Heyy! esakkimuthu
The general idea by allesendro is appreciable. You can easily enhance the transmission length of your system by using amplification as a booster or pre amplifier.
According to example you have given if you are using 20 km SMF having attenuation 0.2dB/km then total loss is 4 dB. So if you are using a 4 dB optical amplifier then it will boost the signal to overcome the degradation by fiber. and if you are using edfa or raman 10dB gain then you will transmit your signal for longer transmission length within acceptable BER limits. again it all depends upon parameters of fiber and noises in amplifiers too. More the power, more the non linear impairments. Check your system for non linear parameters of fiber and noise in amplifiers you are using.