Hello Lukas,
As far i am concerned, You can increase power upto 15mW in free space optical communication link. I had provided you with a link of ITU-T standardization..Did you check it???
It provides you an essential set up parameters for your free space optics designs. Shall check your osd file in working hours tomorrow.
For now i suggest you to increase power at source and use optical amplifier instead of EDFA if you are using one.
Hope it is helpful.
Regards