Fig. 1. (a) Optical micrograph of the end-face of the empty photonic crystal fibre preform. (b) Optical micrograph of part of the partly-filled preform, showing some multimode fibres whose large cores will form nodes in the bandgap fibre’s cladding, and the one single-mode fibre (lower left) that will form the effectively-undoped core of the bandgap fibre. (c) Scanning electron micrograph of the end-face of fibre A drawn from a filled preform. The high-index nodes appear light in the backscattered-electron image because Ge is a stronger scatterer of electrons than Si. (A carbon coating was used under the SEM, as it is more transparent than gold.) The residual single-mode core is barely visible within the low-index bandgap-guiding core in the centre.

Fig. 2. (a) Output near-field images from 10 m of fibre A at indicated wavelengths for wide illumination (hence the nodes are lit up). A bandgap-guided mode is present in the central core for green and orange light. (b) Transmission spectra of fibre A, measured (black curve) and calculated (broken curve) for 0.2 m and measured (blue curve) for 10 m. The wavelengths of the images in (a) are marked as coloured circles on the corresponding curve.

Fig. 3. (a) Output near-field image from 1.8 m of fibre B at 650 nm wavelength for tightly-focused illumination (hence only the bandgap-guided core mode is lit up). (b) The intensity pattern calculated for fibre B at the same wavelength. Note the six faint satellite spots around the main lobe of the pattern in both images.