Optimal slit width for high-precision orbital angular momentum measurement using angular double-slit interferometry

We demonstrate an optimization of angular double-slit interferometry for accurate measurement of orbital angular momentum (OAM) of vortex beams.

By scanning the dynamic double slits, the topological charge (TC) magnitude is directly determined from the oscillation frequency of the on-axis intensity.

Based on repeated experimental investigations, we establish a critical criterion for slit width selection: to avoid phase truncation or period overlap, the angular width of each slit must exactly match the spiral phase period 2{\pi}/|l|.

Under this optimal condition, the interference pattern exhibits the highest visibility and the measurement error is minimized.

Experiments for l = 5, 10, and 15 are performed as representative examples, and the universality of this criterion is confirmed.

Furthermore, by introducing an additional phase shift, the sign of the TC is unambiguously determined, as demonstrated for l = 10 and l = -15.

This simple, robust method provides a high-precision pathway for OAM metrology.