  Integrated electro-optic (EO) frequency combs are essential components for
future applications in optical communications, light detection and ranging,
optical computation, sensing and spectroscopy. To date, broadband on-chip EO
combs are typically generated in high-quality-factor micro-resonators, while
the more straightforward and flexible non-resonant method, usually using single
or cascaded EO phase modulators, often requires high driving power to realize a
reasonably strong modulation index. Here, we show that the phase modulation
efficiency of an integrated lithium niobate modulator could be dramatically
enhanced by passing optical signals through the modulation electrodes for a
total of 4 round trips, via multiple low-loss TE0/TE1 mode multiplexers and
waveguide crossings, reducing electrical power consumption by more than one
order of magnitude. Using devices fabricated from a wafer-scale stepper
lithography process, we demonstrate a broadband optical frequency comb
featuring 47 comb lines at a 25-GHz repetition rate, using a moderate RF
driving power of 28 dBm (0.63 W). Leveraging the excellent tunability in
repetition rate and operation wavelength, our power-efficient EO comb generator
could serve as a compact low-cost solution for future high-speed data
transmission, sensing and spectroscopy, as well as classical and quantum
optical computation systems.
