Robust volcano-shaped gold-coated holographic gratings for dual-grating spectral beam combining of mid-infrared OPO lasers.

Power scaling of mid-infrared (MIR) lasers through spectral beam combining (SBC) has been limited by the performance of SBC gratings for over two decades. This study demonstrates a breakthrough by developing a volcano-shaped gold-coated holographic SBC grating optimized for the 3-4 μm range, achieving a maximum diffraction efficiency of 96.92% with an average efficiency exceeding 95% across a 1.5 μm spectral bandwidth. The self-developed 50 mm × 50 mm large-aperture grating demonstrates an excellent wavefront as low as 0.18 waves. Moreover, the engineered grating exhibited high robustness under laser irradiation exceeding 1000 W/cm², 4 times that of the nominal commercial MIR-ruled gratings. Furthermore, this work analyzed the fundamental relationship in SBC architecture between the diffraction characteristics of MIR gratings and the linewidth properties of MIR lasers. Dual-grating SBC based on those high-performance gratings demonstrated a 92.5% combined efficiency for OPO lasers operating at 3.16 μm and 3.89 μm while maintaining nearly identical beam quality, showing 54% higher efficiency than the conventional single-ruled grating SBC scheme. The demonstrated technological breakthroughs provide transformative solutions for advanced high-power MIR laser systems.