Broadband and flat-top integrated electro-optic frequency combs on a thin-film lithium niobate platform.

Integrated optical frequency combs (OFCs) and femtosecond pulse sources play pivotal roles in advancing optical communications, ranging, and spectroscopy. So far, most chip-scale OFCs are based on semiconductor mode-locked lasers or Kerr nonlinearity in microresonators. However, these approaches face inherent limitations in operation wave band and repetition rate tunability. Here, we demonstrate broadband and flat-top integrated electro-optic (EO) frequency combs by cascading one intensity modulator and two phase modulators on a thin-film lithium niobate chip. The measured EO combs feature over 70 comb lines within the 10-dB bandwidth, continuously tunable repetition rate from 5 GHz to 25 GHz, and central wavelength range over 120 nm. In addition, we obtain femtosecond pulses at different repetition rates with the narrowest pulse width of 565 fs. Our work paves the way for future compact, cost-effective, and reconfigurable on-chip light sources.