Fig. 1. Characteristic of the specially designed WGMR. (a) The transmission spectrum of the WGMR. (b) The backward Rayleigh scattering spectrum of the WGMR. (c) The schematic layout of the WGMR with the over-coupling structure (OCS). a_cw and a_ccw are the amplitudes of the CW and CCW modes of the resonator, respectively. Bⁱⁿ and B^out are the incident and transmitted lights in the resonator. (d) High resolution photograph of the WGMR.

Fig. 2. Schematic layout of the self-built WGMR fiber lasers. (a) The structure of the laser is filtered only by the WGMR. (b) The DPG is connected with the laser in (a). In all fiber lasers, the length and absorption coefficient of EDF are 0.8 m and 24 dB/km @1550 nm. ISO, optical isolator; CIR, optical circulator; OC, optical coupler; PC, polarization controller.

Fig. 3. Optical spectrum of the self-built lasers. (a), (b), and (d) show the spectra of laser $①$, laser $②$, and laser $③$, respectively, and (c) is an enlarged partial view of (b).

Fig. 4. Spectrum of the self-built laser $④$. (a) The optical spectrum and (b) the radio frequency beating spectra.

Fig. 5. Lasing spectrum of single-frequency laser $④$ in 60 min. The power of the pump was 90 mW.

Fig. 6. Power output versus power of pump. Solid dots, the measured maximum; circles, the measured minimum; dotted curves, average of the maximum and minimum. (a) The power of the laser $①$ fluctuated abnormally. (b) Pink, blue, and green marks indicated laser $②$, laser $③$, and laser $④$, respectively.

Fig. 7. Schematic layout of the DSHI and the spectrum of the beating linewidth. (a) The DSHI used for the measurement of the laser linewidth. OC, optical coupling; AOM, acoustic optical modulator; AWG, arbitrary waveform generator for radio frequency; PD, photodiode; ESA, electronic spectrum analyzer. (b) The spectrum of the beating linewidth. Gray dashed curve, the spectrum measured. Blue solid curve, the spectrum from Lorentz fitting.

Table 1. Specifications for Four Self-Built WGMR Fiber Lasers