We present a dual-core fiber that can function as a wavelength selector for applications in dense wavelength division multiplexing (WDM) systems. The fiber consists of an erbium-doped core and an undoped core. The principle of operation relies on the large nonlinear effect of the Er-doped core when it is pumped by a 980-nm laser diode. The selection of wavelength is achieved by adjusting the pump power launched into the doped core. Wavelengths with spacing of 1 nm can be selected with cross talks as low as –28 dB.