We have designed and prepared the multilayer anti-reflection (AR) and diamond-like carbon (DLC) hard coatings for a large scale polycrystal gallium arsenide (Poly-GaAs) infrared optical window to improve its transmittance and surface hardness. The coatings were deposited on 100 mm diameter Poly-GaAs substrates and their optical properties were analyzed. The optimum optical designs are found to be IRX/Ge/YF₃ with buffer layer Y₂O₃ for AR coating, and DLC/Ge/Y₂O₃ and DLC/BP for DLC hard coating, respectively. Their transmittances are 98% for AR coating and 85-87% for DLC coating, which are 5-10% higher than the values for the conventional infrared protective ZnS windows used for airborne infrared thermal imaging systems.