Next generation Passive Optical Networks (ngPONs) that offer increased bandwidth and distance-reach to a higher number of customers may require some modifications in the network infrastructure and in the technology of its devices. Nevertheless, these changes should not require a significant increased cost nor upgrading complexity. Optical Network Units (ONUs) at customer premises of currently deployed PONs include a fixed laser at a non-controlled wavelength, launching the light into the upstream fibre or into a single fibre via a coarse WDM multiplexer. In future PONs, Wavelength division multiplexing (WDM) can be effectively used to upgrade the overall PON capacity in several ways. Thus, new generation ONUs may be wavelength-controlled, wavelength-tuneable or wavelength-agnostic. WDM allows superposing different TDMA PONs over the access fibre line. The number of wavelengths then corresponds to two times the number of TDMA PON systems. This solution offers desirable characteristics for an access infrastructure as the use of one single fibre for both upstream and downstream transmission reduces the network size and connection complexity.