Wavefront Correction–Enabled Stable Multi-Channel Coherent Combining for Microwave Photonic Communication in All-Fiber Links

Optical microwave signals transmitted through all-fiber links are highly susceptible to environmental disturbances. Multi-channel optical combining often induces wavefront distortion, leading to instability in output power and spectral characteristics. This paper presents a model-free wavefront correction method based on the Stochastic Parallel Gradient Descent (SPGD) algorithm, applied to ultra-wideband microwave photonic phased array subarray delay systems. By monitoring the combined signal, the algorithm adaptively adjusts the optical phase of each input channel in real time to enhance coherence. Theoretical analysis shows that, after combining, the wavefronts of different channels exhibit high correlation due to shared optical paths, enabling effective correction even when output paths are excluded from the feedback loop. An injection-locked optical local oscillator is employed to generate beat interference in the optical domain, enabling real-time monitoring of sideband power stability. Experimental results verify that the method significantly suppresses spectral power fluctuations and improves spectral stability. QPSK demodulation at 100MHz bandwidth achieves an error vector magnitude (EVM) of 11.06% without observable symbol decision degradation. To the best of our knowledge, this is the first demonstration of stable wavefront control of optical sidebands carrying microwave signals using a model-free algorithm in an all-fiber microwave photonic architecture.