Two-photon absorption of nonchain HF laser radiation in germanium single crystals

Some results of experimental and numerical studies on the transmission of a nonchain high-frequency (HF) laser beam through germanium (Ge) single crystals of differing thickness and specific resistance are presented. Based on the experimental data for the HF(DF) lasing spectrum, the coefficient of two-photon absorption in germanium has been estimated as K$_2$ = 55 $\pm$ 10 cm/GW at $\lambda$ = 2.8 $\mu$m. The results are in good agreement with theory. The developed experimental data-processing software has enabled the numerical study of the transmission of a high-power beam with $\lambda$ = 2.6–3 $\mu$m through Ge at any laser-pulse-impact moment. It has been shown that thin germanium coatings can efficiently smoothen the distribution of energy over the beam aperture for high-power laser radiation with $\lambda$ = 2.6–4 $\mu$m.