Fig. 1. Schematic topology of the proposed PCFs structure. The air holes in the silica background are arranged in a triangular configuration of lattice constant Λ with the central air hole missing, and all air holes have a uniform diameter d. The first and second innermost air-hole rings (colored green and yellow, respectively) are rotated around the fiber core with angles of α degrees and β degrees relative to their original positions, respectively. And the original positions of air holes in the inner rings are denoted as dashed circles in the right panel.

Fig. 2. Chromatic dispersion D as a function of wavelength λ with Λ=2.3  μm and d=0.61  μm for changing one of the design parameters: (a) α is changing from 0° to 30°, while β is fixed, and (b) β is changing from 0° to 30°, while α is fixed.

Fig. 3. Fundamental mode transverse electric field intensity (Et2) distributions at 1.45 μm (upper figures) and 1.75 μm (lower figures) wavelengths, for nearly zero-dispersion flattened PCFs with Λ=2.3  μm and d=0.61  μm for (a) α=0° and β=0°, (b) α=30° and β=0°, (c) α=0° and β=30°, (d) α=0° and β=0°, (e) α=30° and β=0°, and (f) α=0° and β=30°.

Fig. 4. Chromatic dispersion D as a function of wavelength λ with Λ=2.3  μm and d=0.61  μm for changing α from 0° to 59°, while β has an optimized value of 20°.

Fig. 5. Chromatic dispersion D as a function of the wavelength λ, for changing structure parameters α or β from 0° to 30° while reserving the other parameters, for (a) normal ultra-flattened-dispersion PCF with Λ=2.7  μm and d=0.75  μm, and (b) abnormal ultra-flattened-dispersion PCF with Λ=2.2  μm and d=0.54  μm.

Fig. 6. Chromatic dispersion as a function of the wavelength λ, for the changing α (a) normal ultra-flattened-dispersion PCF with β=16°, Λ=2.7  μm, and d=0.75  μm, and (b) abnormal ultra-flattened-dispersion PCF with β=22°, Λ=2.2  μm, and d=0.54  μm.

Fig. 7. Chromatic dispersion D as a function of the wavelength λ with Λ=2.3  μm and d=0.61  μm; α or/ and β are varied from their optimum values of α=0° and β=20°.