Air-core photonic band-gap fibres without polarisation degeneracy A. Argyros 1,2 , I.M. Bassett 1,2 , M.A. van Eijkelenborg 1 , M.C.J. Large 1 , R.C. McPhedran 3 1 Australian Photonics CRC, Optical Fibre Technology Centre, University of Sydney, 206 NIC Australian Technology Park, Eveleigh, NSW 1430, Australia. 2 School of Physics, University of Sydney, NSW 2006, Australia. 3 CUDOS & School of Physics, University of Sydney, NSW 2006, Australia. a.argyros@oftc.usyd.edu.au Abstract: An air-core holey fibre design that supports a single, circularly symmetric, polarisation non-degenerate mode is presented. The mechanism through which polarisation non- degeneracy is achieved, the consequent advantages, applications and fabrication methods will be discussed. The majority of single-mode optical fibres available have the property that the mode they support is polarisation degenerate, i.e. the mode is in fact two orthogonally polarised modes. This, when combined with unavoidable birefringence caused by perturbations along a fibre, can give rise to problems such as polarisation mode dispersion and polarisation fading in interferometers. These can be avoided through the use of polarising fibres, which support only one linear polarisation, but these fibres inherently lack circular symmetry and alignment issues arise when coupling or sensing applications are considered. Recently, an air-core band-gap fibre design was reported which supports a single polarisation non- degenerate mode, whilst retaining circular symmetry. The design was a Bragg fibre that supported a single TE-polarised mode (a TE-Bragg fibre) [1]. In TE- Bragg fibres, the condition for Bragg reflection is forced to coincide with the Brewster angle condition (TM polarised light is not reflected at the Brewster angle), resulting in TM and hybrid polarisation modes suffering very high confinement losses, whilst TE modes can be supported with orders of magnitude lower confinement loss. The superposition of the Bragg and Brewster angle conditions serves to close the TM band gap, leaving the TE band gap virtually unaffected. Furthermore, the number of TE modes supported can be reduced to one by appropriately adjusting the core size [2]. Such a fibre has the advantages of air-guidance and singlemodedness, whilst potentially eliminating both polarisation and alignment problems and is, to the authors’ knowledge, the only design with all these properties. It would essentially allow for light to be treated as a scalar quantity. The design in [1], however, cannot be fabricated using existing techniques, as conditions on the refractive indices in the fibre [1] are not satisfied by any materials currently used in optical fibres (for an example of Bragg fibre fabrication see [3]). One approach to fabricating TE-Bragg fibres is to use a holey fibre design in which the average refractive index approximates the index profile of the TE-Bragg fibre [4]. The fibre would be a ring-structured fibre consisting of a hollow core (to eliminate index guiding) surrounded by rings of holes to mark the low-index layers of the TE-Bragg fibre. Index guiding analogues of such fibres have already been fabricated using polymers [4]. FIG. 1: Refractive index profile of a ring-structured TE- Bragg fibre, consisting of rings of holes (white) in a host material (black). The values used were n = 1.49 for the host material, Λ i = 0.403 μm, Λ e = 0.578 μm, hole diameter d = 0.335 μm and core radius r co = 2.89 μm. The ring-structured analogue of the TE-Bragg fibre is shown in Fig. 1, the design being chosen so that the azimuthal arithmetic mean of the refractive index closely resembles the index profile of the TE-Bragg fibre. This approach was shown to be valid in the past for index guiding fibres [4]. Unlike conventional triangular lattice band gap fibres (e.g. [5]), two hole spacing parameters must be specified in this case: Λ i is the intra-ring hole spacing and Λ e the inter-ring