Optically induced long-period fiber gratings for guided mode conversion in few-mode fibers

We propose and demonstrate guided mode conversion by an optically induced long-period fiber grating (OLPG). The beating of two guided modes excited by a nanosecond pulsed laser beam is used to induce a refractive index grating via the Kerr effect. This grating converts a counter-propagating continuous-wave beam from the LP(01) to the LP(11) mode. Numerical simulations using a beam propagation method show that a full conversion is possible. Experimentally, a mode conversion with an efficiency of about 50% is firstly demonstrated.