Hi Koll,
In alitsu file, he gave input power source = ‘0’dbm (i.e 1 milli watts) given to the modulator input and getting output = -3dbm approximately (by default), modulated input is given to the optical fiber with length = 100km,attenuation = 0.2db/km and you will get a attenuation of 20db/km, so totally till now you have a power value of = -23dbm. To make this zero, he has given value of = 24db(i.e gain = 20db, noise figure=4db, summing up gives= 24db). At the output of the optical amplifier, you will get 24-23 = 1db. Again this output is given to the next optical fiber with length of = 20km, attenuation = 0.2db/km,attenuation value = 4db, then output power from the second fiber= 5db(1db output from amplifier+4db). Finally given attenuator with value=10db, then output = 10+5=15db.
All I have given is approximate values, simulate your file and check it.