## 1 Introduction
However, a disadvantage of the most methods of investigation of single crystal defect structure is that the preparation of special samples (thin polished plates with thickness of \(\sim\)1 mm or less) is required that cannot further be used for NLO element production. Moreover, the control and visualization methods used in the process of manufacture should be fast and contactless, which is characteristic exactly for the holographic methods. We have already published some results of works in this direction. Thus, for example, in [10] the method of digital holography was used to investigate the process of optical damage of a single crystal. In [11], it was proposed to use the digital holography method to visualize the internal inhomogeneities of the nonlinear ZnGeP\({}_{2}\) crystal, and the feasibility was demonstrated to evaluate shapes and typical sizes of defects. In [12], examples of visualization of such defects as growth strias, microcrack accumulations, and needle-like defects were presented. This paper presents the results of verification of data on voluminous defects obtained by the digital holography method for a set of optical inhomogeneities that are necessary for manufacturing of optical elements. The present work is aimed at verification of the holography method by a comparison of the results obtained with the data obtained by other methods. It has been suggested to use the method of digital IR holography in a new field of volumetric defect detection in single crystal materials, and the possibility of its application for studying optical, chemical, and mechanical volumetric inhomogeneities has been demonstrated experimentally. This is illustrated by experiments with ZnGeP\({}_{2}\) semiconductor materials extremely difficult for experimental investigation because of their large refractive indices (exceeding 3) and non-transparency in the visible range of the spectrum. The practical importance of the present work is that, unlike the previous methods of volumetric defect detection in ZnGeP\({}_{2}\) single-crystal materials, the proposed method obviates the necessity of special preparation of thin samples with thickness less than 1 mm and can be used to investigate samples with thickness up to 50 mm. Digital Holography Method as a Technique for Investigating the Volumetric Defects in Single Crystals  
Modern CCD (or CMOS) cameras have sufficient resolution for digital hologram recording. Figure 1 shows the in-line scheme of digital Gabor's hologram recording used in the present work. Light from laser source (1) passes through collimator (2) forming a desired cross section of the beam transmitted through investigated sample (3). In our case, the collimating objective has a focal length of 70 mm. As a result, an interference pattern is formed by the reference wave (part of radiation passed through the homogeneous sample volume without scattering on inclusions) and the object wave (part of radiation scattered by the inclusions). Camera (4) recorded this interference pattern (the hologram) and stores it in the computer memory. The digital hologram represents digitized values of the interference field of the reference and object waves [13]. This two-dimensional data file \(U(x_{1},y_{1})\) characterizes the field distribution in the hologram plane under illumination by a plane wave. Then numerical calculation [13] of diffraction integral (1) is used to calculate \(U(x_{2},y_{2})\)--the field distribution in the \((x_{2},y_{2})\) plane located at distance \(z\) from the hologram plane \((x_{1},y_{1})\). \[U(x_{2},y_{2})=\frac{1}{i\lambda z}\int_{-\infty}^{\infty}\int_{-\infty}^{ \infty}U(x_{1},y_{1})e^{i\frac{2\pi}{\lambda}z}e^{\frac{i\pi}{\lambda}((x_{1} -x_{2})^{2}+(y_{1}-y_{2})^{2})}dx_{1}dy_{1}, \tag{1}\]  
where \(\lambda\) is the wavelength, \(z\) is the distance between the hologram plane and the image restoration plane. Changing \(z\), the image of the investigated sample volume can be formed. In this case, inclusions are visualized in each restored image of the single crystal cross section.