Fig. 1. Small fiber core

Fig. 2. Effective refractive index curve of LP₀₁ mode

Fig. 3. Effective refractive index curves of LP₀₁ modes with different d₁/Λ

Fig. 4. Offset directions of the air hole medium column near the core area. (a) Radially outward; (b) radially inward

Fig. 5. Effective refractive index curves of LP₀₁ modes with different Δx in the near-core air hole dielectric column

Fig. 6. Effective refractive index curves of LP₀₁ modes with different Δn in the near-core air hole dielectric column

Fig. 7. Schematic diagram of high-order mode PCF structure

Fig. 8. Effective refractive index curve of LP₀₂ mode

Fig. 9. Effective refractive index curves of LP₀₂ modes with different d₁/Λ and d₂/Λ. （a） Effective refractive index curves of LP₀₂ modes with different d₁/Λ;（b） effective refractive index curves of LP₀₂ modes with different d₂/Λ

Fig. 10. Effective refractive index curves of LP₀₂ modes with different Δx₁ and Δx₂. （a） Effective refractive index curves of LP₀₂ modes with different Δx₁; （b） effective refractive index curves of LP₀₂ modes with different Δx₂

Fig. 11. Effective refractive index curves of LP₀₂ modes with different Δn₁ and Δn₂. （a） Effective refractive index curves of LP₀₂ modes with different Δn₁; （b） effective refractive index curves of LP₀₂ modes with different Δn₂

Fig. 12. Basic structure of ADC-PCF mode converter

Fig. 13. Schematic diagrams of core structure. （a） Small core； （b） large core

Fig. 14. Mode effective refractive index curves of small core (LP₀₁ mode) and large core (LP₀₂ mode)

Fig. 15. Schematic diagrams of optimized structure of ADC-PCF mode converter. (a) 2D; (b) 3D

Fig. 16. Mode effective refractive index curves of small core (LP₀₁ mode) and large core (LP₀₂ mode) after structural optimization

Fig. 17. Schematic diagram of monitoring path setting

Fig. 18. Initial field diagram

Fig. 19. Schematic diagrams of mode conversion mode of ADC-PCF mode converter. (a) LP₀₁ mode; (b) LP₀₂ mode

Fig. 20. Energy coupling curves of ADC-PCF mode converter