Hi Abdullah,
Based on your results, in my opinion, it looks like a timing jitter might be the reason of such a gap (the one shown on Pic-1). Do you have this gap when you are transmitting your signal for short distances? Jitter grows cubically with the distance, if I’m not mistaken…
What is the span length in you transm. system? How big is the whole section and do you use a 3R regeneration at the end of it? (can’t see it from your picture…)