  We present a new spatial-spectral mapping technique permitting to measure the
beam intensity at the output of a graded-index (GRIN) multimode fiber with
sub-nanometric spectral resolution. We apply this method to visualize the fine
structure of the beam shape of a sideband generated at 1870 nm by geometric
parametric instability (GPI) in a GRIN fiber. After spatial-spectral
characterization, we amplify the GPI sideband with a Tm-doped fiber amplifier
to obtain a microjoule-scale picosecond pump whose spectrum is finally
broadened in a segment of InF3 optical fiber to achieve supercontinuum ranging
from 1.7 {\mu}m up to 3.4 {\mu}m
