  An array of a finite number waveguides, driven laterally by injecting light
at the outer waveguides, is considered. The array is modeled by a discrete
nonlinear Schr\"{o}dinger equation. It has been shown [Phys. Rev. Lett. 94,
243902 (2005)] that, when the injected light is in the proximity of a
bifurcation point, such a system can be sensitive to small disturbances, making
it possible to act as a light detector. Here, the optimum intensity of the
injected light is discussed, and an analytical approximation is presented.
