Actively Mode-Locked Monolithic Semiconductor Laser with a Tunable Repetition Frequency

In conventional active mode locking [1], a periodic gain/loss or phase modulation can produce a coherent train of optical pulses, provided that it is synchronized to the natural laser repetition rate to within a few percent of frequency detuning. Here, we present a novel regime, where the repetition frequency of a mode-locked semiconductor laser can be tuned continuously and significantly, both above and below the repetition rate imposed by the cavity dimensions. This is achieved by inducing a strong spatiotemporal gain/loss modulation along the entire laser cavity, with the injected microwaves generating a frequency-dependent modulation standing wave. In Fig. 1(a), we display the optical pulse (blue) and modulation wave (red) in the co-propagating reference frame. For high modulation frequencies (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{f}_{\text {mod }}&gt;\mathrm{f}_{\text {rep }, 0}$</tex>, top), there is a positive delay between the peak of the optical and the modulation wave, meaning that the leading edge of the optical pulse is amplified and its trailing edge is attenuated. This pulse pulling effect increases the group velocity of the optical pulse, while the opposite occurs for low modulation frequencies (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{f}_{\text{mod}}&lt;\mathrm{f}_{\text{rep}, 0}$</tex>, bottom), where the group velocity is reduced. This way, for any arbitrary modulation frequency, the repetition rate of the periodic intracavity waveform is adjusted to be synchronized to the external modulation frequency, producing a continuously tunable frequency comb.