A WDM passive optical network enabling multicasting with color-free ONUs

We propose a wavelength-division-multiplexed (WDM) passive optical network (PON) to provide conventional unicast data and downstream multicast function. At the optical line terminal (OLT), for each WDM channel, a dual-drive Mach-Zehnder modulator (DDMZM) is used to generate a sub-carrier double-sideband differential-phase-shift-keying (DPSK) signal. All the central carriers are separated and subsequently modulated to deliver the multicast data, while the remaining sub-carrier DPSK signals carry the downstream unicast traffic. In the optical network units (ONUs), part of the downstream unicast signal power is re-modulated for upstream transmission, which enables source-free ONUs. The proposed scheme is experimentally demonstrated with 1.25-Gb/s downstream unicast, multicast and upstream data.