Dear San, as far as I know, in case of bit rate at the receiver side it will be the same as the transmitter if your system is ok. only the effect of attenuation, dispersion will be there. if the bit rate changes that means you are missing some of the bits transmitted that will be the loss of information. that will be the failure of the system.
The received power is not necessarily always less than the transmitted power. More specifically, if optical amplifiers are used in the transmission line, the power might actually exceed the launched power. However, this might degrade the signal due to the optical amplifier noise figure.