I am right now reading the Okamoto's Fundamentals of Optical Waveguides and encountered some questions on SBS and SRS. In the book, it mentioned a causal relationship between the dispersion relationship of angular frequencies with respect to the wavenumber k and forward and backward scattering: Since the w(+) is almost constant in the vicinity of k=0, so backward and forward scattering are observed almost equally. On the other hand, w(-) satisfies the linear relationship, so only backward scattering is observed (Pg240 & Pg243). I don't quiet get this causal relationship. Could somebody explain that to me?