1. Background: A new class of optical fiber has recently emerged which shows considerable promise; holey fibers have highly tailorable optical properties arising from their design flexibility [1, 2]. Typically holey fibers (HFs) are made from undoped silica, and have a cladding region formed by air holes running along the fiber length. The holes are often arranged in a periodic lattice [as in Fig. 1(a)], and the core is formed by an absent air hole. However the holes do not need to be periodically arranged or even be of constant size for the HF to guide light [3] [see Fig. 1(b)]. Either type of HF can guide because the cladding has a lower effective refractive index than the core. Although this guidance mechanism is conceptually simple, the optical properties of HFs vary dramatically depending on the hole arrangement. This is because the holes are on the same scale as the wavelength, and so the effective cladding index is strongly dependent on both the wavelength and the hole arrangement. A small subset of HFs of the type shown in Fig. 1(a) (i.e. with periodically arranged holes) can guide light via band gap effects [4], but we do not consider this more exacting mechanism here.
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