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

Optical frequency combs 1 , 2 establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications 3 . Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis 4 , 5 and for the generation of terahertz tones of high spectral purity in future wireless communication networks 6 , 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 laser 8 , 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 combs 10 – 13 . Self-starting harmonic frequency comb generation with a THz repetition rate in a quantum cascade laser is demonstrated. The mode spacing uniformity is verified to within 5 × 10 −12 parts of the central frequency. The findings extend the range of applications of quantum cascade laser frequency combs.