Fig. 1. Designed RCYDF refractive index profile structure

Fig. 2. Structure and Yb³⁺ doping concentration distribution of home-made RCYDF

Fig. 3. Multi-wavelength and transverse-mode-switchable fiber laser structure

Fig. 4. Propagation constants of LP₀₁ and LP₁₁ modes and measured transmission spectra of the FMFBG

Fig. 5. Transverse mode field distribution of LP₀₁ and LP₁₁ oscillation modes

Fig. 6. Measured output spectra of the LP₀₁ and LP₁₁ lasing

Fig. 7. Output power of the LP₁₁ and LP₀₁ mode versus pump power

Fig. 8. Measured spectra for lasing at dual-wavelength

Fig. 9. The stability of the LP₀₁ and LP₁₁ mode