This study reviews key technologies of next generation wavelength division multiplexing passive optical networks (WDM-PONs). The authors have studied WDM-PONs with centralised lightwave source and direct detection, where a wavelength-reuse system is employed to transmit the uplink data by using a colourless transmitter at the optical network unit (ONU). WDM-PON system was demonstrated using a Fabry–Perot laser diode as a colourless transmitter at the ONU for differential phase-shift keying (DPSK) downlink signals and on–off keying uplink signals. The constant-amplitude feature of the DPSK modulation could retain a great extinction ratio of the downlink signal and decrease the noise to the uplink signal. The proposed scheme offers a promising solution for future PONs.