Hi Abdullah Baig,
I think logically your results are good, because When you reduce Extinction ratio then BER increases.
In the Mach Zehnder modulator the extinction ratio is an important parameter of the specifications. The Extinction Ratio value is defined as the relation between the maximum and the minimum power levels of the transfer function. For equal power split between two interferometric branches, there is an infinite extinction ratio. It indicates the signal intensity that the transmitter feds to the fiber. Hence, a higher extinction ratio also allows a greater transmission between repeaters in the long distance transmission systems.
In practice fabrication tolerances make it very difficult to obtain an equal power split between two branches and therefore we get a finite extinction ratio which degrades the quality of the signal. A typical value is between 20-40 dB.