The standard definition for noise figure (NF) is the degradation of signal to noise ratio (SNR). That is, if the output noise power of the system is increased more than the output signal power, then this implies a significant noise figure and a degredation of SNR.
For an RF photonic link, there are a couple assumptions that result in a slightly altered definition and calculation for noise figure. One assumption is that the input noise is the thermal noise (kT), such as would be detected from an antenna receiver. It is also the case that RF photonic links may be employed in a case where the input signal power level is not defined. In simple telecommunications aplications, it is standard to expect a certain input power level, but as a communications system at a radar front end for instance, the input signal is not known. We can use the gain of the link as a relationship between output signal and input signal instead of a known input and output signal power.
It is a goal of the link designer in those cases to ensure that all true signals can be distinguished from noise. For these reasons, we may also think of noise figure in the following definition:
Noise figure (NF) is the difference between the total equivalent input noise and thermal background noise.
The equivalent input noise is the output noise without considering the gain of the link.
For the noise figure calculation, we have then:
NF = 10*log_10( EIN / GkT ),
where EIN is the equivalent input noise, G is the link gain, k is Boltzmann’s constant, and T is the temperature in Kelvin.
Equivalent input noise (EIN) is as follows:
EIN = GkT + <I^2>*Z_PD,
where <I^2> is the current noise power spectral density at the output of the link and Z_PD is the photodetector termination impedance.
These together, we have noise figure:
NF = 10*log_10(1+(<I^2*Z_PD)/GkT)