Non-permanent optically induced long-period gratings for energy transfer between transverse fiber modes using femtosecond pulses

For mode conversion between co-propagating transverse modes, long-period gratings, typically written into the fiber by UV radiation, are commonly used and can be applied for dispersion compensation or sensors. Recently, it was shown that long-period gratings can also be transiently generated by high intensity light [1]. In that case, the periodic structure originated from multimode interference, and the refractive index modulation was caused by the Kerr-effect from the resulting intensity pattern. High-energy nanosecond pulses were required to achieve the necessary intensity, and the length of the pulses allowed using a counter-propagating write beam.