As seen earlier, the laser shows the relaxation oscillations. Due to these oscillations the light output during the pulse does not remain constant. The oscillations decay from the leading edge of the pulse to the trailing edge of the pulse.
For low data rates, the duration of the relaxation oscillation is much smaller compared to the bit period. However, as the data rate increases, the intensity fluctuation due to the relaxation oscillation may extend over the entire bit-1.
The relative intensity noise can be treated as the shot noise, as this noise also has multiplicative nature.
The RIN parameter can be defined as
In presence of RIN is modified to , where is the standard deviation of the RIN and is given as
The parameter Q is
Now since , we get
The power penalty is then defined as
A plot power penalty due to RIN is shown in Fig. For BER of , Q = 7, and the power penalty is negligible if The power penalty increases rapidly with .
For , the power penalty is infinite meaning the receiver will never function satisfactorily no matter how much transmitter power is increased.
is modified to 
, where 
is the standard deviation of the RIN and is given as 
, we get 
, Q = 7, and the power penalty is negligible if 
The power penalty increases rapidly with 
. 
, the power penalty is infinite meaning the receiver will never function satisfactorily no matter how much transmitter power is increased. 
