The mode-coupling properties of tunable long-period fiber gratings (LPGs) formed in photonic crystal fibers (PCFs) are presented. The mode coupling from the fundamental core mode to a cladding mode of a PCF is obtained by use of periodic mechanical pressure. The strength and the wavelength of the resonant peak are tuned by adjusting the grating period and the pressure applied on the PCF. Contrary to the conventional fiber case, the resonant wavelength of the PCF LPG is decreased by increasing the periodicity.
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