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A novel modelocking technique is presented in which the intensity-dependent mode-coupling dynamics of a long-period fiber grating is used to achieve modelocking in a passive optical fiber laser. By an appropriate choice of the grating period, a resonant coupling occurs between co-propagating core and cladding modes, causing the low-intensity wings of the pulse to be transferred to the cladding mode and be attenuated. In contrast, the higher intensity peaks of a pulse are detuned from resonance by the nonlinearity and remain largely unaffected. Numerical studies of this pulse-shaping mechanism show that the laser produces stable mode-locked soliton-like pulses which are limited in bandwidth by the smaller of either the grating transmission bandwidth or amplifier bandwidth.