  Hollow core fibers are considered as promising candidates to deliver intense
temporally overlapping picosecond pulses in applications such as stimulated
Raman scattering (SRS) microscopy and endoscopy because of their inherent low
nonlinearity compared to solid-core silica fibers. Here we demonstrate that,
contrary to prior assumptions, parasitic signals are generated in Kagom\'e
lattice hollow core fibers. We identify the origin of the parasitic signals as
an interplay between the Kerr nonlinearity of air and frequency-dependent fiber
losses. Importantly, we identify the special cases of experimental parameters
that are free from parasitic signals, making hollow core fibers ideal
candidates for noise-free SRS microscopy and endoscopy.
