In a laser, the control of its spectral emission depends on the physical dimensions of the optical resonator, restricting it to a set of discrete cavity modes at specific frequencies1–4. Without modifying the optical cavity, this results in substantial gaps in the obtainable laser emission spectrum, as well as a fixed repetition rate, limiting the device’s usability in various experiments and applications where a considerable degree of tunability is required in the spectral or temporal domain. Here we overcome this fundamental limit by demonstrating a monolithic semiconductor laser5–7 with a continuously tunable repetition rate from 4 GHz up to 16 GHz, by using a microwave driving signal that induces a spatiotemporal gain modulation along the entire laser cavity8,9, generating intracavity mode-locked pulses10–13 with a continuously tunable group velocity14. At the output, frequency combs15,16 with continuously tunable mode spacings are generated in the frequency domain, and coherent pulse trains with continuously tunable repetition rates are generated in the time domain17. Our results pave the way for fully tunable chip-scale lasers and frequency combs, which will be advantageous for use in a diverse variety of fields, from fundamental studies to applications such as high-resolution and dual-comb spectroscopy18,19.
