## 4 Experimental Validation and Results Analysis
### Measured Performance and Spectral Matching Evaluation of the Optical Filter  
To verify the actual spectral modulation performance of the designed optical filter within the solar illumination module, systematic measurements were conducted using a UV-NIR spectrophotometer (Shimadzu Corporation, Kyoto, Japan) and a spectral radiometer (ASD Inc., Boulder, CO, USA). The design objective of the filter was to produce a  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Components**} & \multicolumn{3}{c}{**Parameter of Apodizator**} \\ \cline{2-4}  & **Focal Length** & **Effective Aperture** & **Mechanical Aperture** \\ \hline Collimating Lens & 85 & 22 & 25 \\ Aperture Stop & \(\backslash\) * & 7 & 25 \\ Focusing Lens & 25 & 7 & 25 \\ \hline \hline \end{tabular}  
* The aperture stop serves as a mechanical limiter, with no optical focusing function, and is used solely to define the system’s input aperture.  
\end{table}
Table 2: Parameter of apodizator (mm).  
Figure 6: Measured far-field energy distribution.  
high-uniformity, low-fluctuation spectral output within the 1540-1560 nm communication band, closely approximating the standard solar spectral distribution.  
The filter transmittance curve was measured using a Shimadzu UV-3600 spectrophotometer (Shimadzu Corporation, Kyoto, Japan), with a spectral range of 185-3300 nm and a resolution of 0.5 nm. The shaped spectral distribution was measured using an ASD FieldSpec spectral radiometer (ASD Inc., Boulder, CO, USA), which covers a wavelength range of 350-2500 nm with a wavelength accuracy of \(\pm\)0.5 nm. The laser output power was adjusted from 0 to 2 W in 50 mW increments, and the spectral radiance within the 1540-1560 nm band was recorded at each power level.  
Figure 7 presents a comparison between the theoretical design curve and the measured transmittance of the filter. The shaded area indicates the theoretical tolerance band, accounting for fabrication deviations caused by multilayer coating thickness variations. The yellow curve represents the measured transmittance of the fabricated filter sample. It can be observed that at 1540 nm and 1541 nm, the measured transmittance slightly exceeds the upper tolerance limit, resulting in deviations from the intended spectral shaping at these wavelengths. This discrepancy is mainly attributed to the high precision required in controlling the transmittance at each wavelength point, which poses significant challenges in multilayer coating fabrication and spectral matching.  
Figure 8a shows the system's spectral radiance distribution after spectral shaping by the designed filter, covering an output power range of 600-2000 mW. Compared to the original laser output (Figure 4), the shaped spectrum exhibits significantly improved smoothness with reduced fluctuation amplitude. The spectral consistency is notably enhanced within the typical working power range of 750-1750 mW.  
To quantitatively evaluate the shaping performance, Figure 8b compares the spectral uniformity before and after applying the filter. The original laser output achieves a maximum uniformity of approximately 76% at high power (2 W), but drops below 30% at low power levels, indicating strong instability. After introducing the filter, the spectral uniformity across the 750-1750 mW range increases to over 75%, with a peak value exceeding 80% and an average spectral match of 78%. These results demonstrate that the designed filter provides effective spectral modulation and enables consistent reconstruction of the solar spectral profile across a wide power range, thereby offering a reliable foundation for realistic Sun outage background simulation.  
Figure 7: Comparison between the theoretical transmission design curve (with tolerance band) and measured results of the spectral filter.