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# High-efficiency and broadband on-chip electro-optic frequency comb generators

## Abstract

Developments in integrated photonics have led to stable, compact and broadband comb generators that support a wide range of applications including communications1, ranging2, spectroscopy3, frequency metrology4, optical computing5,6 and quantum information7,8. Broadband optical frequency combs can be generated in electro-optical cavities, where light passes through a phase modulator multiple times while circulating in an optical resonator9,10,11,12. However, broadband electro-optic frequency combs are currently limited by low conversion efficiencies. Here we demonstrate an integrated electro-optic frequency comb with a conversion efficiency of 30% and an optical span of 132 nm, based on a coupled-resonator platform on thin-film lithium niobate13. We further show that, enabled by the high efficiency, the device acts as an on-chip femtosecond pulse source (336 fs pulse duration), which is important for applications in nonlinear optics, sensing and computing. As an example, in the ultrafast and high-power regime, we demonstrate a frequency comb with simultaneous electro-optic and third-order nonlinearity effects. Our device paves the way for practical optical frequency comb generators and provides a platform to investigate new regimes of optical physics that simultaneously involve multiple nonlinearities.

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## Relevant articles

• ### Mirror-induced reflection in the frequency domain

Nature Communications Open Access 22 October 2022

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## Data availability

The datasets generated and analysed during this study are available from the corresponding authors upon reasonable request.

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## Acknowledgements

We thank C. Wang for helpful discussion. This work is supported by AFOSR FA9550-19-1-0376 (A.S.-A.); AFOSR FA9550-19-1-0310 (A.S.-A. and Y.H.); DARPA LUMOS HR0011-20-C-137 (M.Y., L.S., R.C. and M.L.); NASA 80NSSC21C0583 (M.Y. and R.C.); AFRL FA9550-21-1-0056 (N.S.); NSF ECCS-1839197 (D.Z.); ARO W911NF2010248 (Y.H.); DOE DE-SC0020376 (N.S. and M.L.); Harvard Quantum Initiative (HQI) postdoc fellowship (D.Z.); Maxim Integrated (now Analog Devices) (B.B. and J.M.K.); and Inphi (now Marvell) (B.B. and J.M.K.). N.S. acknowledges support by the AQT Intelligent Quantum Networks and Technologies (INQNET) research program. Device fabrication was performed at the Harvard University Center for Nanoscale Systems. The views, opinions and/or findings expressed are those of the author 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

Y.H. and B.B. conceived the idea. Y.H. developed the theory and fabricated the devices. M.Y. and Y.H. carried out the measurements. B.B. and Y.H. performed the numerical simulations. Y.H. wrote the manuscript with contributions from all authors. N.S., D.Z., R.C., A.S.-A., L.S. and M.Z. helped with the project. M.L. and J.M.K. supervised the project.

### Corresponding authors

Correspondence to Yaowen Hu or Marko Lončar.

## Ethics declarations

### Competing interests

M.Z. and M.L. are involved in developing lithium niobate technologies at HyperLight Corporation. The remaining authors declare no competing interests.

## Peer review

### Peer review information

Nature Photonics thanks Songtao Liu, Xiaoxiao Xue and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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

## Extended data

### Extended Data Fig. 1 Tunability of pump frequency and turning on-off using the heater.

Spectra of the device pumped at 1530 nm (purple trace) and 1603 nm (blue trace and red trace). By tuning the resonances of cavity 1 to match the resonances of cavity 2, the device can be pumped at different wavelengths. The cut-off at wavelengths that are far blue-detuned (purple trace) is due to TE-TM (transverse electric-transverse magnetic) polarization crossing, which affects the TE-designed cavity 2, and can be minimized by additional dispersion engineering35. The comb can also be turned off simply by tuning the resonance of cavity 1 to be mis-aligned from cavity 2. The blue and red traces show the on and off comb states by changing the heater voltages without changing the laser or microwave drive, showing an excellent extinction ratio.

### Extended Data Fig. 2 Minimizing the TE-TM crossing via dispersion engineering.

The cut-off of the comb spectrum around 1400 nm originates from the TE-TM polarization crossing. Due to birefringence of lithium niobate, the TE modes that propagate along the y- and z-direction of the thin-film lithium niobate crystal axes have different indices no,TE and $$n_{e,TE}$$, respectively, while the indices of TM modes are $$n_{o,TM}$$ for both directions. When the TE mode circulates inside the micro-resonator, it experiences different averaged TE indices ranging from ne,TE to $$n_{o,TE}$$ at different bending points of the resonator. As a result, in our current geometry ($$w = 1.2\,\mu m$$, h = 350 nm, and t = 250 nm), the TM mode has an index that is between the value of no,TE and $$n_{e,TE}$$ at wavelengths below ~1450 nm (Left panel), leading to a degeneracy between the TM index and averaged TE indices. This index degeneracy can cause polarization-crossing, which can be pushed toward lower wavelength via dispersion engineering. For example, for a geometry with w = 1.2 μm, h = 350 nm, and t = 150 nm, the range that $$n_{o,TM}$$ is in between the $$n_{o,TE}$$ and $$n_{e,TE}$$ is pushed to ~1250 nm.

### Extended Data Fig. 3 Measurement setup for Figs. 2 and 3.

The coupled-resonator device is characterized using the above setup. In the experiment of Fig. 2b, an EDFA is used to obtain higher pump power. In the experiment of Figs. 2c and 3b, the EDFA is not used. PC, polarization controller; DUT, device under test; EDFA, Erbium-doped fiber amplifier; OSA, optical spectrum analyzer; PD, photodetector.

### Extended Data Fig. 4 Device parameter analysis.

a, b, Transmission spectrum of a single cavity 1 (a) and 2 (b) with the same fabrication parameters as the coupled-resonator device. c, Transmission spectrum of a coupled-resonator device on the through port. d, Transmission spectrum of a coupled-resonator device when microwave is on. (c) and (d) are measured on two different coupled-resonator devices with the same fabrication parameters. The background oscillation is due to the Fabry-Perot resonance formed in the bus waveguide due to the reflection at the two facets of the chip. The extracted parameters give a theoretical conversion efficiency of 28%.

### Extended Data Fig. 5 Frequency spectrum before and after passing the EDFA.

The spectrum shows the frequency comb before (top panel) and after (bottom panel) amplification by the EDFA in the time-domain pulse measurement of Fig. 4b.

### Extended Data Fig. 6 Tuning the dual-pulse in one roundtrip time.

The high conversion-efficiency allows us to measure the signal in the time-domain under varied optical detuning. Unlike dark-pulse Kerr combs or platicons in a coupled-resonator Kerr frequency comb generator20, which exhibits a completely different mechanism in both spectral and temporal domains compared to their single-resonator counterparts, our coupled-resonator structure preserves the time-domain features of the single-resonator EO combs. The output signal shows that there are two pulses in one round-trip time and the delay between the two pulses can be tuned by changing the optical detuning, which is identical to the conventional single-resonator EO comb.

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Hu, Y., Yu, M., Buscaino, B. et al. High-efficiency and broadband on-chip electro-optic frequency comb generators. Nat. Photon. 16, 679–685 (2022). https://doi.org/10.1038/s41566-022-01059-y

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• DOI: https://doi.org/10.1038/s41566-022-01059-y

• ### Mirror-induced reflection in the frequency domain

• Yaowen Hu
• Mengjie Yu
• Marko Lončar

Nature Communications (2022)