Broadband flat thin-film lithium niobate electro-optic frequency comb generator fabricated with photolithography.

Flat electro-optic frequency combs, as coherent spectral sequences with equidistant distribution, exhibit significant potential in high-capacity optical communication and reconfigurable microwave photonics. This study develops a monolithically integrated, fully packaged electro-optic comb device on a thin-film lithium niobate (TFLN) platform using an efficient photolithography scheme, overcoming the size and loss constraints of traditional discrete cascaded components. The synergistic interaction between cascaded amplitude modulator (AM) and three phase modulators (PMs) balances the optimization between comb-line count and flatness. Theoretical modeling reveals that flatness nonlinearly depends on intensity modulation parameters, while comb-line count linearly correlates with phase modulation coefficients. Experimentally, a coherent spectrum with 27 comb lines is generated, achieving 25 GHz equidistant spacing and 3 dB in-band flatness. Tests show a total insertion loss of 10.2 dB, an intrinsic 3 dB electro-optic bandwidth of 56 GHz, and high bias stability via integrated NiCr thermal resistors. Compared to conventional discrete-component schemes, this device achieves significant improvements in comb-line spacing expansion, power efficiency optimization, and system integration, offering a highly stable dynamic light source solution for high-speed optical communication and programmable photonic systems.