## 4 Conclusions  
The digital holography method used to investigate the volumetric defects of the ZnGeP\({}_{2}\) single crystal structure has allowed us to detect defects and defect pileups of actual ingots with thickness of about 10-50 mm. This makes it possible to cut crystals into nonlinear optical fragments with minimal concentrations of volumetric defects.  
The experimental stand described in this work, including the CCD camera with pixel size of 7.4 \(\upmu\)m and the laser diode emitting radiation at a wavelength of 1.064 \(\upmu\)m together with the developed software have allowed us to detect and to identify the volumetric defects with minimal linear size of \(\sim\)15-20 \(\upmu\)m.  
Our experimental investigations of the ZnGeP\({}_{2}\) single crystal samples have demonstrated that the digital holography is promising for application to technological control and quality assurance certification of optical crystals. It has been shown that results of holographic tests allow:  
* Diagnostics of inclusions and optical inhomogeneities to ascertain the compliance of the single crystal with the optical quality characteristics, including sizing;
* Diagnostics of materials intended for operation in the optical IR range;
* Optical focusing by the virtual method (numerically, in the post-experiment regime) for wide range of variations of optical thicknesses without limitations peculiar to actual zooming systems connected with finite depth of focusing of zooming systems;
* Construction of the 3-D defect distribution in the investigated volume.  
During our research, we have verified the data on voluminous defects obtained by the method of digital holography by their comparison with the results of chemical etching of the single crystal surface and X-ray topography. The minimal size of inclusions detected by the method of digital holography was experimentally determined. For the first time, the dislocation rows have been detected in the nonlinear crystal volume.  
Figure 6: Hologram of the plate of unannealed material with thickness of 50 mm cut perpendicularly to the growth direction (**a**), holographic image restored from the chosen fragment of the hologram of the crystal plane located at a depth of 9 mm from the plate surface (**b**), zoomed image of a number of dislocations reconstructed from the restored image (**c**), and X-ray topogram of the cross section cut of the initial fragment of the single crystal ingot (result of investigations performed in [21] using X-ray transmission topography) (**d**).  
**Author Contributions:** conceptualization, V.D. and A.G.; methodology, V.D., I.P.; software, S.P., A.O., A.D.; validation, M.Z., A.G.; formal analysis, A.G., V.D.; investigation, N.Y., S.P., M.Z.; resources, V.D., I.P.; data curation, A.G.; writing--original draft preparation, N.Y., A.G.; writing--review and editing, V.D.; visualization, A.D., N.Y., M.Z.; supervision, V.D.; project administration, V.D.; funding acquisition, A.O. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The study was performed within the framework of the State Assignment of the Russian Federation (Project No. 8.2712.2017/4.6).  
**Conflicts of Interest:** The authors declare no conflict of interest.