I. Introduction
Nonlinear directional coupler (NLDC) have attracted considerable attention on the researching domain of nonlinear waveguide devices because of its potential application to ultrafast all-optical signal processing [1], [2]. However, it has been observed that ultrafast all-optical directional coupler's switching occur over a relatively broad range of powers in contrast to ideal switching, which is characterized by a step function. Fortunately, relatively ideal switching characteristics have been realized in an active fiber coupler [3]. Active fiber couplers can be executed by doping with rare-earth elements, for example, erbium ions in one or more cores of ordinary passive fiber couplers. The limitations on the operation of the device, caused by the linear loss, can be eliminated by introducing gain in the fiber cores of the active fiber couplers. Meanwhile, erbium-doped silica, whose nonlinear coefficient is much larger than undoped silica, can provide high nonlinearity; the erbium doped in the fiber coupler can overcome the requirement of high operation power.