Integrated femtosecond pulse generator on thin-film lithium niobate.

Integrated femtosecond pulse and frequency comb sources are critical components for a wide range of applications, including optical atomic clocks 1 , microwave photonics 2 , spectroscopy 3 , optical wave synthesis 4 , frequency conversion 5 , communications 6 , lidar 7 , optical computing 8 and astronomy 9 . The leading approaches for on-chip pulse generation rely on mode-locking inside microresonators with either third-order nonlinearity 10 or with semiconductor gain 11,12 . These approaches, however, are limited in noise performance, wavelength and repetition rate tunability  10,13 . Alternatively, subpicosecond pulses can be synthesized without mode-locking, by modulating a continuous-wave single-frequency laser using electro-optic modulators 1,14-17 . Here we demonstrate a chip-scale femtosecond pulse source implemented on an integrated lithium niobate photonic platform 18 , using cascaded low-loss electro-optic amplitude and phase modulators and chirped Bragg grating, forming a time-lens system 19 . The device is driven by a continuous-wave distributed feedback laser chip and controlled by a single continuous-wave microwave source without the need for any stabilization or locking. We measure femtosecond pulse trains (520-femtosecond duration) with a 30-gigahertz repetition rate, flat-top optical spectra with a 10-decibel optical bandwidth of 12.6 nanometres, individual comb-line powers above 0.1 milliwatts, and pulse energies of 0.54 picojoules. Our results represent a tunable, robust and low-cost integrated pulsed light source with continuous-wave-to-pulse conversion efficiencies an order of magnitude higher than those achieved with previous integrated sources. Our pulse generator may find applications in fields such as ultrafast optical measurement 19,20 or networks of distributed quantum computers 21,22 .