Linear‐Wave Radiation and Intra‐Cavity Spectral Reshaping Induced by Gas‐Molecule Absorption in Ultrafast Mid‐Infrared Laser

Mid‐infrared mode‐locked lasers, operating in finger‐print region of various gas molecules, are of great interest in fields of laser spectroscopy, remote sensing, and frequency metrology. The absorption strengths of gas molecules at mid‐infrared wavelengths are orders of magnitude higher than their overtones in the near‐infrared or visible region, providing higher detection sensitivity and longer sensing range. Meanwhile, the strong molecule (H 2 O and CO 2, for example) absorption, especially at atmospheric environment, is widely realized as a deterioration factor for the stable operation of mid‐infrared lasers, being a critical bottleneck for laser‐performance improvement. However, the mechanism of some phenomena induced by strong absorption in the mid‐infrared laser are not be comprehensively investigated so far. Here, we study both theoretically and experimentally the phenomenon of linear‐wave radiation due to strong intra‐cavity molecule absorption, and find that the nonlinear interplay between the radiated linear wave and the laser pulse, over co‐propagating inside the laser cavity, can result in spectral spiking and reshaping effects of the laser output. Our findings shed some light on the self‐consistency dynamics of ultrafast laser pulses at strong‐absorption conditions, and the demonstrated laser system can be potentially applied for nonlinear spectroscopy and gas detection with improved sensitivity and broadband coverage.