Fig. 1. Tapered fiber-microsphere coupling system: (a) photo of the coupling system; (b) microspheres fabricated by using the CO2 laser heating method (right, diameter of 300 μm) and by using the arc discharging method (left, diameter of 200 μm); (c) structure of the HM.

Fig. 2. Measured surface topography of the SiO2 microsphere with a surface roughness of Ra=30  nm.

Fig. 3. Measured WGM transmission spectra of the SiO2 microsphere with (a) D≈359  μm and d≈3  μm, where the 3 dB bandwidth≈1.2  pm. (b) D≈371  μm and d≈3  μm, where the 3 dB bandwidth≈2.4  pm.

Fig. 4. Tunable WGM transmission spectra (a) varies with different pump powers, where the respective peak wavelength is given in (b), and shows a blue-shift for the decreased pump power.

Fig. 5. Experimental setup of the single-wavelength fiber ring laser based on the tapered fiber-microsphere coupling system.

Fig. 6. Mode-hopping-free single-wavelength laser spectra measured for two hours.

Fig. 7. Measured tunable laser spectra. With the pump power decreased, the laser wavelength is blue-shifted.

Fig. 8. Mode distribution (a) in the SiO2 microsphere and (b) in the HM (the layer’s refractive index is 2.5, and the layer thickness is 0.14 μm), where the coupling condition and the sphere radius are fixed. (c) Comparison of the WGM transmission spectra.

Fig. 9. Comparison of the WGM transmission spectra with (a) d=0.22  μm, where the layer refractive index is n=2.0, 2.1, 2.2, 2.3, and 2.5; (b) d=0.14  μm, where the layer refractive index is n=2.0, 2.1, 2.2, 2.3, and 2.5; and (c) n=2.4, where the layer thickness is d=0.1, 0.14, 0.18, 0.22, and 0.26 μm.

Fig. 10. Mode distribution in the HM with a coating thickness of 0.27 μm and a refractive index of 2.5: (a) under coupling with a gap of 0.6 μm, (b) critical coupling with a gap of 0.3 μm, and (c) over coupling with a gap of 0 μm.