Researchers from the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences proposed a novel method using a Shack-Hartmann wavefront sensor (SHWFS) combined with "extended sources" to accurately measure the atmospheric coherence length r. This research provides technical support for evaluating atmospheric turbulence in real-time, especially when observing targets that lack point-like light sources or are obscured by high background noise. The results were published in the journal Photonics.

Atmospheric turbulence is the "invisible hand" that degrades the quality of optical imaging. As light travels through the fluctuating layers of the Earth's atmosphere, its phase becomes distorted, causing images seen through telescopes to appear blurry or shaky. The atmospheric coherence length r is the core physical quantity used to measure this turbulence strength; it determines how much an adaptive optics system needs to compensate. Traditional methods typically rely on "point sources" like distant stars. However, during daytime observations, laser communications, or when observing the Moon and planets—which are "extended sources" with surface textures—traditional measurement schemes often perform poorly or fail entirely.

To solve this problem, the research team explored how to extract turbulence information directly from images of extended sources. A Shack-Hartmann sensor consists of an array of micro-lenses. When light passes through, it forms a pattern of spots on a detector. For a point source, these spots are discrete dots; for an extended source, each micro-lens forms a sub-image of the target. The researchers calculated the relative displacement, or centroid deviation, between these sub-images to back-calculate the wavefront distortion caused by the atmosphere, thereby determining the r value.

During the research process, the team constructed a precise experimental model. They addressed the challenge of sub-image matching accuracy by employing the Normalized Cross-Correlation (NCC) algorithm combined with parabolic interpolation technology. This approach allowed them to track image jitter with sub-pixel precision. The researchers not only simulated various turbulence intensities in a controlled laboratory environment but also validated the algorithm using actual outdoor observation data. They compared results across different sub-aperture sizes and signal-to-noise ratios to ensure the robustness of the data processing.

The experimental results demonstrated that when dealing with extended source targets, the measured atmospheric coherence length was highly consistent with results obtained from the traditional Differential Image Motion Monitor (DIMM) method. Even in scenarios where target features were faint or environmental noise was high, the system maintained stable measurement frequency and high precision. This success proves that the Shack-Hartmann sensor possesses exceptional environmental adaptability in non-point source scenarios.

The practical significance of this research lies in its diverse application scenarios. In air-to-ground laser communication, this technology can sense atmospheric quality in real-time to optimize data transmission rates. In astronomy, it allows scientists to use planetary surface features as references, obtaining high-definition images even without a bright "guide star." This work lays an important foundation for building more intelligent and efficient adaptive optics systems in the future.