Fig. 1. Cross-section micrograph of the RCF.

Fig. 2. Mode field distributions of the LP₀₁ (HE₁₁^x/y) and LP₁₁ (TE₀₁, TM₀₁, and HE₂₁^e/o) modes in the RCF.

Fig. 3. Relationships between the effective refractive indices of the first- and second-order modes in the RCF and the incident laser wavelength.

Fig. 4. Schematic diagram of a CVB generation system based on RCFBG.

Fig. 5. Reflection spectra of the RCFBG and the SMFBG.

Fig. 6. (a1)–(a5) Offset fusion splicing and (b1)–(b5) intensity distribution of the transmission modes in the RCF when it was fusion spliced with different offsets.

Fig. 7. Output spectra of the laser at a pump power of 508 mW for the (a) LP₀₁ and (b) LP₁₁ modes.

Fig. 8. Intensity distributions of the LP₁₁ mode laser when the RCF is fusion spliced with different core offsets.

Fig. 9. (a1) Intensity distributions of the TE₀₁ mode and (a2)–(a5) after passing through a linear polarizer; (b1) Intensity distributions of the TM₀₁ mode and (b2)–(b5) after passing through a linear polarizer. White arrows indicate the polarizer orientations.

Fig. 10. Mode conversion efficiencies for (a) TE₀₁ and (b) TM₀₁ modes.

Fig. 11. Intensity distribution across the beam center of the TM₀₁ mode.