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Experimental verification of femtosecond transverse mode conversion induced by non-permanently written long-period gratings

Summary form only given. Mode conversion in few-mode fibers has been excessively studied and utilized, e.g., for dispersion compensation. For this purpose long-period gratings (LPG) have been permanently written into a fiber using UV or pulsed radiation. Recently, mode conversion has been demonstrated by transiently writing the grating structure into the fiber core via the optical Kerr effect with counter-propagating high power nanosecond pulses. An energy transfer between two probe modes with an efficiency of about 50% could be measured, limited by the damage threshold of the fiber front facet. A femtosecond laser source offers the possibility to reach the necessary peak powers at by a factor of 1000 reduced pulse energy. Therefore, we demonstrate the use of femtosecond pulses for writing optically induced long-period gratings (OLPG) to convert energy from one transverse probe mode to another. Due to the short interaction length of counter-propagating femtosecond pulses, our approach is changed to a co-propagating one, compared to our earlier work, using polarization to distinguish between probe and write beam.