Hey Eman,
You will get a much better BER if you use a low pass filter on the output of the Photodetector PIN component. Use a cutoff frequency of 0.75*Bit rate to extract the signal.
It is usually a better idea to change the Bit rate globally, by double-clicking anywhere on the layout or using the Project Browser in the bottom left corner. This will automatically update the bit sequence generators and also calculate a correct time window for visualizers etc. You will also notice that the low pass filters automatically have a cut-off frequency of 0.75*Bit rate.