We investigate nonlinear couplers whose operation is based on Raman or Brillouin scattering. A quantum approach is used. Strong coherent laser pumping is assumed and linearized Heisenberg-Langevin equations of motion are solved. The influence of linear coupling constants, losses and phase mismatches on the dynamical behaviour of the coupler is discussed. Quantum statistical properties of optical modes involved in the interaction are investigated within the framework of the generalized superposition of coherent signal and quantum noise. Most attention is devoted to the possibility of generation of non-classical light. It is shown that such light can be generated only in compound modes according to our model and influence of various parameters on the generation is discussed. Optimal conditions for the generation of squeezed light are found.