Hello Jyoti Raina,
I already mentioned that BER analyzer uses algorithm for the calculation of Q-factor and Min BER, for more details about algorithm and formula please go through pdf “Optisystem components library” page no. 134-138. There everything is explained in theoretical as well as in mathematical form. you will find the Optisystem components library pdf at the installation directory. Hope this will definitely help.
regards,
Dhiman