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 clocks¹, microwave photonics², spectroscopy³, optical wave synthesis⁴, frequency conversion⁵, communications⁶, lidar⁷, optical computing⁸ and astronomy⁹. The leading approaches for on-chip pulse generation rely on mode-locking inside microresonators with either third-order nonlinearity¹⁰ 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 platform¹⁸, using cascaded low-loss electro-optic amplitude and phase modulators and chirped Bragg grating, forming a time-lens system¹⁹. 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.