Selecting Optical Frequency Combs for Multi-Channel Coherent Communications

Multi-channel coherent communications—the backbone of 800G, 1.6T, and future 3.2T optical transport—demand spectrally pure, stable, and scalable light sources. Optical frequency combs have emerged as a compelling alternative to arrays of discrete lasers, offering dozens of equally spaced, phase-locked carriers from a single continuous-wave pump. However, selecting the right comb source requires careful evaluation of linewidth, tone-to-tone coherence, power uniformity, and the modulator technology used to encode data onto each comb line. Across our work with TFLN Devices, we have seen how thin-film lithium niobate intensity modulators can both generate and process optical frequency combs with exceptional efficiency. Below we outline key criteria for choosing an optical frequency comb for your coherent architecture.

Comb Generation Methods: Electro-Optic vs. Kerr Combs

The two dominant approaches for generating an optical frequency comb are electro-optic (EO) modulation and Kerr microcombs. EO combs rely on cascaded phase and intensity modulation, typically using high-bandwidth modulators. This method produces combs with excellent linewidth (equal to the pump laser), high tone stability, and precise frequency spacing—ideal for coherent wavelength-division multiplexing (WDM). Kerr combs, while compact, often suffer from higher phase noise and require complex pump stabilization. For multi-channel coherent communications, we recommend EO combs driven by low-Vπ modulators. Our TFLN Devices—specifically the 67 GHz and 110 GHz intensity modulators—deliver < 3.0 V half-wave voltage and < 4.5 dB insertion loss, enabling flat comb generation over 20 nm or more with minimal RF drive power.

Key Metrics: Bandwidth, Loss, and Uniformity

Once you have selected an optical frequency comb source, the next consideration is how to modulate each comb line independently or collectively. For systems employing subcarrier modulation or direct comb line switching, the modulator’s bandwidth and optical loss directly impact the aggregate data rate. Our TFLN Devices provide two bandwidth options: 67 GHz for 800G applications and 110 GHz for 1.6T and beyond. The ultra-low insertion loss (< 4.5 dB) means less pump power is wasted, allowing a single comb source to feed multiple parallel modulators without amplification. Comb line power uniformity across the C‑band is another critical factor; we have measured less than 0.5 dB variation over 40 channels when using our modulators in a cascaded configuration. This simplifies receiver-side equalization and reduces digital signal processing overhead.

Integration with Coherent Receiver Arrays

The true value of an optical frequency comb in multi-channel coherent communications emerges when paired with a parallel coherent receiver array. Each comb line serves as a local oscillator for a respective channel, eliminating the need for per‑channel tunable lasers. However, the comb’s relative phase noise between tones must be low enough for high-order modulation formats (16QAM, 64QAM). EO combs generated by TFLN Devices exhibit exceptionally low phase noise due to the linear electro‑optic response of lithium niobate. We have successfully demonstrated comb-driven coherent links at 800G per wavelength with error‑vector magnitudes below 5%.

Building Scalable Comb-Based Links

Selecting the right optical frequency comb is not a one‑size‑fits‑all decision. EO combs driven by low‑loss, high‑bandwidth modulators offer the best path to scalable multi‑channel coherent systems. With TFLN Devices providing 67/110 GHz bandwidth, < 4.5 dB loss, and < 3.0 V Vπ, we enable compact, power‑efficient comb generation and modulation. At Liobate, we are committed to delivering superior TFLN intensity modulators and supporting your transition to comb‑based coherent architectures. We invite system architects to evaluate how our devices can unlock the full potential of optical frequency combs for your next‑generation transport solutions.