Light serves as both a high-fidelity information carrier and an ideal measurement probe for information processing. In singular optics, phase singularities—special "dark regions" in optical fields—are ideal sensing probes due to their extreme sensitivity to variations in physical parameters. However, their practical application in dynamic sensing has long been limited by the speed of singularity detection. Conventionally, the rotational Doppler effect based on orbital angular momentum of vortex beams has been the mainstream method for measuring dynamic rotation. Yet, this approach not only demands strict optical alignment and is prone to failure under off-axis conditions, but also struggles to distinguish rotation direction and becomes ineffective in complex motion scenarios, thus restricting its broader applicability.
In a new paper published in Light: Science & Applications , a team of scientists, led by Professors Jianping Ding and Hui-Tian Wang from National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, China, and co-workers have developed a novel "optical singularity protractor" for high-performance rotation sensing. Moving beyond the traditional approach that relies on the interferometric measurement of composite vortex beams, the team introduced a fundamental shift in sensing strategy: rather than analyzing the full probe beam, they now decode the rotational frequency shift by tracking the dynamic trajectories of phase singularities in the composite vortex probe. This approach extends singularity-based metrology from static position measurements to dynamic rotation measurements. The researchers established the physical relationship between the rotational motion of an object and the velocity of singularities in the probe beam. This enables the direct extraction of the object's rotation speed by measuring the angular displacement of these singularities, functioning like the pointer of a protractor.
To overcome the long-standing challenge of rapidly detecting and tracking the evolution of phase singularities, the team innovatively employed a neuromorphic event sensor. Unlike a conventional camera, this sensor responds asynchronously only to changes in logarithmic light intensity gradients. This event-driven mechanism aligns perfectly with the characteristic of phase singularities, which feature near-zero intensity but extremely high intensity gradients in their vicinity. The sensor efficiently captures the sparse event point cloud generated by the moving singularities. Based on this data, the system reconstructs the time-resolved positional and directional vectors of the singularities with microsecond-scale resolution, ultimately enabling precise extraction of the rotational frequency shift.
The authors highlight the advances and applications of their method, noting that their optical singularity protractor offers three distinct advantages over conventional approaches: “(1) It eliminates the need for stringent optical axis alignment by directly computing the rotation center and speed from the event point cloud of singularity motion; (2) it achieves high-precision, high-purity measurements by leveraging the inherent accuracy and sparsity of event-driven detection; and (3) it demonstrates robust performance in complex scenarios, as validated across varying rotation speeds, off-axis dynamics, and nonlinear directional switching.”
They conclude that this work establishes a novel optical rotation sensing framework based on singularity trajectory analysis and event-driven perception. This paradigm shift moves beyond traditional Doppler-based sensing toward singularity dynamics, exemplifying the deep integration of neuromorphic photonics with topological structured light.
Looking ahead, the team forecasts that this technique “paves a new avenue for high-precision metrology using topological structured light in dynamic scenarios and facilitates the construction of novel singularity-based neuromorphic optical systems,” with potential breakthroughs anticipated in semiconductor metrology, computational imaging, and compact remote sensing.
Light: Science & Applications
Optical Singularity Protractor for Rotating Metrology with Neuromorphic Sensing