440-GBaud All-Electronic Signaling Enabling Single-Wavelength Net Rate over 1 Tb/s per Modulation Dimension

The ability to generate high-symbol-rate signals has been instrumental in pushing the speed limits of optical fiber communications. Over the past 50 years, the reported electronic symbol rates at the Optical Fiber Communication Conference (OFC) have improved from 123 MBaud in 1975 to 440 GBaud in 2025. This paper reviews the historical evolution of symbol rates in both laboratory demonstrations and commercial products, with a particular focus on the trend in the coherent era. We highlight a crucial technique that has improved symbol rates in laboratories over decades – electronic multiplexing and discuss its role in future coherent systems. Laboratory symbol rates have plateaued around the 200-GBaud level for nearly a decade, and this level is now transitioning into 1.6-Tb/s commercial products. Here, we report a record Nyquist symbol rate of 440 GBaud, doubling the state-of-the-art using a newly designed digital-band-interleaving (DBI) transmitter providing an analog bandwidth of 221 GHz. The signal drives a thin-film Lithium Niobite modulator and exhibits a flat-top spectrum across 440-GHz optical bandwidth without optical equalization. A high transmitter quality supports probabilistically shaped 16-level amplitude modulation that achieves a net rate over 1 Tb/s per modulation dimension. The 200-GHz class DBI system can support research and development at both component and subsystem levels for next-generation single-channel 400-GBaud client optics targeting 800 Gb/s or coherent optics at 3.2 Tb/s.