Saturation of the Raman amplification by self-phase modulation in silicon nanowaveguides

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.