Cross-phase-based detection of fractional vortex over a broad measurement range

Orbital angular momentum (OAM), one fundamental physical degree of freedom of light, is carried by optical vortices. Their theoretically infinite eigenstates have fueled extensive research, yet topological charge (TC) measurement remains a core and indispensable prerequisite for the practical application of optical vortices. Compared with conventional integer-order vortices, fractional vortices (FVs) exhibit far more complex spatial profiles, which introduce substantial difficulties for their TC measurement. To address this issue, we propose an FV detection method based on an improved second-order cross-phase. By utilizing the proposed cross-phase, the FV is decomposed into misaligned bright and dark stripes, wherein the inherent topological information embedded in the FV is mapped into regular, intuitively readable optical field intensity distributions. Experimental results confirm that FVs spanning TC values from -51 to +51 (a total 102-order dynamic range) can be quantified with a maximum absolute error below 0.04. This work provides a convenient approach for wide-range FV identification, offering prospects for FV-based optical communication and high-dimensional quantum information processing.