Broadband Waveguide Electro-Optic Comb Enabled by Mode Circulation

Integrated electro-optic (E-O) frequency combs built on the lithium niobate-on-insulator (LNOI) platform have emerged as a promising tool for diverse applications. Compared with high-quality-factor microresonator schemes, waveguide-based E-O combs deliver enhanced flexibility and higher efficiency. However, they often exhibit restricted spectral bandwidth due to their non-resonant optical characteristics. In this paper, we present a broadband waveguide-based E-O comb by using mode-circulation. The inherent mode hybridization of the anisotropic LNOI waveguide is suppressed effectively by employing a novel Z-propagation designed mode multiplexer, thereby enabling the scaling of the mode-circulating E-O comb to four mode-channels (TE0, TE1, TE2, and TE3 modes). By integrating the mode-circulating scheme with a push-pull type GSG traveling-wave electrode configuration, and carefully designing multimode phase modulators and delay line waveguides for each optical loop, the modulation index of the E-O comb is enhanced by a factor of eight. Experimentally, we successfully generated 128 comb lines covering a wavelength range of ~26.5 nm by driving the fabricated E-O comb with a radio frequency (RF) signal of 25 GHz and 34 dBm power. The comb exhibits a modulation index enhancement factor of approximately 7.5, a voltage-length product of 0.62 V·cm, and an optical loss of 7 dB. Spectral measurements are carried out by utilizing the reconfigurability of the present E-O comb, and a spectral resolution of 2 MHz (~0.016 pm) is achieved within the wavelength range of 1530–1570 nm. Furthermore, the time-to-frequency mapping method is used to generate a broadband flat-top comb with 41 lines and 4 dB flatness. The present E-O comb offers the advantages of high energy efficiency, broad bandwidth, and reconfigurability, and will play a vital role in a wide range of applications, such as high-speed optical communications, high-precision measurements, and optical computing.