  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 significant limitations. Here, we show that the
advantages of each platform can be combined. We introduce a novel kind of
soliton, called 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.
