Phase Noise of Optically Generated Terahertz Signals Employing an InP-based Photonic IC

Photonic integrated circuits (PICs) play a vital role in terahertz (THz) systems due to their compact and low-cost nature. For practical THz applications, broadband, stable and widely tunable THz signals are required. These can be provided by photonic approaches in which two optical carriers are down-converted by a photodiode, also known as optical heterodyne detection. In this work, we present the experimental characterization of a custom-developed InP-based PIC, which features two free-running distributed-feedback (DFB) lasers. By tuning the laser current, the beat frequency can be continuously tuned between 325 GHz to 850 GHz. The phase noise of the beat signal at 325 GHz measured after down-conversion is $-80 \mathrm{dBc} / \mathrm{Hz}$ at an offset frequency of 10 MHz. It is proven by using the beat-note method, that the phase noise of the optically generated THz signal is composed of the phase noises of the individual DFB lasers.