  We study the spatio-temporal instability generated by a universal unstable
attractor in normal dispersion graded-index multimode fiber (GRIN MMF) for
femtosecond pulses. Our results present the generation of geometric parametric
instability (GPI) sidebands with ultrashort input pulse for the first time.
Observed GPI sidebands are 91 THz detuned from the pump wavelength, 800 nm.
Detailed analysis carried out numerically by employing coupled-mode pulse
propagation model including optical shock and Raman nonlinearity terms. A
simplified theoretical model and numerically calculated spectra are
well-aligned with experimental results. For input pulses of 200-fs duration,
formation and evolution of GPI are shown in both spatial and temporal domains.
The spatial intensity distribution of the total field and GPI sidebands are
calculated. Numerically and experimentally obtained beam shapes of first GPI
features a Gaussian-like beam profile. Our numerical results verify the unique
feature of GPI and generated sidebands preserve their inherited spatial
intensity profile from the input pulse for different propagation distances
particularly for focused and spread the total field inside the GRIN MMF.
