As to the backscatter instability which is one of nonlinear three-wave resonant interactions, the reflection coefficient(r) in the case of homogeneous-slab medium is calculated, assuming all the three wavepackets have negligible damping caused by medium. The expression has turned out to be such that $r=tanh^2KA_pL$, where K, Ap, and L are the constant coupling coefficient, the constant pump wave amplitude, and the thickness of the medium engaged in the interaction each. When this result is interpreted in terms of the Brillouin backscattering in a so-called underdense plasma in controlled fusion, we obtain rather an unfitting result mainly in the independence of the reflection coefficient on the incident laser-light intensity. We see the incompatibility rise chiefly from neglecting the damping of pump-wave in the plasma. In contrast to the former results by others in the limit of large pump-wave damping, our result might be regarded as that for cases of negligible pump-wave damping, in general stimulated backscattering phenomena.
