We report what is believed to be the first demonstration of optical fiber g
ratings written in photonic crystal fibers. The fiber consists of a germani
um-doped photosensitive core surrounded by a hexagonal periodic air-hole la
ttice in a silica matrix. The spectra of these gratings allow for a detaile
d characterization of the fiber. In particular, the gratings facilitate cou
pling to higher-order leaky modes. We show that the spatial distribution an
d the effective index of these modes are determined largely by the design o
f the lattice and that the grating spectra are unaffected by the refractive
index surrounding the fiber. We describe these measurements and correspond
ing simulations and discuss their implications for the understanding of suc
h air-hole structures. (C) 1999 Optical Society of America.