Analysis and Reduction of Phase Noise Effects in Multi-Channel Microwave Photonic Systems

Microwave photonic applications such as beamforming and filtering entail the photonic summation of optically-carried RF signals. The most commonly used components to perform these functions are fiber-optic couplers, arrayed waveguide gratings and optical concentrators, whose common characteristic is to gather these optical signals on a single output before converting them into an RF signal with a photodiode. The spatial superposition of several optical fields causes photonic interference leading to the emergence of noise in the RF domain. This paper focuses on an experimental study of the impact of optical interference and phase decorrelation between delayed optical fields in an optical fiber interferometer, considering two different combining technologies. When a fiber coupler is employed, the full overlap of the spatial fields induces a high-level noise on the electrical signal generated after the photodiode. To solve this problem, the use of a microwave photonic summation device based on an optical power concentrator and a high-speed planar photodiode permits a strong mitigation of the optical interference. The results of the experiments presented reveal that the proposed approach can reduce phase noise effects on the resulting electrical sum signal by a factor of 40 dB.