The first photonic crystal fiber (PCF) opened a new era for fiber optics.1 The potential device applications of this novel, microstructured fiber in optical-communication systems and sensing technologies have attracted research attention and intensive study worldwide. Most previous studies have concentrated on PCFswith a photonic crystal (PC) cladding surrounding a homogeneous core. If the wavelength of the light falls in the forbidden band gap of the PC, the light is confined to the core, which can even be hollow. In contrast, index-guiding PCFs confine light in the same way as traditional fibers do, by exploiting total internal reflection at the effective-index mismatch between core and cladding. Indexguiding fibers that employ a PC cladding were initially appealing because the fundamental space-filling mode index of the cladding lattice varies strongly with wavelength. However, as shown in our earlier work,2 a fiber with a PC core can also guide light. A birefringent fiber with a finite, 1D-PC core has even been fabricated.3 By the same token, only a few numerical modeling efforts have been extended to a 2D-PC core,4 and we know of no report on fabrication and characterization of silica-based fibers with a 2D-PC core. Moreover, grating devices based on such fiber are rare, both theoretically and experimentally. Here, we report the first fiber Bragg grating (FBG), to our knowledge, written on silica-based, all-solid, nanostructure-core fiber (NCF). Instead of the PC cladding, the fiber core is formed by a periodic 2D array of high-index rods. To make the fiber, we used a doped-silica preform to build the periodic structure of the core, which we then overclad with a pure silica jacket. The maximum refractive-index difference between the germanium-doped region and pure silica is approximately 3%. We then drew the preform into a cane and stacked the cane in a hexagonal pattern. The stack was jacketed in a silica tube and drawn to form the nanostructure core. The resulting fiber was etched in 50% hydrofluoric acid for 2min, to an outer Figure 1. (a) Scanning electron microscope (SEM) picture of the fabricated nanostructure-core fiber. (b) The details of the core region. The black regions are germanium-doped. (c) Contour plot of the measured mode profile of the guided mode for the fiber shown in (a). (d) Attenuation spectrum, measured using the cutback method.