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 VO1+ 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. •Demonstrated an annealing process that reduces NIR absorption by 73% in spinel and yields spinel optics with transmission values within 1% of the Fresnel limit.•Developed a model and understanding of Fe impurity oxidation in MgAl2O4.•Equibiaxial strength testing and corresponding Weibull analysis reveals that the spinel optics fabricated in this study have a higher design strength per given failure probability than sapphire optics.