Quantum cascade laser (QCL) frequency combs are a promising candidate for chemical sensing and biomedical diagnostics, requiring only milliseconds of acquisition time to record absorption spectra without any moving parts 1,2,3,4 . They are electrically pumped and have a small footprint, making them an ideal platform for on-chip integration 5 . Until now, optical feedback is fatal for frequency comb generation in QCLs and destroys intermodal coherence 6 . This property imposes strict limits on the possible degree of integration. Here, we demonstrate coherent injection locking of the repetition frequency to a stabilized RF oscillator. For the first time, we prove that the spectrum of the injection locked QCL can be phase-locked, resulting in the generation of a frequency comb. We show that injection locking is not only a versatile tool for all-electrical frequency stabilization, but also mitigates the fatal effect of optical feedback on the frequency comb. A prototype self-detected dualcomb setup consisting only of an injection locked dualcomb chip, a lens and a mirror demonstrates the enormous potential for on-chip dual-comb spectroscopy. These results pave the way to miniaturized and all-solid-state midinfrared spectrometers.

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Coupled clocks are a classic example of a synchronization system leading to periodic collective oscillations. This phenomenon already attracted the attention of Christian Huygens back in 1665, who described it as a kind of "sympathy" among oscillators. In this work we describe the formation of two types of laser frequency combs as a system of oscillators coupled through the beating of the lasing modes. We experimentally show two completely different types of synchronizations in a quantum dot laser -in-phase and splay states. Both states can be generated in the same device, just by varying the damping losses of the system. This effectively modifies the coupling among the oscillators. The temporal output of the laser is characterized using both linear and quadratic autocorrelation techniques. Our results show that both pulses and frequency-modulated states can be generated on demand. These findings allow to connect laser frequency combs produced by amplitude-modulated and frequency-modulated lasers, and link these to pattern formation in coupled systems such as Josephson-junction arrays.