said by TomS_:
All of the PON implementations I have seen work by sending the same downstream signal to all ONTs, and this is achieved using 1xN way splitters. The ONTs pick out the data that belongs to them, and ignore the rest.
In the upstream direction the ONTs use a TDM based protocol, having time slots in which they can transmit.
Tom, you are absolutely correct. Both IEEE 802.3ah and ITU-T G.984 GPON standards dictate that method.
If some company wanted to use even simple WDM (let alone "dense" WDM) on top of GPON passive infrastructure, they have to use custom OLT/ONU designs. I am curious to see a link to such a commercial product - if they actually exist.
P.S. ITU uses ATM frames downstream while IEEE uses 802.3 frames. Most OLT/ONUs are "universal" and can support either ITU/IEEE method. However, they can't support WDM without a complete re-design.