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We study the nonlinearities in silicon-on-insulator (SOI) optical waveguides, which include two-photon absorption (TPA), free-carrier absorption, and spontaneous and stimulated Raman scattering (SRS). We show experimentally that free carriers generated by TPA in the SOI waveguides produce large optical loss at room temperature. The experimental results confirmed the presence of relative optical signal amplification from SRS, but it was found that net gain was hardly achieved because the stimulated Raman gain was less than the induced loss from TPA-generated free carriers at room temperature with continuous-wave pumping source in a SOI rib waveguide. We also experimentally investigated the temperature dependence of Raman scattering in the SOI waveguide and observed the Raman gain to be greater than TPA-generated free-carrier absorption loss at 77 K.