Post-process annealing of MgAl₂O₄ spinel optics for high energy laser windows

Magnesium aluminate (spinel) has been investigated as a low-absorption material for applications such as high-strength windows, armor, and high energy laser (HEL) apertures. To develop spinel for HEL applications, it is critical to lower the absorption of the lasing medium at the operating wavelength to reduce optical path distortion. In this work, post-processing of spinel via annealing is investigated to lower the absorption at 1.064 μm. Specifically, spinel optics are annealed in ambient air, oxygen, and nitrogen at temperatures of 1000–1400 °C and their transmission, absorption and photoluminescence properties are measured. This annealing process reduces the absorption by 73% from 463 ppm/cm to 126 ppm/cm and results in optics that have transmission within 1% of the Fresnel limit over the 1.0–2.0 μm wavelength range. The resulting optical characterizations imply a direct oxidation of Fe²⁺ to Fe³⁺ and the formation of V_O¹⁺ at distorted octahedral sites to mediate this reaction that results in lower optical absorption in the near-IR (NIR). Further, performance of equibiaxial flexure testing and corresponding Weibull analysis indicates that spinel manufactured using this method can outperform sapphire windows from a design strength perspective.