A wavelength division multiplexed-passive optical network includes an optical line terminal for generating downstream optical signals of discrete wavelengths and for receiving upstream optical signals of discrete wavelengths, a remote node, coupled to the optical line terminal, a wavelength division unit settled to reflect a predetermined wavelength, and a plurality of optical network units. Each optical network unit has an optical source which is oscillated in a multi-mode and is self-injection locked by the predetermined wavelength provided thereto, thereby to generate the upstream optical signal in a single mode to be provided to the remote node.