  We present a multimode Hamiltonian formulation for the problem of
opto-acoustic interactions in optical waveguides. We establish a Hamiltonian
representation of the acoustic field and then introduce a full system with a
simple opto-acoustic coupling that includes both photoelastic/electrostrictive
and radiation pressure/moving boundary effects. The Heisenberg equations of
motion are used to obtain coupled mode equations for quantized envelope
operators for the optical and acoustic fields. We show that the coupling
coefficients obtained coincide with those established earlier, but our
formalism provides a much simpler demonstration of the connection between
radiation pressure and moving boundary effects than in previous work [C. Wolff
et al, Physical Review A 92, 013836 (2015)].
