Hi,
Inorder to compensate for the system degradation, the signal power has to be increased to
achieve the same SNR or BER performance as that of an ideal system. This increase in
power is called the Power Penalty.
There are two types of signal degradation which can contribute to the power penalty.
1) Degradation during propagation in the optical fiber
2)Degradation due to peripheral electronic and optic components in the system like the
lasers, photo-detectors, couplers etc.
Due the fluctuations the mean of the peak current is reduced decreasing the SNR.
The SNR can be restored by increasing the pulse amplitude. Hence there is power penalty
for timing jitter. It can be noted that the power penalty very rapidly increases with the timing jitter. When
the rms value of the timing jitter is about 0.15, the power penalty becomes infinite.
The power penalty may become significant if the semiconductor laser is biased above the
threshold. For lasers biased below threshold, the extinction ratio is typically 0.05 and the
power penalty is less than 0.4 dB.
Thanks