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A new method for fabricating long-period gratings (LPGs) in photonic crystal fibers (PCFs) is presented. A tightly focused femtosecond infrared laser beam scans across a PCF periodically, resulting in periodic filling of cladding air-holes through a micro-explosion and redeposition process and forming an LPG. Strong resonance of ~26 dB has been demonstrated with a grating length of <;5 mm. An annealed LPG demonstrated stable operation at 800°C for over 30 h.