FTTH - WDM

Most of the FTTH architectures which I found on the internet has WDM module to multiplex 1310nm , 1490nm and optional 1550nm wave lengths in to the fiber. so my question is

Cant we use a passive optical coupler instead of WDM module

If WDM multiplexer has been used in the OLT then which equipment act as WDM de-multiplexer ... ONU ?

BPON, EPON, GEPON, and GPON have the same basic wavelength plan and use the 1,490 nanometer (nm) wavelength for downstream traffic and 1,310 nm wavelength for upstream traffic. 1,550 nm is reserved for optional overlay services, typically RF (analog) video.

Not sure it's usually referred to as WDM especially as without the optional wavelength there's only a single transmission wavelength each way. Certainly WDM-PON is a different thing to the above.

The ONT is the consumer end and it has to do the demultiplexing.

Passive optical devices can split and combine wavelengths or be directional.