Raman backscattering (RBS) in plasma is the basis of plasma-based amplifiers and is important in laser-driven fusion experiments. We show that saturation can arise from nonlinearities due to coupling between the fundamental and harmonic plasma wave modes for sufficiently intense pump and seed pulses. We present a time-dependent analysis that shows that plasma wave phase shifts reach a maximum close to wave breaking. The study contributes to a new understanding of RBS saturation for counter-propagating laser pulses.