## 3 Experiment and Discussion
### Initial Pixel Alignment of the DOPC System and Initial Conjugate Phase Acquisition
#### 3.1.2 Initial Pixel Alignment of the DOPC System  
The initial phase \(\varphi_{0}\) by the two-step digital phase shift holography method has enough precision for compensation. In contrast, pixel misalignment has a greater influence on the experiment. Several pixelmisalignments can greatly restrict the final optimization. Thus, it is necessary to carry out the initial pixel alignment of the DOPC system before optimization. In the DOPC system, the optimal performance requires a pixel alignment between object light and conjugate light on the surface of the scattering medium and pixel alignment between the SLM and CCDs. The SLM should be perpendicular to the incident reference beam. The power of the beam, back-propagated through the MOI-3, is measured by a power meter placed between L3 and M2, and the back-propagating signal is maximized by adjusting the rotation platform of the SLM carefully. In such a way, the phase conjugate beam and the object beam are roughly in a total optical path and act at the same position as the scattering medium (SM).  
Then, pixel-to-pixel alignment is required between CCD1 and the SLM. The SLM plane is directly imaged on the CCD1 plane via L8. When the pixel dimensions of CCD1 and the SLM are 6 and 8 microns, the pixel-size matching can be addressed by adjusting \(\mu\) and \(v\) based on  
\[\mu+\
u=f\times\left(M+1\right)^{2}/M, \tag{7}\]  
where \(\mu\) is the length between SLM and L8, \(\
u\) is the length between CCD1 and L8, \(f\) is the focal length of L8 (\(f=100\) mm), \(M\) is equal to the ratio between \(d_{CCD1}\) and \(d_{SLM}\). \(d_{CCD1}\) is the pixel dimensions of CCD1, and \(d_{SLM}\) is the pixel dimensions of the SLM.  
After that, a phase pattern of a negative United States Air Force (USAF) target was loaded in the SLM, and the corresponding image was obtained by CCD1, as shown in Figure 3a,b. According to the relationship between the pattern and the image, we can determine the pixel correspondence between the SLM and CCD1. We subtracted the inverted image of CCD1 from the pattern loaded by the SLM, as shown in Figure 3c. The magenta part represent s the pattern loaded by SLM, and the green one represent s the inverted image of CCD1. The green and magenta ones should be completely consistent in size and position, which means that the pixel alignment between the SLM and the CCD is achieved. The misalignment of pixels can be greatly eliminated by fine-tuning the pixel correspondence, as shown in Figure 3c,d. In this way, the rough pixel alignment between the SLM and CCD1 was achieved. Additionally, the initial pixel alignment of the DOPC system is finished.  
Figure 2: Experimental setup for the DOPC system. M1-3 are reflecting mirrors; H is the half-wave plate; SPF represents the spatial filter; A1–2 are the beam attenuators; BS1–4 are beam splitters; L1–8 are aspherical lenses, and the corresponding focal lengths are 100, 100, 100, 50.8, 100, 200, 100, and 100 mm, respectively; SM is the scattering medium; SLM is the spatial light modulator; MO1–3 are microscope objectives; Q is the quarter-wave plate; CCD1 and CCD2 are charge-coupled devices.