Theoretical investigation of Mirrorless Lasing in Continuous Wave-driven Alkali Vapors*

We present a theoretical investigation of mirrorless lasing in alkali atom vapors coherently pumped by continuous-wave beams, utilizing two-photon transitions to achieve directional steady-state gain in infrared transitions. Our approach employs a two-photon pumping scheme of the nS_1/2 to nD_5/2 transition to induce population inversion between intermediate levels, leading to the generation of directional gain along the beams’ axis. We analyzed the conditions required for steady-state population inversion using the Lindblad master equation formalism and developed a finite-difference time-domain (FDTD) scheme to simulate the coupled light-atom dynamics in a warm vapor cell.

 A key focus of this work is understanding the directional characteristics of the emitted fields and the transition from incoherent amplified spontaneous emission (ASE) to coherent mirrorless lasing. We demonstrate how our approach accurately models both regimes, even in the absence of a predefined mode structure typical of cavity-based lasing systems. Our results enable the generation of coherent backward-propagating signals in thermal alkali vapors, establishing a foundation for experimental implementation and advancements in remote sensing technologies, including improved signal-to-noise ratios.