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Self-starting harmonic frequency comb generation in a quantum cascade laser

Nature Photonics volume 11, pages 789–792 (2017)Cite this article

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Abstract

Optical frequency combs1,2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications3. Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis4,5 and for the generation of terahertz tones of high spectral purity in future wireless communication networks6,7. Here, we demonstrate self-starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser8,9. Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be uniform to within 5 × 10−12 parts of the central frequency. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs10,11,12,13.

In recent decades, several techniques to generate optical frequency combs (OFCs) have been demonstrated based on different nonlinear mechanisms that fulfil the mode-locking condition. Originally, passively mode-locked lasers based on saturable absorption and Kerr lensing were used to create short light pulses, and were subsequently shown to also constitute frequency combs. This type of mode locking is an example of amplitude-modulated mode locking, so-named for the temporal behaviour of the electric field of the emitted light. However, these techniques usually result in elaborate optical systems. More recently, new routes promising chip-scale comb generators have been investigated based on optically pumped ultrahigh-quality-factor crystalline microresonators14,15,16 and broadband quantum cascade lasers (QCLs) with specially designed multistage active regions10,17. In both cases, the essential underlying mechanism responsible for the generation of OFCs is cascaded four-wave mixing (FWM) enabled by a third-order χ (3) Kerr nonlinearity. The temporal behaviour of these OFCs is not restricted to ultrashort pulses, but can represent rather sophisticated waveforms due to a non-trivial relationship among the spectral phases of the comb teeth. In fact, the output of a QCL-based frequency comb resembles that of a frequency-modulated laser with nearly constant output intensity10,18.

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Fig. 1: Harmonic and fundamental comb operation in QCLs.
Fig. 2: Spectral evolution of FP QCLs.
Fig. 3: Mode spacing uniformity of the harmonic state.
Fig. 4: Harmonic comb operation in presence of native dispersion.

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Acknowledgements

This work was supported by the DARPA SCOUT programme through grant no. W31P4Q-16-1-0002. The authors acknowledge support from the National Science Foundation under award no. ECCS-1614631. Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Assistant Secretary of Defense for Research and Engineering or of the National Science Foundation. M.P. and D.K. thank J.B. MacArthur for the assembly of RF filters, A.Y. Zhu for sputtering gold on a QCL submount, and N. Rubin for careful reading of the manuscript.

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  1. Dmitry Kazakov and Marco Piccardo contributed equally to this work.

Authors and Affiliations

  1. Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, 02138, USA

    Dmitry Kazakov, Marco Piccardo, Paul Chevalier & Federico Capasso

  2. Department of Information Technology and Electrical Engineering, ETH Zurich, 8092, Zurich, Switzerland

    Dmitry Kazakov

  3. Department of Physics and Astronomy, Texas A&M University, College Station, TX, 77843, USA

    Yongrui Wang & Alexey Belyanin

  4. Pendar Technologies, 30 Spinelli Place, Cambridge, MA, 02138, USA

    Tobias S. Mansuripur

  5. Thorlabs Quantum Electronics (TQE), Jessup, MD, 20794, USA

    Feng Xie, Chung-en Zah & Kevin Lascola

Authors
  1. Dmitry Kazakov
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Contributions

D.K. and M.P. conceived, designed and implemented the experiments and wrote the manuscript with feedback from the other co-authors. P.C. contributed to the realization of the experiments. F.X., C.Z. and K.L. provided the QCL devices. Y.W. and A.B. developed the theoretical model and contributed to the theoretical part of the manuscript. T.S.M., P.C., A.B., M.P., D.K. and F.C. discussed the data. All work was done under the supervision of F.C.

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Correspondence to Marco Piccardo or Federico Capasso.

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Kazakov, D., Piccardo, M., Wang, Y. et al. Self-starting harmonic frequency comb generation in a quantum cascade laser. Nature Photon 11, 789–792 (2017). https://doi.org/10.1038/s41566-017-0026-y

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