The understanding I have. The interposer is the dielectric layers between the silicon chip and the photonic integrated chip which it also made up of dielectric layers so the same material set. So I would assume the fiber enters from the side/edge of the PIC opposite the silicon chip. 

OK, this would make sense. So an external GaAs/InP laser plunges into the waveguids of the interposer. The interposer leads the light to the Si-Chip which mudulates the info to the light. Interposer then mixes the different (moduled) lights to one laser beam. 

Ah, it would be really nice if POET will put an illustration on their web pages to keep customers / shareholders informed!