Harnessing Fick's distribution in thermally diffused polycrystals for in-situ optimization of laser efficiency

Transition-metal (TM) doped II-VI ceramics like Cr²⁺:ZnS/ZnSe have garnered significant attention as mid-infrared (MIR) laser gain media due to their broad emission spectrum and power scalability. Here, we report the doping profile engineering for laser ceramics via an empirical revision of Fick's diffusion model for Cr:ZnS, and its application to in-situ laser performance optimization. Using a 30% output coupler, the system achieved a maximum laser output power of 5.8 W and a maximum slope efficiency of 53.5%, reaching 81% of the intrinsic Stokes limit. The output power is further amplified to nearly 10 W with an optimized trade-off between gain and thermal management, as previously noted, which again enabled the ion concentration selection in a single-pass master oscillator power amplifier (MOPA) configuration. This approach is broadly applicable to thermally diffused gain media, paving the way for advancing high-efficiency and high-performance MIR laser sources for both fundamental research and industrial/medical applications.