Feedforward Compensation of Laser Frequency and Phase Noise for Photonics-Based Sub-THz Generation

Optical heterodyne detection, in which two laser beams are coupled and then detected by using a photo-detector, is an attractive photonic method for generating sub-THz signals. However, the signals generated by heterodyning two free-running lasers suffer from poor frequency stability and large phase noise. In this letter, we propose and demonstrate a feedforward frequency and phase compensation of two free-running lasers heterodyned for sub-THz generation. We detect the frequency drift and phase fluctuations of the beating signal of the two laser lights after photonic frequency down-conversion and then Compensate them using an optical IQ modulator. Our theoretical analysis and experimental demonstration carried out for a 165-GHz signal show that the frequency stability and phase noise characteristics of the sub-THz signal generated by our proposed scheme are limited by those of RF oscillators used for compensation.