Lasing in the bad-cavity regime has promising applications in quantum 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 weak optical feedback. The laser power can be as high as tens of μW 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 future discoveries in optical clocks, cavity QED, continuous-wave superradiant laser, and explorations of quantum many-body physics.

由于激光频率对腔长波动的敏感性降低，坏腔状态下的激光在量子精密测量和频率计量中具有广阔的应用前景。迄今为止，相关研究主要集中在传统腔体上，其精度足够高，使得谐振线宽与腔体的自由光谱范围相比足够窄，但仍处于坏腔状态。然而，精细度接近2极限的腔体的激光输出从未通过实验获得。在这里，我们展示了一种极其糟糕的腔激光器，分析了限制腔精度的物理机制，并报告了有史以来最差的激光腔，其精度达到2.01。光学腔的反射率接近于零，仅提供微弱的光学反馈。激光功率可高达数十μW ，光谱线宽达到几kHz，比增益带宽窄一千倍以上。此外，腔体牵引力的测量显示牵引系数为 0.0148，这是连续波激光器有史以来达到的最低值。我们的研究结果为未来新型超稳定激光器开辟了前所未有的创新视角，这可能会引发光学钟、腔 QED、连续波超辐射激光器以及量子多体物理探索的未来发现。