Noise Figure in a Microwave Photonic Link

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)

Noise Sources in RF Photonic Links

Identifying the noise sources in an RF Photonic link allows us to determine the performance of the link and helps us to identify critical components to link and device design to develop a high performance link. Below is an intensity modulated optical link. Other modulation schemes in RF photonic links may be discussed at a later point.

Since the output of the RF photonic link is the photocurrent generated by the photodetector, the noise sources are a current noise power spectral density.

Noise sources from the laser:

Laser RIN (relative intensity noise) is the fluctuation of optical power. Relative intensity noise is the noise of the optical power divided by the average optical power in a laser. RIN noise originates from spontaneous radiative carrier recombination and photon generation.

Noise sources from the modulator:

Noise in a modulator is due to thermal noise of electrode termination and ohmic loss in the electrodes.

Noise sources from the photodetector:

Shot noise occurs as a result of the quantization of discrete charges or photons. Noise is also generated by the photodetector termination.

Total current noise power spectral density of the RF photonic link:

RF Photonic Links

RF Photonic links (also called Microwave Photonic Links) are systems that transport radiofrequency signals over optical fiber. The essential components of an RF photonic link are the laser as a continuous-wave (CW) carrier, a modulator as a transmitter and the photodetector as a receiver. A low-noise amplifier is often used before the modulator.

Optical fiber boasts much lower loss over longer distances compared to coaxial cable, and this flexibility of optical fiber is one advantage over conventional microwave links. Another advantage of RF photonic links are their immunity to electromagnetic interference, which plays a more significant role in electronic warfare (EW) applications. RF Photonic links are employed in telecommunications, electronic warfare, and quantum information processing applications, although the performance requirement in each of these situations vary. In telecommunications, a high bandwidth is required, while in EW applications having high spurious-free dynamic range (SFDR) and a low noise figure (NF) is critical. In quantum information processing applications, a low insertion loss is critical.

In EW scenarios, unlike in telecommunications, the expected signal frequency and signal power is unknown. This is because typically, an RF photonic link is found as a radar receiver. In a system with high SFDR and low NF, distortion is minimized, the radar has stronger reliability and range, and smaller signals can be registered. Here is a demonstration of two scenarios with different SFDR and NF:

Low SFDR, High NF:

High SFDR, Low NF: