Temporal solitons in a coherently driven active resonator

Optical frequency combs are lightwaves composed of a large number of equidistant spectral lines. They are important for metrology, spectroscopy, communications and fundamental science. Frequency combs are most often generated by exciting dissipative solitons in lasers or in passive resonators, both of which suffer from important limitations. Here we show that the advantages of each platform can be combined. We introduce a novel kind of soliton (called an active cavity soliton) hosted in coherently driven lasers pumped below the lasing threshold. We use an active fibre resonator and measure high-peak-power solitons on a low-power background, in excellent agreement with simulations of a generalized Lugiato–Lefever equation. Moreover, we find that amplified spontaneous emission has negligible impact on the soliton’s stability. Our results open up novel avenues for frequency comb formation by showing that coherent driving and incoherent pumping can be efficiently combined to generate a high-power ultra-stable pulse train. By adding a carefully designed amplification section in a passive resonator, but pumping it below the lasing threshold, ultra-stable high-power cavity solitons can be formed, effectively removing the important barrier of having to work in low-loss environments.