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Simulations of the effect of the core ring on surface and air-core modes in photonic bandgap fibers

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Abstract

We show through computer simulations that the thin silica ring that surrounds the air core of a photonic-bandgap fiber introduces surface modes. The intensity profile and dispersion of these modes indicate that they are the modes of the waveguide formed by the ring surrounded by air on one side and the photonic crystal cladding on the other. The ring also induces small perturbations of the fundamental core mode. Coupling to those surface modes, which have propagation constants close to that of the core mode, are likely to induce substantial loss to the core mode. By reducing the thickness of the ring and/or by suitably selecting its radius the propagation constants of the surface modes can be moved farther from that of the core mode and the loss reduced.

©2004 Optical Society of America

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Figures (9)

Fig. 1.
Fig. 1. (a) Cross-section of an air-core photonic-bandgap fiber with a core radius such (R=0.9Λ) that the core does not support surface modes, and (b) same fiber with a thin silica ring around the core.
Fig. 2.
Fig. 2. (a) Cross-section of a generic preform for an air-core photonic-bandgap fiber, consisting of a stack of silica tubes with the seven center tubes removed to form the fiber’s air-core, and (b) photograph of a fiber drawn from such a preform.
Fig. 3.
Fig. 3. Calculated ω-k diagram of the air-core fiber of (a) Fig. 1(a) (no ring), and (b) Fig. 1(b) (ring present).
Fig. 4.
Fig. 4. Intensity contour lines of the fundamental core mode of (a) the fiber of Fig. 1(a) (no ring), and (b) the fiber of Fig. 1(b) (ring present), both calculated at kzΛ/2π=1.7. The relative intensity on the contours varies from 0.1 to 0.9 in increments of 0.1.
Fig. 5.
Fig. 5. Intensity contour lines of two exemplary surface modes of the fiber of Fig. 1(b) (ring around the core).
Fig. 6.
Fig. 6. (a) Cross-section of an air-core photonic-bandgap fiber with a core radius such (R=1.13Λ) that the core supports surface modes, and (b) same fiber with a thin silica ring around the core.
Fig. 7.
Fig. 7. Calculated ω-k diagram of the air-core fiber of (a) Fig. 6(a) (no ring), and (b) Fig. 6(b) (ring present).
Fig. 8.
Fig. 8. Dispersion curves of core modes and surface modes, and intensity profiles of four exemplary surface modes simulated with the simplified air-core fiber geometry in references 7 and 12.
Fig. 9.
Fig. 9. Photonic crystal terminated by a thin slab used to model very thin PBF core ring.
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