Letter | Published:

Broadband electro-optic frequency comb generation in a lithium niobate microring resonator

Nature (2019) | Download Citation

Abstract

Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communication, precision metrology, timing and spectroscopy1,2,3,4,5,6,7,8,9. At present, combs with wide spectra are usually generated by mode-locked lasers10 or dispersion-engineered resonators with third-order Kerr nonlinearity11. An alternative method of comb production uses electro-optic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability12,13,14. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. Here we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large EO response, ultralow optical loss and highly co-localized microwave and optical fields15, while enabling dispersion engineering. Our measured EO comb spans more frequencies than the entire telecommunications L-band (over 900 comb lines spaced about 10 gigahertz apart), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 hertz to 100 megahertz), and we use this feature to generate dual-frequency combs in a single resonator. Our results show that integrated EO comb generators are capable of generating wide and stable comb spectra. Their excellent reconfigurability is a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy16 to optical communications8.

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

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Acknowledgements

This work is supported by the National Science Foundation (award numbers ECCS-1609549, ECCS-1740291 E2CDA, ECCS-1740296 E2CDA and DMR-1231319); the Harvard University Office of Technology Development (Physical Sciences and Engineering Accelerator Award); and Facebook, Inc. Device fabrication is performed at the Harvard University Center for Nanoscale Systems, a member of the National Nanotechnology Coordinated Infrastructure Network, which is supported by the National Science Foundation (award number ECCS-1541959).

Reviewer information

Nature thanks Yanne K. Chembo, Paulina Kuo and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Author notes

  1. These authors contributed equally: Mian Zhang, Brandon Buscaino, Cheng Wang

Affiliations

  1. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA

    • Mian Zhang
    • , Cheng Wang
    • , Amirhassan Shams-Ansari
    • , Christian Reimer
    • , Rongrong Zhu
    •  & Marko Lončar
  2. HyperLight Corporation, Cambridge, MA, USA

    • Mian Zhang
    •  & Christian Reimer
  3. Edward L. Ginzton Laboratory, Department of Electrical Engineering, Stanford University, Stanford, CA, USA

    • Brandon Buscaino
    •  & Joseph M. Kahn
  4. Department of Electronic Engineering and State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Hong Kong, China

    • Cheng Wang
  5. Department of Electrical Engineering and Computer Science, Howard University, Washington, DC, USA

    • Amirhassan Shams-Ansari
  6. The Electromagnetics Academy at Zhejiang University, College of Information Science and Electronic Engineering, Zhejiang University, Hangzhou, China

    • Rongrong Zhu

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Contributions

M.Z., B.B., C.W., J.M.K. and M.L. conceived the experiment. M.Z., C.W. and A.S.-A. designed and fabricated the devices. B.B. performed theoretical modelling and numerical simulations of the integrated EO comb. M.Z. and R.Z. designed waveguide dispersion. M.Z., C.W., A.S.-A. and C.R. carried out the device characterization. M.Z. and B.B. performed the data analysis. M.Z. and B.B. wrote the manuscript with contributions from all authors. J.M.K. and M.L. supervised the project.

Competing interests

The authors declare competing interests: M.Z., C.W., C.R. and M.L. are involved in developing lithium niobate technologies at HyperLight Corporation.

Corresponding authors

Correspondence to Joseph M. Kahn or Marko Lončar.

Extended data figures and tables

  1. Extended Data Fig. 1 Dispersion engineering and tailored EO comb phase-matching condition.

    a, Simulated dispersion for an air-clad lithium niobate ridge waveguide with top width w = 1,800 nm, film thickness t = 600 nm and etch depth h = 550 nm. b, The phase-matching condition for generating EO comb sidebands. The grey area shows the region of phase matching, with round-trip modulation strength β. With a 50-GHz microwave drive and β = 1.2π, an EO comb spanning 1.3 octaves can be generated. c, Simulated group velocity dispersion (GVD) for an air-clad lithium niobate waveguide with a different geometry optimized for an octave-spanning comb with small microwave driving amplitude. d, An octave-spanning EO comb is shown with β = 0.3π. e, Another example of an air-clad lithium niobate waveguide with dispersion engineered for broad comb generation. f, Such EO comb generation features a flat response over 600 nm. A broad comb spanning less than an octave can be generated in devices with small microwave modulation amplitudes and high-Q optical resonators.

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DOI

https://doi.org/10.1038/s41586-019-1008-7

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