Microstructured Fibre Taper with Constant Outer Diameter

Abstract

Microstructured Optical Fibres (MOFs) can be tapered in many ways depending on the length scale. Short taper of a few centimetres in length is commonly produced using the flame brush technique on a conventional fibre tapering rig (Bilodeau et al., 1988). Mesotaper of up to tens of metres in length can be produced using improved tapering rig with a ceramic microheater (Vukovic et al., 2008). It is also possible to taper optical fibre on a standard fibre draw tower, both for conventional step index fibre (Chernikov et al., 1993) and for MOFs (Tse et al., 2006b). Longitudinal variation of the MOF structure can lead to a comprehensive control of dispersion and nonlinearity, for spectral control under general conditions (Tse et al., 2008). The possibility of different long-length fibre-taper designs can lead to exciting applications in nonlinear fibre optics, such as uniform and stable supercontinuum generation for telecommunications spectral slicing (Chen et al., 2009a; Dudley & Coen, 2002; Genty et al., 2009; Vukovic & Broderick 2010), adiabatic soliton compression (Hu et al., 2006; Tse et al., 2006b, 2008), mode conversion (Town & Lizier, 2001) and pulse transformation (Broderick, 2010). The early dispersion-decreasing microstructured fibres were fabricated by stacking of glass capillaries with radial (2D) designs consisted of uniform air hole size (Travers et al. 2007; Tse et al., 2006b). The preforms were drawn into canes (diameter in mm scale), and finally tapered down either by changing the pressure of all the air holes, or by reducing the outer diameter (OD) of the fibre during the drawing process. Both methods led to variation of hole and core sizes along the fibre. The disadvantage of such tapering schemes is that when the features reduce in size, the associated confinement loss increases (Marks et al. 2006; Nguyen et al., 2005). Therefore, a large number of rings of air holes are needed to reduce the loss to a low level; the fabrication of these MOFs is labour intensive. Moreover, decrease of the outer diameter of the fibre may reduce the mechanical strength of the fibre and handling may became difficulty, at the same time induce complication when connecting to standard fibres. It has been reported that selective holes within the microstructure region of MOFs can be independently pressurised during the fibre drawing process (Couny et al. 2008). Together with the possibility to vary the pressure of the holes during the drawing process (Tse et al.,