Analysis of signal to noise ratio in photonic beamformers

The goal of this paper is to evaluate the impact of photonic beamformers on the amplitude and the signal-to-noise ratio (SNR) of RF signals received by a phased array antenna. To this end, the two-port definitions of RF gain and noise figure are generalized to include multiple-port combining devices such as beamformers. These metrics are then applied to several simple photonic and hybrid RF/photonic beamformer architectures. The gain and noise figure are determined in large part by the components at the photonic interfaces, namely the laser, modulator and photodetector, and therefore all component parameters must be held constant when comparing different beamformer architectures. As expected, reducing optical loss in the beamformer is essential for achieving high RF gain and SNR at the beamformer output. While the addition of optical amplifiers increases the effective beamformer gain, it also introduces noise which drives up the noise figure. Efficient combining in the beamformer via WDM (wavelength division multiplexing) and/or RF combiners with high coupling efficiency such as Wilkinson combiners can yield a high effective gain without adding additional noise