Dear Vamsi,
there are few effects that show on the eye diagram such as dispersion, nonlinearity, jitter, distortion and interference, etc… Typically, the pulses’ dispersion appears on the x-axis as width of the eye crossing. The width of the eye is also influenced by transmitter’s jitter. The width of low and high levels on the y-axis are related to the received pulses levels, which are affected by nonlinearity, interference and noise.
This description is a general information. you may check the datasheets of the BER and eye analyzers in OptiSystem and optical communication books for more information.
Regards,
Ahmad