Signal-to-Noise Ratio of True-Time Delay Beamformers Based on Microwave Photonic Links Using an Incoherent Broadband Optical Source

The signal-to-noise ratio (SNR) of true-time delay beamformers based on microwave photonic links using an incoherent broadband optical source (IBOS) is investigated. A comprehensive theory considering the bias angle of the Mach-Zehnder modulator employed in the microwave photonic link is presented to analyze the SNR. When the RF signal at the fundamental frequency is fed by the beamformer, the SNR is a function of the bias angle, the frequency of the applied RF signal, the modulation index, the system dispersion, and the shape of the IBOS. We find that by biasing the MZM near the minimum transmission point, the SNR at the fundamental frequency is maximized. On the other hand, by exactly biasing the MZM at the minimum transmission point, the RF signal at the double frequency can be fed by the beamformer with maximum power. The maximum SNR at the double frequency is a function of the frequency of the applied RF signal, the system dispersion, and the shape of the IBOS while its dependence on the modulation index is eliminated. In both cases, the relationship between the SNR and the frequency of the RF signal, the system dispersion, and the shape of the IBOS is also investigated. An experiment is carried out to verify the theoretical analysis, and the experiment results agree well with the calculation.