## 4 Experimental Results and Discussion  
### Time Reversal Experimental Results  
Figure 3a is an off-axis holographic interference pattern acquired by a CMOS camera when the diffuser is not used during the phase extraction process. The spacing of the light (dark) fringes is about 28 pixels, which satisfies the two-dimensional sampling theorem of the image. After two-dimensional Fourier transform, the spectrogram is obtained (see Figure 3b), and it is clearly visible that the spectrogram is divided into three regions. According to the off-axis digital holography theory, the three  
regions in the spectrogram correspond to \(\hat{U}_{3}\), \(\hat{U}_{1}+\hat{U}_{2}\), \(\hat{U}_{4}\) in Equation (9) from left to right. As shown in Figure 3c, a suitable frequency domain filter is constructed to only make the spectrograms with \(\widetilde{RO^{*}}\) left. The inverse Fourier transform of the spectrograms left obtains \(\widetilde{ROC^{*}}\), and its phase distribution is shown in Figure 3d. During the time reversal, the conjugate phase pattern was loaded on the SLM, and the conjugate reference light was injected to obtain a light wave \(\widetilde{O^{*}}\) which was conjugated with the original optical phase; thus, the time reversal of the object light is realized. After completing the basic construction of the off-axis digital holographic optical phase conjugate system, the performance of the system to achieve light wave time reversal was tested. After controlling the two-dimensional precision stage to move OL2 (objective lens 2) along the \(X\)-axis and \(Y\)-axis (see Figure 2) respectively, the influence of the propagation path variation of the object light to the phase conjugate reduction was tested. Moving OL2 along the \(X\)-axis with a step size of 50 \(\upmu\)m changes the collimation ability of the object light. When the reference light is directly reflected from the SLM to be modulated, the spot size on the CCD changes. If the conjugate phase of the object light is loaded on the SLM, the light will return along the original path and form a time-reversed light wave of the object light, thus the size of the spot detected on CCD remains the same. Figure 4b is a curve which shows the relationship between the lights spot size recorded by the CCD and the displacement along the \(X\)-axis. It can be seen from the figure that when the displacement along the \(X\)-axis is in the range of (\(-\)350 \(\upmu\)m, 350 \(\upmu\)m), the conjugate light achieves a better phase conjugate reduction effect than the direct reflected light. In addition, moving OL2 along the \(Y\)-axis with a step size of 10 \(\upmu\)m changes the spot position of the unmodulated light on the CCD. Figure 5b shows the spots position change of the unmodulated reflected light as well as the time-reversed conjugate light on the CCD. According to the phase conjugation theory, the time-reversed modulated light can return along the original path, so ideally, the position of the spot detected on the CCD does not change with the change of the position of OL2. It can be seen from Figure 5b that the off-axis digital holographic phase conjugate system has a good phase conjugate reduction effect when the displacement along the \(Y\)-axis is in the range of (\(-\)80 \(\upmu\)m, 80 \(\upmu\)m). Figure 3: (**a**) Interference pattern; (**b**) spectrum of the interference pattern; (**c**) spectrum after filtering; (**d**) phase conjugation pattern. During the experiment, we adjusted the relative position of Lens 3 and the CMOS camera, and also changed the imaging resolution of the CMOS camera to the SLM to observe the influence of those actions above the phase conjugate reduction quality. Figure 6 shows a curve of the intensity of the spot collected by CCD changes as a function of the displacement of Lens 3. As can be seen from the figure, when the displacement error of Lens 3 is within the range of (-1 mm, 1 mm), the intensity does not change much.