There are two fundamental noise mechanisms in a photodetector:
- shot noise
- thermal noise
Receiver Shot and Thermal noise.osd details the signal degraded by thermal and shot noise in the PIN photodetector. The low-pass filter has a cutoff frequency with the same value as the bit rate.
Figure 1: Receiver Shot and Thermal noise
The upper system has the photodetector without thermal noise; the only noise generated at the output is the shot noise. In Figure 2, you can see that the shot noise is signal amplitude dependent.
Figure 2: Receiver Shot noise
The lower system has the photodetector with shot noise; the only noise generated at the output is the thermal noise. In Figure 3, you can see that the thermal noise is signal amplitude independent.
Figure 3: Receiver thermal noise