Enhanced detection of rotational Doppler shift from sunlight

The rotational Doppler effect, for which the frequency shift is proportional to the light's orbital angular momentum $\ell$ and the object's rotational speed ($Δf \propto\ell Ω$), has proven to be a powerful tool for detecting the speed of rotational objects. However, the current detection technique is mainly based on coherent laser sources. There is scarce mention of using partially coherent light sources, let alone sunlight. In this work, we collect sunlight and direct it into the laboratory, where it is modulated into a partially coherent probing source and then realize rotational Doppler shift detection. Our study reveals that in low-light conditions, where background noise is stronger than the signal, the superposition of rotational Doppler signals at different wavelengths can significantly enhance the signal strength and improve the signal-to-noise ratio, enabling accurate measurement of the rotational speed of objects. Our research provides experimental validation for the application of sunlight in rotational Doppler shift detection, demonstrating its potential value for passive remote sensing.