Effect of laser beam diameter on near infrared laser damage threshold of Ge optical materials

Germanium is widely used as lens or windows in infrared optical systems, however, germanium optical elements may be damaged by melting under high energy laser irradiation. Therefore, it is necessary to carry out theoretical and experiment research on laser damage threshold of the germanium optical material. In this paper, the effect of laser beam diameter on the damage threshold of germanium was analyzed by numerical calculation. Besides, the difference of damage threshold represented using line power density and area power density was compared. It was found that when the diameter ratio of beam spot to sample was 0.02 to 0.09, the damage threshold decreased by 27.9% and 85.6%, respectively, when using W/cm and W/cm2 as the unit correspondingly. Considering the difference between the size of the beam and the element in optical system, the line power density is more suitable for extrapolation and comparison. In addition, the numerical results were verified by damage threshold experiment under the continuous laser of 1080 nm, which indicates the damage threshold of germanium is 263W/cm, 280W/cm and 290W/cm respectively, and the beam diameter is 0.5mm, 1mm and 2mm correspondingly. It was found that when the diameter ratio of beam spot to sample was 0.021 to 0.083, the damage threshold increased by 10.3% and decreased by 85.6%, respectively, when using W/cm and W/cm2 as the unit correspondingly. These results provide data support for the design and application of germanium optical elements to ensure the reliability of the high energy laser system.