Abstract

For effective device design and for elucidation of underlying mechanisms, it is important to determine whether the input-intensity-dependent response of a nonlinear directional coupler¹ (NLDC) is predominantly refractive or absorptive.² This is readily determined under practical conditions by studying the response of asymmetric NLDCs. A propagating-beam model,³ including effects of saturation of both refractive and absorptive nonlinearities, shows (Fig. 1a) that for a NLDC less than one half beat length long with a higher mode index in the input guide (positive asymmetry), the coupling initially increases with increased input power. This is because the nonlinearity, taken as negative, drives the guides toward synchronism. For negative asymmetry, the coupling initially decreases. For a coupler with saturable nonlinear absorption, however, asymmetry has little effect [Fig. 1(b)].

© 1990 Optical Society of America