  Fourier-transform spectroscopy is an indispensable tool for analyzing
chemical samples in scientific research as well as chemical and pharmaceutical
industries. Recently, its measurement speed, sensitivity, and precision have
been shown to be significantly enhanced by using dual frequency combs. However,
wide acceptance of this technique is hindered by its requirement for two
frequency combs and active stabilization of the combs. Here we overcome this
predicament with a Kerr-lens mode-locked bidirectional ring laser that
generates two frequency combs with slightly different pulse repetition rates
and a tunable yet highly stable rate difference. This peculiar lasing principle
builds on a slight difference in optical cavity length between two
counter-propagating lasing modes due to Kerr lensing. Since these combs are
produced by the one and same laser cavity, their relative coherence stays
passively stable without the need for active stabilization. To show its
utility, we demonstrate broadband dual-comb spectroscopy with the single laser.
