optical link butget question

dear friends i need ans for this one .
fout stations are avilable A,B,C,D.
for both A to B
C to D i am using fiber with Numerical aperature 0.20
α=2.0
attenuation 0.1 db/km

from B to C i am using fiber with Numerical aperature 0.17
α=1.9
attenuation=0.12db/km

i am using 2 couplers with 0.5 db loss each.
safety margin 7 db is desired
it is a duplex system

now i need what is minimum optical power must be launched (Pt) so that the system work satisfactorly



any body good in link design please give ans for this one.