Fig. 1. Fiber structures and refractive index distributions of five baht money and RRF. Cross sections of (a) ordinary fibers and (b) RRF;(c) picture of five baht money^[33]; refractive index distributions of RRF in (d) x-axis and (e) y-axis directions

Fig. 2. Calculated neff of ordinary step-index fiber and rectangular-assistant ring-core fiber for different modes. (a) Ordinary step-index fiber; (b) rectangular-assistant ring-core fiber

Fig. 3. Relationship among n_eff, rectangle length a, and rectangle width b. (a) Relationship between n_eff and rectangle length a for b=2 μm; (b) relationship between n_eff and rectangle width b for a=19 μm

Fig. 4. Relationship among Δneff between LP₀₁ and LP_11,a, rectangle length a, and rectangle length b at different wavelength. (a) Relationship between Δneff and rectangle length a for b=2 μm; (b) relationship between Δneff and rectangle width b for a=19 μm

Fig. 5. Relationship among Δneff between LP_11,a and LP_11,b,rectangle length a, and rectangle length b at different wavelength. (a) Relationship between Δneff and rectangle length a for b=2 μm; (b) relationship between Δneff and rectangle width b for a=19 μm

Fig. 6. Modal profiles of RRF

Fig. 7. Scheme of preparation of RRF

Fig. 8. Cross section of rounded RRF

Fig. 9. neff of each mode varies with ra

Fig. 10. Effect of offset of rectangle center point in x and y directions on fiber mode properties. (a) x direction; (b) y direction