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# Integrated femtosecond pulse generator on thin-film lithium niobate

## Abstract

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 gain11,12. 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 modulators1,14,15,16,17. 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 measurement19,20 or networks of distributed quantum computers21,22.

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## Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

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## Acknowledgements

This work is supported by the Defense Advanced Research Projects Agency (DARPA) under contract nos HR0011-20-C-0137, ARO W911NF2010248, ONR N00014-18-C-1043 and AFOSR (FA9550-19-1-0376 and FA9550-20-1-0297). Device fabrication was performed at the Harvard University Center for Nanoscale Systems. D.Z. acknowledges support by the Harvard Quantum Initiative post-doctoral fellowship. The research D.B. performed was supported by an appointment to the Intelligence Community Postdoctoral Research Fellowship Program at Harvard University, administered by Oak Ridge Institute for Science and Education through an interagency agreement between the US Department of Energy and the Office of the Director of National Intelligence. The views, opinions and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the US Government.

## Author information

Authors

### Contributions

M.Y. and Y.O. conceived the idea. M.Y. designed the chip with the help of D.B., C.R., R.C., P.K., L.H. and M.Z. D.B., C.R., R.C. and L.H. fabricated the devices. D.B. fabricated the grating device with the help of R.C. M.Y. carried out the measurement and analysed the data with the help of P.K., D.B., R.C., D.Z. and Y.O. M.Y. performed numerical simulations with the help of Y.O. H.R.G. and L.J. provided the DFB laser. L.S. and Y.H. helped with the fabrication. M.Y. wrote the manuscript with contribution from all authors. A.L.G. and M.L. supervised the project.

### Corresponding author

Correspondence to Marko Lončar.

## Ethics declarations

### Competing interests

C.R., P.K., L.H., M.Z. and M.L. are involved in developing lithium niobate technologies at HyperLight Corporation.

## Peer review

### Peer review information

Nature thanks Kenneth Kin-Yip Wong and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data figures and tables

### Extended Data Fig. 1 The electro-optic response of the amplitude modulator on the time-lens chip.

The response is measured and calculated based on the S21 of the VNA and the calibration of the fast photodetector and radio-frequency (RF) cables, when the AM is biased at the quadrature point and under small signal modulation (top). The electro-optic S21 response of the AM indicates a 3-dB bandwidth of >45 GHz (bottom). Since the impedance mismatch results in a shape fall off in the response at near d.c. frequencies, the electro-optic S21 is often referenced to an RF frequency (2 GHz in our case). MZI: the Mach–Zehnder interferometer; AM: amplitude modulator; PM: phase modulator; EDFA: erbium-doped fibre amplifier; PD: photodetector; VNA: vector network analyser.

### Extended Data Fig. 2 Characterization of the half-wave voltage (Vπ) of the ‘recycling’ phase modulator.

a, The experimental set-up for characterizing Vπ. The optical spectra are recorded as we sweep the RF frequency from 2 to 40 GHz at a step of 50 MHz. Vπ is extracted based on the pump power depletion in the optical spectrum, which is determined by the magnitude squared of the zeroth-order Bessel function |J0(β)|2, where β = πV/Vπ and V is the driven voltage. b, The extracted Vπ as a function of the RF frequency. c, The Vπ around 30.1 GHz, which shows the lowest Vπ of 2.5 V at 30.1 GHz. The 3-dB power bandwidth, corresponding to $$\sqrt{2}\,{V}_{{\rm{\pi }}}$$, is 1.4 GHz (29.4 to 30.8 GHz). d, The frequency period of the Vπ (free spectral range = 2.84 GHz). It is determined by 1/τ, where τ is the optical delay time from the first ground-signal (GS) pair to the second GS pair in the phase modulator. e, The Vπ at each resonant RF frequency. In the experiment, we operate at 10.075 and 30.135 GHz, which correspond to 2.2 and 2.5 V, respectively. The results are in good agreement with the theory, which is based on the measured RF properties (index, loss, impedance) and optical group refractive index. OSA, optical spectral analyser.

### Extended Data Fig. 3 Numerical simulation of the optical spectrum and the temporal pulses of the time-lens output.

The optical spectrum (left) shows a 10-dB optical bandwidth of 12.5 nm, and the temporal pulse shows a FWHM of 526 fs after compression (right), which is in excellent agreement with the experimental results in Fig. 2d at 30.135 GHz. In the simulation, the dispersion medium used is the single-mode fibre (SMF-28) with a length of 59 m. The temporal-domain behaviour shows a pulse compression factor of 31.5 before and after the dispersion operator. a.u., arbitrary units.

### Extended Data Fig. 4 Autocorrelator trace of wavelength multiplexed pulse sources.

The experimental trace is obtained from the intensity autocorrelator when two pulse trains are simultaneously synthesized using two continuous-wave lasers at wavelengths of 1,543.75 and 1,556.83 nm, corresponding to Fig. 3a.

### Extended Data Fig. 5 Four-wave mixing in HNLF using the pulse train as the pump and a continuous-wave laser as the probe.

ae, Five optical spectra are recorded when the time-lens pulse train at 1,549 nm is sent to the HNLF along with a continuous-wave (CW) laser at wavelengths of 1,600, 1,620, 1,640, 1,660, and 1,680 nm (ae), respectively. In addition to the nonlinear broadening around the pump wavelength, mini-comb spectra are generated around the wavelength of the continuous-wave laser due to the cross-phase modulation (XPM) from the high-peak-power pulses. Four-wave mixing (FWM) between the pulse pump and the continuous-wave laser probes results in broad comb spectra at the idler wavelength on the lower wavelength side of the pump. The broadest conversion bandwidth is around 40 nm when the continuous-wave laser is at 1,640 nm.

### Extended Data Fig. 6 Stability measurement of the 30-GHz electro-optic comb spectrum over 3.5 h at a time step of 30 s.

Both power and the optical spectrum are recorded. a, The output comb power (top) and the standard deviation of the central 20 comb lines’ power (bottom) as a function of time. The output comb power features a periodic variation every 20 min (dips) due to the slow and periodic mechanical misalignment between the lensed fibre and chip facets. The stable operation of the optical spectrum on chip is verified by excellent spectral flatness during this time period as well as the flat-top optical spectra (b) at each randomly sampled point labelled in a.

### Extended Data Fig. 7 Characterization of the on-chip chirped Bragg grating.

a, Experimental set-up. The device is measured using a tunable continuous-wave laser and a circulator. b,c, The reflection and transmission spectrum of a 2.5-mm Bragg grating. The grating 3-dB bandwidth is 1,544–1,573 nm, which covers the optical bandwidth of the time lens output in Fig. 2 (shaded). The transmission within the bandwidth is limited by our detector noise floor. d, The optical spectrum of the time lens after reflection from the grating. The spectrum in Fig. 2 was used as an input to the grating. e, The extracted fringe period from b and the group delay as a function of wavelength. The fringe is a result of the Fabry–Pérot cavity effect due to the reflection between input facet and grating. The slope of the group delay indicates a total dispersion value of 1.62 ps nm−1. f, The insertion loss as a function of wavelength, extracted from the reflection spectrum and laser input power spectrum. Since the wavelength can be mapped to the physical propagation distance in the grating device, the propagation loss is fitted to be 0.033 dB mm−1.

## Supplementary information

### Supplementary Information

The file contains information of the supplementary methods, including the design and characterization of the waveguide crossing (Supplementary Fig. 1) and inverse taper (Supplementary Fig. 2) in the time-lens chip, the time-lens pump spectrum for the four-wave-mixing experiment in a highly nonlinear fibre (Supplementary Fig. 3), and the characterization of the dispersion-engineered waveguide for pulse compression (Supplementary Figs. 4 and 5). There are five Supplementary figures in total.

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Yu, M., Barton III, D., Cheng, R. et al. Integrated femtosecond pulse generator on thin-film lithium niobate. Nature (2022). https://doi.org/10.1038/s41586-022-05345-1

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• DOI: https://doi.org/10.1038/s41586-022-05345-1