Laser ignition (LI) allows for precise manipulation of ignition timing and location and is promising for green combustion of automobile and rocket engines and aero-turbines under lean-fuel conditions with improved emission efficiency; however, achieving completely effective and reliable ignition is still a challenge. Here, we report the realization of igniting a lean methane/air mixture with a 100% success rate by an ultrashort femtosecond laser, which has long been regarded as an unsuitable fuel ignition source. We demonstrate that the minimum ignition energy can decrease to the sub-mJ level depending on the laser filamentation formation, and reveal that the resultant early OH radical yield significantly increases as the laser energy reaches the ignition threshold, showing a clear boundary for misfire and fire cases. Potential mechanisms for robust ultrashort LI are the filamentation-induced heating effect followed by exothermal chemical reactions, in combination with the line ignition effect along the filament. Our results pave the way toward robust and efficient ignition of lean-fuel engines by ultrashort-pulsed lasers.

激光点火（LI）可以精确控制点火时间和位置，并有望在贫油条件下实现汽车和火箭发动机以及航空涡轮的绿色燃烧，并提高排放效率；然而，实现完全有效和可靠的点火仍然是一个挑战。在这里，我们报告了通过超短飞秒激光点燃稀薄的甲烷/空气混合物的成功率达100％的实现，该飞秒激光长期以来一直被认为是不合适的燃料点火源。我们证明了最小点火能量可以降低到亚mJ水平，这取决于激光丝的形成，并揭示了当激光能量达到点火阈值时，产生的早期OH自由基产率显着增加，显示了着火和失火的明确边界案件。强大的超短LI的潜在机制是细丝诱导的加热效应，然后是放热化学反应，再加上沿细丝的线点火效应。我们的研究结果为通过超短脉冲激光稳健而高效地点燃稀薄燃料发动机铺平了道路。