Highly Robust Optical Phase Decorrelation in Microwave Photonic Summation Systems Using Mode-Coupling Receiver

The summation of multiple optical carriers into a single output before photodiode (PD) is a widespread structure in several microwave photonic (MWP) applications [1] . However, the optical phase interference/decorrelation induced by imbalance fibre lengths can alter the phase relation among optical carriers on the combined signal, and thus, the phase noise appears on the resultant RF signal after photodetection [2] . Some solutions such as multicore fibre, optical concentrator with a planar PD, etc, can mitigate the interference at the cost of input number, operation frequency, and transmission length [3] , [4] . Therefore, we propose a mode-coupling receiver (MCR) [5] as a key component to achieve the optical combination with lower phase noise and insertion loss. The employed MCR combines an N single-mode fibre (SMF) into a one multi-mode fibre (MMF) using ( N -1) 2×2 optical couplers, as can be seen in Fig. 1 (a) . Of utmost importance for this architecture is that the input signals are taking several paths to concurrently reach the MMF (red lines in Fig. 1 (a) ). This allows to compensate the optical insertion losses and to cancel the optical interferences obtained on the active area of the PD at one location by the other interference for respecting the energy conservation law. In this case, optical interference isolation among optical carriers is performed, and subsequently, avoiding phase noise limitations in several optical transmission domains such as passive optical network, phase antenna array beamforming, and RF transversal filtering.