Hello@Avondale 

I'm afraid power line units could not work with the optical cable, the ' transmitter' or main powerline units need wire to router through Ethernet cable. 

Please confirm the shed and the main building are under the  same power meter, or they could not pair with each other. Considering powerline signal strength loses over distance powerline devices performance in the house extension could be unsatisfactory.  If the extender does not performance good, you may need to consider other ways to extender your network,  like an AP wire to main router through Ethernet cable, and put AP inside the shed.