  We experimentally show that the self-phase modulation of picosecond pump
pulses, induced by both the optical Kerr effect and free-carrier refraction,
has a detrimental effect on the maximum on-off Raman gain achievable in silicon
on insulator nanowaveguides, causing it to saturate. A simple calculation of
the Raman gain coefficient from the measured broadened output pump spectra
perfectly matches the saturated behavior of the amplified Raman signal observed
experimentally at different input pump powers.
