Investigating trade-offs between loss and modality in HC-PBGFs by varying the core size. The figure shows (top row) examples of fibres with thin core surround (and thus reduced incidence of surface modes) and increasing core element number: (A) 3 cell, showing a single mode behaviour (after ), (B) 7 cell, (C) 19 cell and (D) 37 cell (after ). For the first three fibres the corresponding simulated air-guided modes are also shown in the row below. As the core size increases, a reduction of the loss and an increase of the number of air-guided modes are theoretically predicted (see Table 1). The images are collected using a scanning electron microscope and the length bar corresponds to 10 µm.