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.

Optical System - Figure 1 - Receiver Shot and Thermal noise

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.

Optical System - Figure 2 - Receiver Shot noise

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.

Optical System - Figure 3 - Receiver thermal noise

Figure 3: Receiver thermal noise