Hi Shify,
An optical interleaver simply considers the DWDM channels as odd and even channels and then combines them into one signal stream. I have attached an image clearly showing what it does.
The “WDM interleaver MUX” component in OptiSystem library is the same as an optical interleaver, and “WDM interleaver Demux” is the same as an optical de-interleaver. You can refer to my following answer:
Regards