Ultra-narrow linewidth electro-optical mixing via SOA-MZI using Pound-Drever-Hall locking

This paper presents a comprehensive study on achieving ultra-narrow linewidth in an electro-optical up-conversion system based on a Semiconductor Optical Amplifier Mach–Zehnder Interferometer (SOA-MZI), operating under both standard and differential modulation schemes. The system employs a 19.5 GHz optical pulse clock (OPC) for electro-optical sampling, enabling up-mixing of an intermediate frequency (IF) signal to a radio frequency (RF) range extending up to 78.5 GHz. Experimental results show that measured linewidths of the electro-optically mixed signal range from 220 kHz at 20 GHz to 280 kHz at 78.5 GHz in standard mode, and from 120 kHz to 180 kHz in differential mode. The system achieves a stable optical output power of 29.5 dBm (standard) and 35.5 dBm (differential) at 1550 nm, with an optical conversion gain of up to 45.5 dB, power stability of 0.2–0.4% RMS, and near-Gaussian beam profiles with 94–98% circularity. Applying the Pound–Drever–Hall (PDH) locking technique significantly improves spectral purity, reducing linewidth to 5 kHz at 20 GHz and 10 kHz at 78.5 GHz in standard mode, and as low as 1 kHz and 3 kHz, respectively, in differential mode. These results highlight the effectiveness of combining PDH feedback with SOA-MZI-based up-conversion, offering high efficiency, broad bandwidth, and exceptional stability, making the system a promising candidate for millimeter-wave and terahertz communication applications.