## 4 Discussion
Digital holography requires rather fine adjustment of the optical system and imposes restrictions on the maximal angle formed by the object and reference waves that is limited by the size of the matrix pixel, as well as by the imperfections of the object shape. For instance, an imperfect flatness of the waveguide sidewalls causes curvature of the interference fringes, even in the absence of an elastic wave (see the curvature of initially vertical interference fringes in Figure 2). In this particular case, distortion of the transmitted wavefront was not critical and allowed us to reconstruct phase distributions nearly over the entire waveguide area within the field of view. Nevertheless, stronger distortions of the probe wavefront by the object can disallow reconstructing the phase distribution of the transmitted wavefront, even in the absence of disturbance. Note that such problem can arise also when monitoring shape changes of objects with rough, non-reflective surfaces [51; 52; 53] or in characterizing discontinuous deep surfaces [54]. The solution can be the utilization of additional optical methods, e.g., multiplexed or frequency-sweeping digital holography [54; 55; 56; 57]. Conversely, holographic interferometry is based specifically on the visualization of object changes that significantly expand the capabilities of this technique in investigation of objects that introduce significant distortions to the probe wavefront. This also allows conclusions to be drawn on the characteristics of strong disturbances, such as, in our case, shock waves produced by explosive evaporation of the metallic foil (Figure 5a). Investigation of such objects using digital holography poses significant difficulties due to a large amount of sharp gradients and singularities of phase distribution (see Figure 5b,c). The fast-gated digital camera with relatively large pixel size utilized in our experiments did not allow for the detection of rapidly oscillating strain wave components (see phase images in Figure 5c), although the position of first diffraction order was almost optimal for recording high-resolution phase images (see the image in Fourier domain in the inset of Figure 1b). The minimal size of a reliably resolvable high-frequency oscillating component was about 1.5 mm. Meanwhile. holographic interferometry utilizing a similar optical setup allows for the detection of smaller details of the strain wave (down to parts of mm) due to much higher resolution of the photographic films (being, however, still insufficient for resolving shock waves). Therefore, the technique of classical holographic interferometry is more suitable for the investigation of shorter disturbances when submillimeter resolution is required. We note, also, that efficient resolution of the optical system depends on the required magnification or reduction of the recorded area on the frame, with reduction causing a resolution decrease and, vice versa, magnification providing its increase. In the case of long disturbances, the image reduction is hardly avoidable. According to our experience, in general, the two major characteristics that should be considered for making a decision on the particular realization of the holographic approach that should be used for imaging are the spatial and temporal resolutions. If high spatial resolution is not required and the major object of interest introduces only smooth phase gradients, then digital holography is preferable due to an opportunity for automatic and relatively simple data processing. On the other hand, if the object requires much higher temporal resolution of the order of several nanoseconds or less then the pulsed laser approach should rather be used for holographic imaging. Besides that, digital holography should be implemented if the experimental workflow requires immediate availability of the results right after hologram recording.