial contacted positions.

Fig. 5. (a)–(c) Evolution of emitted laser spectra when the MBR is coupled with the tapered fiber at different diameters (d = 2.9, 3.2, and 3.4 µm); (d) calculated propagation constant as a function of the microfiber radius and the radial order of the MBR. Insets show the field distributions of different orders of radial modes.

Fig. 6. (a) Evolution of emitted laser spectra as the wavelength of the pump laser increases; (b) wavelength shifts of the emitted laser mode with the increasing wavelength of the pump laser. The fitting line shows a linearity of 99.96% with a tuning range of 0.102 nm.