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It is shown that long-period gratings (LPGs) can be written in a pure-silica photonic crystal fiber without physical deformation by using the glass structure change. LPGs with different grating periods are made by the glass structure change induced by arc discharge. We find that the resonance wavelengths decrease with increasing the grating period in contrast with LPGs fabricated in conventional fibers. The second-order core mode begins to be supported in the long-wavelength region, and two resonance peaks appear there. This is due to a large increase of the index difference between the pure-silica core and the holey cladding against wavelength.