Critical power in a symmetric nonlinear directional coupler

An alternative theoretical analysis directed to obtaining a more accurate expression for the critical power in the symmetric nonlinear directional coupler, or other nonlinear, five-layer waveguiding structures, is reported. Our analysis works with the nonlinear supermodes of the waveguide, and it is presented as an improvement of the preceding study by Silberberg and Stegeman [1]. Numerical simulations comparing both techniques, and by means of the beam propagation method, are reported for a few cases. A set of three representative, power dependent parameters is proposed, in order to give a comprehensive view of the power flow between both branches in the device as a function of the excitation conditions.