239417.fig.001a
(a)
239417.fig.001b
(b)
239417.fig.001c
(c)
239417.fig.001d
(d)
Figure 1: Typical structured optical fibres into which gratings have been written: (a) low-loss (<4 dB/km) 4-ring all-silica photonic crystal optical fibre; (b) 12-ring photonic crystal fibre with triangular core and photosensitive, high-NA step-index germanosilicate center; (c) simple Fresnel fibre; and (d) air-clad fibre with Y -doped core.