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This letter focuses on interaction of copropagating and counterpropagating pulses inside silicon-on-insulator waveguides using finite-difference time-domain (FDTD) simulations. To the best of our knowledge, this is the first time that copropagating and counterpropagation regimes have been analyzed and compared within an extended FDTD model that takes into account linear dispersion of silicon as well as the effect of stimulated Raman scattering. Our analysis shows unambiguously that the second-order Stokes and anti-Stokes sidebands of sufficiently high intensities can develop from noise when the two pulses are copropagating, but these sidebands are absent owing to an inherent phase mismatch when these pulses counterpropagate. We study the evolution of interacting pulses in the temporal and frequency domains and compare the FDTD results with those obtained by integrating a generalized nonlinear Schrodinger equation.