# Fast Computational Ghost Imaging with Laser Array Modulation
## 4 Experimental Verification  
Next, the proposed method for generating speckle generation and change of patterns is verified experimentally. To this end, an optical setup is assembled. Figure 4 shows a schematic and a picture of the experimental setup. A laser diode mounted on movable stages is utilized as an alternative for the laser array. The stages can control the position of the diode in both horizontal and vertical directions. Moreover, instead of a single photo-detector, an image sensor is used to measure the object signals in order to avoid signal loss caused by alignment errors associated with the correction optics. \(B_{i}\) is derived from the sum of all pixel values. In the mathematical model of thermal ghost imaging, measured intensity on the photo detector is defined as the spatial integrals of optical intensity through the object. That means optical signals through the object in CGI can be approximated to incoherent signal. This is because coherence of speckle illumination patterns is weak due to optical scattering. Therefore, \(B_{i}\) obtained by the above process can be considered to be the same as that by the photo detector. The geometry of this setup is different from that of CGI to avoid the influence of the SLM misalignment. However, it is useful for verification of the effects of laser array modulation in CGI. A ground glass is introduced for random phase modulation. The intensity distributions of the speckle  
Figure 4: (**a**) Schematic of the optical setup for experimental verification of the proposed method and (**b**) picture of the optical setup. Figure 3: Simulation results: (**a**) target object, (**b**) reconstructed image obtained by conventional CGI; figures (**c,d**) show the images recon-structed with the proposed method for \(L_{2}\) = 2000 mm and 400 mm, respectively. patterns are measured with a beam splitter and an image sensor. The pixel intensities obtained by the image sensor are denoted as \(l_{i}(x,y)\). The wavelength of the laser is 532 nm, and the pitch of the laser emission (\(\Delta b\)) is set to 800 \(\upmu\)m. The values of (\(M_{e}\), \(N_{e}\)) are set to (30, 30). Thus, the total number of measurements is 900. Figure 5 shows the results of the experiment. Figure 5a is a picture of the target, and Figure 5b is the result obtained by image reconstruction. From Figure 5b, it can be seen that the symbol \"5\" is successfully reconstructed in the image. The reconstructed image is tilted and shifted with respected to the target. This is because the speckle distributions illuminating the target do not completely match the reference patterns. With more accurate alignment, the images are reconstructed correctly. Also signal processing based on differential ghost imaging [6] and normalized ghost imaging [7] are useful for not only the conventional GI but also the proposed method. In this experiment, the target symbol is selected from the USAF 1951 chart. Attempts to measure large parts of the chart did not yield successful reconstruction because the transparent area of the chart was too wide. In Ref. [7], it is shown that signal to noise ratio (SNR) of reconstructed images depends on transparent area in the target. The wider the transparent area is, the lower SNR is. The reason is analyzed in the Ref. [7]. In Ref. [25], also, the relation between the transparent area and quality of the reconstructed images is studied, and visibility is defined for assessing the reconstructed images. According to Ref. [25], a huge number of measurements are required to improve the visibility. The proposed method is suitable for high speed measurements. Therefore, our method is considered to be useful for improvement of visibility. In Ref. [26], a VCSEL array with 1200 emitters was used for confocal interferometry. Such an array device can be employed in the proposed method. From the experimental verifications, it is shown that the proposed scheme for renewal of speckle illumination patterns by means of laser array modulation is useful for CGI.