Raman gain and nonlinear optical absorption measurements in a low-loss silicon waveguide

We fabricated a low-loss $(\sim 0.22\mathrm{dB}∕\mathrm{cm})$ rib waveguide (WG) in silicon-on-insulator with a small effective core area of $\sim 1.57\mu {\mathrm{m}}^2$ and measured the stimulated Raman scattering gain in the WG. We obtained $2.3\mathrm{dB}$ Raman gain in a $4.8\text{-}\mathrm{cm}$ -long S-shaped WG using a $1455\mathrm{nm}$ pump laser with a cw power of $0.9\mathrm{W}$ measured before the WG. In addition, we observed nonlinear dependence of Raman gain and optical propagation loss as a function of the pump power. Our study shows that this mainly is due to two-photon absorption (TPA) induced free carrier absorption in the silicon WG. We experimentally determined the TPA induced free carrier lifetime of $25\mathrm{ns}$ , which agrees well with our modeling.