## 2 Experimental Methodology
### Recording of Wave Evolution in Solid Waveguides
#### 2.2.1 Classical Holographic Interferometry  
In experiments performed by holographic interferometry, waves were generated and recorded using pulsed ruby lasers (pulse duration 20 ns, pulse energy up to 0.5 J, wavelength 694 nm). The optical schematic of the experimental setup is shown in Figure 1a. Synchronization of pump and probe laser pulses was provided by a delayed pulse generator, allowing variation of the delay time between laser pulses in a wide range. The field of view was 30-50 mm in diameter. Two holograms were sequentially recorded on a single frame: one before excitation of the initial shock wave and second corresponding to a certain location of the strain wave in the waveguide. Subsequent illumination of these two holograms with a copy of the reference beam provided simultaneous reconstruction of copies of two object beams, their interference resulted in the formation of a holographic interferogram. Double-exposure holograms were recorded on high-resolution holographic film with maximal sensitivity in the red spectral range and interferograms were digitized at the reconstruction. To record interferograms with the system of carrier fringes, a thin wedge was introduced into the object beam between exposures of the two holograms, causing a slight tilt of the object beam of the second hologram. During optical reconstruction of the double-exposure hologram the interference of the two object beams resulted in the formation of the system of equidistant carrier fringes on the interferogram. Variations in the optical path of the object beam produced by the propagating elastic wave caused fringe deformations within the wave area. The analysis of these deformations allowed for determining wave parameters. Note that in experiments with elastic waves in one-dimensional waveguides (rods and bars) due to a uniformity of strain distribution in the direction perpendicular to that of wave propagation, it is convenient to align carrier fringes in parallel with the waveguide axis. That makes further analysis most straightforward and illustrative and provides accurate data. The strain wave profile was determined in our experiments simply by digitizing the fringe shape, the soliton width equaled to the length of the disturbed area, and the amplitude was calculated using the equation [43]:  
\[A=\frac{\Delta K\lambda}{h(n-1)(1-\upsilon)} \tag{1}\]  
where \(\Delta K\) is the maximal fringe shift, \(\lambda\) is the wavelength of the recording radiation, \(h\) is the waveguide thickness, \(n\) and \(\upsilon\) are the refractive index and Poisson ratio of the waveguide material. Figure 1: Schematics of experimental setups used for the recording of wave patterns by (**a**) holographic interferometry and (**b**) digital holography. L are lenses, M are mirrors, BS are beamsplitters. A diagram illustrating the digital hologram data processing workflow is shown in the inset in (**b**) (the image in the Fourier domain is shown in the logarithmic scale). Note that in the case of extended disturbances with gently sloping fronts, such as strain solitons, the error in measuring the disturbance length becomes significant, especially if stitching of several consecutive frames is applied. A more accurate wavelength characteristic, in this case, is full width at half maximum (FWHM). To ensure the correct stitching of several frames, the carrier fringes on these frames must have exactly the same orientation. Since fringe orientation is defined by the position of the wedge introduced between the two exposures, special attention should be paid to keep the same wedge positions with sufficiently high accuracy. Note that, in contrast with digital holography, the orientation of the carrier interference fringes in holographic interferometry is defined solely by the initial and final wedge positions. Therefore, by precise wedge orientation, we have achieved repeatable position of carrier fringes, allowing for easy stitching of consecutive frames.