## 2 Test Samples and Research Technique  
Two crystals GaSe (sample no. 1) and GaSe:In (sample no. 2) produced by Harbin Institute of Technology, China, were used as the test samples. Sample no. 2 was doped with In. The thickness of sample no. 1 was 3.1 mm, the sample aperture was 8 \(\times\) 5 mm(rectangle), the thickness of sample No. 2 was 4.49 mm, and the sample aperture was 5 \(\times\) 7 mm\({}^{2}\). The working surfaces of the samples were orthogonal to the optical axis of the crystal. Se (6N) and Ga (6N) were used for GaSe synthesis. They were purified firstly in Aqua Regia and then carefully washed in distilled water fused silica ampules, and pyrolytic boron nitride (PBN) loads and crucibles were used for synthesis and growth. The ampules with substances were pumped down 10\({}^{-5}\) mm Hg. The single-temperature method was applied for synthesis. GaSe crystals with a diameter of 30 and 40 mm were grown by vertical Bridgeman (VB) method. The high-temperature zone was 1075 \({}^{\circ}\)C and the low-temperature zone was 920 \({}^{\circ}\)C. The temperature gradient was 4 deg/cm and the pulling rate was 0.5 mm/h. The surface roughness of both samples was identical and was determined using a Bruker Dimension Edge atomic force microscope (AFM). AFM scanning of the samples was carried out on the area of 10 \(\times\) 10 \(\upmu\)m at room temperature. The result of determining the surface roughness of sample no. 1 is shown in Figure 1.  
Figure 1 shows that the maximum difference in height and depth of inhomogeneities on the surface of the samples was 6 nm. A Ho\({}^{3+}\):YAG laser developed at the IAP RAS was used as a source of laser radiation. Lasing was carried out at a wavelength of 2.091 \(\upmu\)m (close to the radiation wavelength of 2.12 \(\upmu\)m produced in the process of degenerate parametric generation based on a KTP crystal) [13]. Lasing was carried out in the active Q-switching mode using an acousto-optic modulator with pulse duration \(\tau\) equal to 15, 18, 20, or 23 ns, depending on the pulse repetition rate \(t\) equal to 10, 12, 14, or 20 kHz, respectively. To avoid changes in the pulse duration and the intensity distribution in the cross section of the generated beam, we pumped the Ho\({}^{3+}\):YAG laser with a thulium fiber laser with a fixed average power of 50 W. The maximum average radiation power generated by the Ho\({}^{3+}\):YAG laser was 30 W in a linearly polarized high-quality Gaussian beam (M\({}^{2}\) was less than 1.2). The scheme of the test setup is shown in Figure 2. The stand consisted of the following elements: 1--Ho\({}^{3+}\):YAG laser; F.I.--Faraday optical isolator; A--attenuator, which was used to adjust the power level (consisted of a half-wave plate and a polarizing mirror M1 with high transmittance in p--polarization and high reflection in s--polarization); T--two-lens Galilean telescope focusing radiation on the test samples; Sample--sample under study fixed in a holder, adjusted in angle and transverse coordinates; P.M.--power meter. The average radiation  
Figure 1: Example of the roughness profile of the test sample no. 1 measured using a Bruker Dimension Edge atomic force microscope (AFM). power \(P_{a}\) was measured using a power meter immediately before installation of the test samples. The studied plates were deviated from the position corresponding to normal incidence by 3-5\({}^{\circ}\) to prevent the reflected radiation from entering the laser exit window. The diameter of the laser beam at the 1/e\({}^{2}\) level in the plane corresponding to the entrance aperture of the tested plate was measured by the Foucault knife-edge test according to the technique described in [14,15]. The diameter of the radiation beam measured in the knife plane (sample installation) was d = (280 \(\pm\) 10 \(\upmu\)m) at the 1/e\({}^{2}\) level. Figure 3 shows the intensity distribution over the laser beam diameter.