  Lasing in the bad-cavity regime has promising applications in precision
measurement and frequency metrology due to the reduced sensitivity of the laser
frequency to cavity length fluctuations. Thus far, relevant studies have been
mainly focused on conventional cavities whose finesse is high enough that the
resonance linewidth is sufficiently narrow compared to the cavity's free
spectral range, though still in the bad-cavity regime. However, lasing output
from the cavity whose finesse is close to the limit of 2 has never been
experimentally accessed. Here, we demonstrate an extremely bad-cavity laser,
analyze the physical mechanisms limiting cavity finesse, and report on the
worst ever laser cavity with finesse reaching 2.01. The optical cavity has a
reflectance close to zero and only provides a weak optical feedback. The laser
power can be as high as tens of $\mu$W and the spectral linewidth reaches a few
kHz, over one thousand times narrower than the gain bandwidth. In addition, the
measurement of cavity pulling reveals a pulling coefficient of 0.0148, the
lowest value ever achieved for a continuous wave laser. Our findings open up an
unprecedentedly innovative perspective for future new ultra-stable lasers,
which could possibly trigger the future discoveries in optical clocks, cavity
QED, continuous wave superradiant laser, and explorations of quantum manybody
physics.
