## 2 Experimental Setup and Methodology
However, given the current resolution limitations, further optimization of the optical setup and detection mechanism is necessary to fully realize its potential in high-precision imaging applications. To ensure proper alignment and collection of the reflected signals, two spherical mirrors are positioned within the setup. One mirror collects the reflected signal from the object and directs it to the GDD, while the other captures the reference signal and directs it to the GDD. This arrangement allows the detector to receive both reference and reflected signals, enabling the calculation of range information through frequency analysis. The experimental setup, as illustrated in Figure 2, is configured with distinct optical path lengths to maintain precise alignment between the main and reference signal paths. This schematic highlights the arrangement that directs the main signal toward the target while ensuring the reference path aligns accurately with the GDD. The calibrated path distances are critical for achieving high-resolution depth measurements and consistent imaging fidelity. The optical paths are configured to precisely direct both the main and reference signals. The target path, measuring 4.65 m, carries the main signal from the source to the object, then to the spherical mirror, and finally to the detector. The reference path, measuring 1.45 m, provides a stable reference signal. With an optical path difference (OPD) calibrated to 3.2 m, this arrangement enables accurate range detection and alignment for subsequent analysis. The off-axis parabolic mirrors in the experimental setup are positioned at their focal points to collimate the MMW beams. In the target path, the collimated beam is directed onto the object to be imaged, ensuring that the MMW signals maintain their focus during interaction with the object. In the reference path, the beam is directed toward the reference mirror. The off-axis placement of reference mirror serves a dual purpose: it ensures that  
Figure 2: Schematic of the experimental setup with calibrated optical path distances for main and reference signal alignment. the reference beam is properly aligned with the GDD and prevents any interference with the target path. By placing Mirror 2 off-axis, we avoid alignment issues that might arise from direct alignment of the reference path with the target path, thus minimizing crosstalk and ensuring accurate signal measurements. This setup guarantees precise collimation and alignment of both beams, which is essential for reliable range detection and image reconstruction in the 3D MMW imaging process. To ensure that the reference signal remained aligned throughout the scanning process, a pointed laser beam was used for optical calibration. The laser was directed along two separate paths and reflected by the off-axis parabolic mirrors, with careful adjustments made to maintain precise alignment of the reference signal during the entire scanning procedure. This method ensured the accuracy and consistency of the signal throughout the experiment. To complement the schematic and enhance clarity, a photo of the lab setup is included as Figure 3a, depicting the Tx 272 transmitter, Magic Hybrid Tee, off-axis parabolic mirrors, and GDD, along with the laser alignment setup used for optical calibration. As it is difficult to capture all equipment and arrangements in a single photo, we have included sub-photos highlighting the optics arrangement for beam splitting and the two propagation paths as in Figure 3b. Additionally, a close-up image of the GDD mounted on the linear translational stage is provided, illustrating its vertical and horizontal step movements, as shown in Figure 3c. In response to the challenges posed by GDDs, we carefully considered their limitations, including the response variability among individual units, and addressed these by using a single GDD in our experimental setup.