  Mode-locked lasers (MLLs) have enabled ultrafast sciences and technologies by
generating ultrashort pulses with peak powers substantially exceeding their
average powers. Recently, tremendous efforts have been focused on realizing
integrated MLLs not only to address the challenges associated with their size
and power demand, but also to enable transforming the ultrafast technologies
into nanophotonic chips, and ultimately to unlock their potential for a
plethora of applications. However, till now the prospect of integrated MLLs
driving ultrafast nanophotonic circuits has remained elusive because of their
typically low peak powers, lack of controllability, and challenges with
integration with appropriate nanophotonic platforms. Here, we overcome these
limitations by demonstrating an electrically-pumped actively MLL in
nanophotonic lithium niobate based on its hybrid integration with a III-V
semiconductor optical amplifier. Our MLL generates $\sim$4.8 ps optical pulses
around 1065 nm at a repetition rate of $\sim$10 GHz, with pulse energy
exceeding 2.6 pJ and a high peak power beyond 0.5 W. We show that both the
repetition rate and the carrier-envelope-offset of the resulting frequency comb
can be flexibly controlled in a wide range using the RF driving frequency and
the pump current, paving the way for fully-stabilized on-chip frequency combs
in nanophotonics. Our work marks an important step toward fully-integrated
nonlinear and ultrafast photonic systems in nanophotonic lithium niobate.
