  Optical frequency comb underpins a wide range of applications from
communication, metrology, to sensing. Its development on a chip-scale platform
-- so called soliton microcomb -- provides a promising path towards system
miniaturization and functionality integration via photonic integrated circuit
(PIC) technology. Although extensively explored in recent years, challenges
remain in key aspects of microcomb such as complex soliton initialization, high
threshold, low power efficiency, and limited comb reconfigurability. Here we
present an on-chip laser that directly outputs microcomb and resolves all these
challenges, with a distinctive mechanism created from synergetic interaction
among resonant electro-optic effect, optical Kerr effect, and optical gain
inside the laser cavity. Realized with integration between a III-V gain chip
and a thin-film lithium niobate (TFLN) PIC, the laser is able to directly emit
mode-locked microcomb on demand with robust turnkey operation inherently built
in, with individual comb linewidth down to 600 Hz, whole-comb frequency tuning
rate exceeding $\rm 2.4\times10^{17}$ Hz/s, and 100% utilization of optical
power fully contributing to comb generation. The demonstrated approach unifies
architecture and operation simplicity, high-speed reconfigurability, and
multifunctional capability enabled by TFLN PIC, opening up a great avenue
towards on-demand generation of mode-locked microcomb that is expected to have
profound impact on broad applications.
