Ultrabroadband Infrared Polarization Beam Combiner Based on Calomel Crystals with High Efficiency Exceeding 92% and Low Wavefront Distortion

Abstract

Midwave and long-wave infrared (MW & LWIR) lasers are essential for applications such as infrared detection and counteraction. However, generating high-power MW & LWIR lasers directly is challenging. Polarization beam combining technology provides an effective method to achieve high-power lasers with excellent beam quality. This study introduces Glan–Foucault type polarization beam combiner (PBC) based on the calomel (Hg₂Cl₂) single crystal (SC), which exhibits an exceptionally wide transmission range (0.38–25 μm), ultrahigh birefringence of 0.533@4.6 μm, and a favorable laser-induced damage threshold (LIDT₍₀₀₁₎ = 8.06 J/cm²@3.5 μm). The designed PBCs demonstrate an outstanding extinction ratio (ER) of up to 40 dB and a theoretical maximal transmittance of 81% across the full spectrum. When two fundamental mode Gaussian beams from the quantum cascade lasers (QCLs) were combined, the PBCs with infrared antireflection (AR) coatings achieved combining efficiencies of 93.5% at 4.6 μm and 92.4% at 9.2 μm, along with beam quality factors M² of 1.23 and 1.17, respectively, without noticeable degradation. This work presents a novel polarization component for efficiently scaling the laser output power and maintaining good beam quality across mid- and far-infrared range.