  Integrated femtosecond pulse and frequency comb sources are critical
components for a wide range of applications. The leading approaches for on-chip
pulse generation rely on mode locking inside microresonator with either
third-order nonlinearity or with semiconductor gain. These approaches, however,
are limited in noise performance, wavelength tunability and repetition rates.
Alternatively, sub-picosecond pulses can be synthesized without mode-locking,
by modulating a continuous-wave (CW) single-frequency laser using a cascade of
electro-optic (EO) modulators. This method is particularly attractive due to
its simplicity, robustness, and frequency-agility but has been realized only on
a tabletop using multiple discrete EO modulators and requiring optical
amplifiers (to overcome large insertion losses), microwave amplifiers, and
phase shifters. Here we demonstrate a chip-scale femtosecond pulse source
implemented on an integrated lithium niobate (LN) photonic platform18, using
cascaded low-loss electro-optic amplitude and phase modulators and chirped
Bragg grating, forming a time-lens system. The device is driven by a CW
distributed feedback (DFB) chip laser and controlled by a single CW microwave
source without the need for any stabilization or locking. We measure
femtosecond pulse trains (520 fs duration) with a 30-GHz repetition rate,
flat-top optical spectra with a 10-dB optical bandwidth of 12.6 nm, individual
comb-line powers above 0.1 milliwatt, and pulse energies of 0.54 picojoule. Our
results represent a tunable, robust and low-cost integrated pulsed light source
with CW-to-pulse conversion efficiencies an order of magnitude higher than
achieved with previous integrated sources. Our pulse generator can find
applications from ultrafast optical measurement to networks of distributed
quantum computers.
