  Ring quantum cascade lasers have recently gained considerable attention,
showing ultrastable frequency comb and soliton operation, thus opening a way to
integrated spectrometers in the midinfrared and terahertz fingerprint regions.
Thanks to a self-consistent Maxwell-Bloch model, we demonstrate, in excellent
agreement with the experimental data, that a small but finite coupling between
the counterpropagating waves arising from distributed backscattering is
essential to stabilize the soliton solution.
