who can help me with optical receiver?

Hi

I am working on an optical receiver, 10gbps. when input power is high, the eye diagram is good, however, when input power is low, the eyediagram is bad. I attach the pics, please help me, any hints will be appreciated. Thx in advance!

regards
Edmund

Help you in what?
What you said is natural phenomena

Added after 35 seconds:

Be more clear in presenting the problem.

in brief, low sensitivity, narrow dynamic range, two '1' lines