Second harmonic generation in multimode graded-index fibers: spatial beam cleaning and multiple harmonic sideband generation

We study experimentally and numerically the spectral and
spatial dynamics of second harmonic generation in an alloptically
poledmultimode graded-index fiber. In contrast with
poled single-mode fibers, in a multimode graded-index fiber a
pump can generate a series of sharp sidebands around its second
harmonic (SH) that originate from the sub-millimetric
periodic evolution of the intensity at the fundamental frequency.
The mutual interaction between the fundamental
and its SH may also strongly affect the spatial distribution
of guided light for both colors: when increasing the pump
power, both fundamental and SH output beams evolve
from disordered multimode speckles into two bell‐shaped
beams.