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 x 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).