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


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.

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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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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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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).

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