Hi alistu sorry for the delay , pls find attached the result of BER analyzer. with your advice my run time for 120 km fiber transmission has reduced to 8 min . i decreased the pump powers for my amplifier. no doubt gain decreased but OSNR improved becuase for high pump powers though i was getting very high gain but OSNR degraded signifcantly because of increased crosstalk due to pump-pump interactions and non-linearities.
Now pls help me improve my BER. as advised i reduced laser linewidth to 2 MHz but no improvement.BER is still not nearing 10^-9.