The key property of optical frequency combs is a stable frequency relation among the comb tones. An ideal optical comb covers a wide spectral range, has tunable spacing, high power-per-tone, many tones, and good power uniformity, and is very compact. The most compact optical combs today are "Kerr frequency combs" that exploit the nonlinear Kerr effect in microresonators. These combs generally cover very large spectral bands, but they have a large comb spacing (resulting in a low number of comb tones) and are not tunable. Another type of frequency comb that is very compact is the "electro-optic (EO) comb" that uses EO modulators to generate comb tones. They have tunable comb spacing compatible with electronics (10s of GHz), but generally have limited spectral width.

Bao and colleagues present a combination of the two compact frequency combs to generate a comb that combines good features of both of them. The result is a comb that is still compact, covers a large spectral band, has convenient (electronics-accessible) comb spacing, and is quasi-tunable. The resulting comb has 5-THz 3-dB bandwidth, tunable spacing of 21, 24, or 27.4 GHz, and comb linewidth around 25 kHz. The authors demonstrate the usability of their new frequency comb by showing data transmission: the individual comb tones are used as transmitter lasers modulated with 10 Gbaud QPSK coherent signals (giving 20 Gbit/s/channel). Apart from optical communications, this versatile comb is of interest in various fields, including for many spectroscopic techniques.

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