The eye diagram consisting of two eyes corresponds to the coded multilevel signal. So you would just have to improve the Q-factor and BER. If you are using 20dB amplifier, I assume you have a 100 km transmission line. You can further increase launched power until the system reaches nonlinear effects (that’s usually when simulation takes a long time). You can decrease the linewidth to, say, 0.1MHz (unless you are reproducing the work of a paper). You can also consider using DSP component to compensate for nonlinearities, dispersion and some other unwanted effects.
Regards