Relative phase noise suppression for two-color optical sources via optoelectronic feedback and feedforward

Highly coherent two-color optical sources play an increasingly important role in microwave photonics and terahertz (THz) generation. However, achieving low relative phase noise at large frequency spacings remains challenging. Here, we propose an optoelectronic feedback-feedforward system that suppresses relative phase noise without relying on optical reference cavities. An unbalanced Mach-Zehnder interferometer (MZI) is employed for phase discrimination. The feedback loop provides effective low-frequency stabilization, while the feedforward path enables further correction of the residual relative phase noise over a broad frequency-offset range using an electrical oscillating cavity and optical frequency shifting. The proposed scheme is experimentally demonstrated at frequency spacings of both 100 GHz and 1.1 THz using free-running lasers. At 10-kHz offset, the relative phase noise is reduced from −50 to −100 dBc/Hz, corresponding to a suppression of 50 dB. Moreover, the relative phase noise remains below −90 dBc/Hz at offsets ranging from 3 kHz to 1 MHz. These results demonstrate a promising approach to low-phase-noise microwave-to-THz signal generation, with the output frequency determined by both the frequency spacing between the two optical tones and the frequency shift introduced by the oscillating signal.