  Dual-comb spectroscopy has emerged as an indispensable analytical technique
in applications that require high resolution and broadband coverage within
short acquisition times. Its experimental realization, however, remains
hampered by intricate experimental setups with large power consumption. Here,
we demonstrate an ultra-simple free-running dual-comb spectrometer realized in
a single all-fiber cavity suitable for the most demanding Doppler-limited
measurements. Our dual-comb laser utilizes just a few basic fiber components,
allows to tailor the repetition rate difference, and requires only 350 mW of
electrical power for sustained operation over a dozen of hours. As a
demonstration, we measure low-pressure hydrogen cyanide within 1.7 THz
bandwidth, and obtain better than 1% precision over a terahertz in 200 ms
enabled by a drastically simplified all-computational phase correction
algorithm. The combination of the unprecedented setup simplicity, comb tooth
resolution and high spectroscopic precision paves the way for proliferation of
frequency comb spectroscopy even outside the laboratory.
