## 1 Introduction
Hence, their sizes, shapes, orientations, and localizations in the single crystal are determined. This procedure can be called a virtual microscope, because the process of restoration of images of volume cross sections is analogous to the process of longitudinal microscope focusing. In this case, the defects are visualized in each restored image of the volume cross section, which makes it possible to determine the defect sizes, shapes, orientations, and positions and hence, to identify the defects. Figure 2 illustrates the image of the ZnGeP\({}_{2}\) plate with thickness of 6 mm containing a volumetric defect at a depth of 2.4 mm recorded with the digital holographic camera. The back face of the plate was at a distance of 61.8 mm from the matrix and at a distance of 67.8 mm from the front face of the plate. The defect (whose shape resembled a butterfly) was at a distance of 64.2 mm from the photodetector matrix, since the best defect focusing was observed exactly at this value of \(z\). Figure 1: In-line scheme of digital Gabor hologram recording comprising laser—1, expander—2, investigated sample—3, and Videoscan-2020/PC CMOS camera—4. In digital holography, the above-described process of information recording and restoration has already been used to study plankton and settling particles in a liquid [14; 15]; it can also be successfully used for diagnostics of defects in optical materials [11; 16]. The unique property of holography is the possibility of recording information about a sufficiently large volume of the medium in a single hologram and at the same time, with high spatial resolution sufficient for obtaining data on each particle, inhomogeneity, or defect in the volume of the medium (including its size, shape, and coordinates). In the examined case of digital holography, a hologram can be transferred via communication lines, and recording of a time sequence of digital holograms makes it possible to create a video of the investigated process based on the holographic data [14]. Thus, digital holography provides unique possibilities for investigation of inhomogeneities of different types in optical materials, especially for diagnostics of optical material of single crystals intended for operation with optical radiation of high intensity (high optical density), including nonlinear optical elements of optical parametric oscillators (OPO). In the present work, results of application of digital holography to investigation of inhomogeneities in ZnGeP\({}_{2}\) single crystals are presented. Since the Gabor scheme imposes reduced requirements on the spatial and temporal coherence of radiation [12; 13], a semiconductor laser diode (denoted by 1 in Figure 1) with wavelength of 1.064 \(\upmu\)m and output power of 100 mW was used as a radiation source. A CCD camera with 1600 \(\times\) 1200-pixel matrix, pixel sizes no more than 7.4 \(\upmu\)m \(\times\) 7.4 \(\upmu\)m, and physical matrix size of 2/3\({}^{\prime\prime}\) was used as a recording camera (denoted by 4 in Figure 1). The long-wavelength boundary of the spectral sensitivity of the employed CCD camera was at \(\sim\)1.1 \(\upmu\)m. The working spectral range of the ZnGeP\({}_{2}\) transmittance in which the absorption coefficient did not exceed \(\sim\)1 cm\({}^{-1}\) was in the range 0.9-8.3 \(\upmu\)m. Thus, sources generating radiation in the spectral region near 1.06 \(\upmu\)m and CCD matrices operating in the visible range can be used to investigate the ZnGeP\({}_{2}\) single crystals by the digital transmission holography method. These problems were considered in detail in [11; 16; 17].