Performance Evaluation of Dual-Drive Mach-Zehnder Modulator and Optomechanical Crystal Cavity Comb Generation for All-Optical Band Conversion in 5G-Advanced Cellular Systems

Multiband operation is a key aspect of emerging 5G-Advanced cellular systems, also named 5.5G, which target seamless provision of multi-gigabit per second connectivity employing sub-6 GHz and mm-wave overlapping coverage. All-optical frequency conversion gives flexibility due to the feasible high-speed reconfiguration in broad bands and large radio signal bandwidth. Optical frequency combs, featuring a spectrum of discrete, equally spaced coherent frequency lines, are crucial for high-precision metrology, spectroscopy, and telecommunications. Their effectiveness in all-optical frequency conversion depends on their stability in terms of frequency drift, phase noise, and power distribution across the comb lines. This paper evaluates experimentally the generation of optical frequency combs employing two distinct technologies: a dual-driven Mach-Zehnder modulator (DD-MZM) and an optomechanical crystal cavity (OMCC), and experimentally compares their performance for all-optical frequency conversion of 5G data streams. The DD-MZM implementation generates a comb with flexible line spacing and comprising several spectrum-flat lines with low phase noise ($-88.5$ dBc/Hz at 1 kHz offset and $-108.3$ dBc/Hz at 100 kHz offset). The OMCC implementation provides a reduced footprint (182 $\mu \mathrm{m}^{2}$) since it is implemented on a silicon chip and has the extra advantage of generating an optical comb without an external local oscillator, which reduces its power requirements (under 1 mW) while providing a phase noise of $-38.3$ dBc/Hz at 1 kHz offset and $-97.1$ dBc/Hz at 100 kHz offset. The polarization stability and jitter of both implementations are also evaluated. The experimental demonstration evaluates the error vector magnitude (EVM) of frequency-converted 3GPP 5G NR signals using both implementations, confirming the successful transmission with EVM smaller than 12.01% for DD-MZM up to the third harmonic and EVM smaller than 17.36% with the OMCC first harmonic.