Impact of free electron laser coherence on imaging quality

Abstract The high temporal and spatial coherence of free electron lasers (FELs) reduces the uniformity of the illumination field, leading to scattering effects that blur the edges of patterns, resulting in diminished accuracy and clarity. Traditional imaging models regard the light source as fully incoherent, making it difficult to assess the impact of partially coherent light fields on imaging. If FELs are used in imaging systems, their coherence must be considered. To address this issue, this study explores the relationship between coherence, imaging quality and speckle contrast through a simulation method based on random phases. The method divides the light beam into temporal and spatial coherence cells, analyzes their interactions, and simulates imaging results under different coherence conditions. Additionally, speckle patterns for various illumination modes are calculated to evaluate their effects on speckle contrast and illumination uniformity. The results indicate that under different illumination modes, illumination uniformity decreases as coherence increases, while speckle contrast increases with higher coherence. In terms of imaging quality, higher coherence leads to an increase in both line edge roughness (LER) and line width roughness (LWR), thereby reducing the imaging quality. Additionally, the narrower the line width, the greater the impact of coherence on the imaging quality, resulting in poorer imaging performance.