Fig. 1. (a) Refractive index profile and electric field intensity distribution of considered bandgap fiber. The core, high-n layer and low-n layer thicknesses are 22 λ, λ, 9 λ, respectively. (b) Confinement loss computed versus the wavelength for different values of ∆n. Also reported is the material loss.

Fig. 2. (a) Refractive index profile and electric field intensity distribution of designed fiber. (b) Effective index n_e of the Bragg mode versus the wavelength compared to clad (pure-silica) index n₂ and core (raised) index n₁ = n₂ + ∆n_core showing n₂ < n_e < n₁ for λ < 0.85 μm, evidencing total-internal-reflection mechanism. (c) Radial electric field distribution canceling at low-/n → high-/n transitions and maximum at high-/n → low-/n transitions characteristic of the bandgap waveguidance mechanism. (d) Confinement loss computed at λ = 0.8 μm versus ∆n_core. ∆n in the cladding layers is 15.10^-3. Also reported is the material loss.

Fig. 3. Refractive index profile and electric field intensity distribution in two fabricated fibers (at λ = 833 nm). The fibers’ outer diameter is 125 μm. Fibers were drawn from different preforms explaining the slightly different RIPs. Circles denote measured field intensity distribution while bold line denotes the field distribution computed taking into account the actual index profile.

Fig. 4. Observed near field intensity patterns for (a) L = 1.2 m and (b) L = 30 m and for various launching conditions.

Fig. 5. Attenuation spectra for (a) L = 30 m, (b) L = 100 m. Also reported is the material loss. Differences in position and level of attenuation peaks arise from different RIPs.

Fig. 6. (a) Observed near field intensity pattern corresponding to the coupling between the Gaussian mode and the LP_8,1 mode of the first annular waveguide. (b) Effective index versus the wavelength for the Gaussian mode and some modes of the first annular waveguide. Also reported is the refractive index of pure silica.

Fig. 7. Bend loss for bandgap fiber (red), step-index fiber (black) and ideal PCF (blue). Dots: measured data, lines: computed values. In-set: index profile and electric field distribution of SIF.