Off-axis aperture-based detection of Orbital Angular Momentum (OAM) modes

Beams carrying Orbital Angular Momentum (OAM) gives structured light a degree of freedom, allowing for high-dimensional encoding in quantum information processing, optical communication and sensing. Laguerre-Gaussian (LG) modes are widely used to represent OAM states but their detection typically requires full-beam access and complex interferometric or holographic techniques. In this work, we present a compact and non-interferometric method for determining the OAM mode using an off-axis triangular aperture. Partial sampling of the beam generates distinct diffraction patterns whose intensity peak structure directly reveals the topological charge. The proposed approach enables reliable OAM identification using only a small spatial portion of the beam spot, making it robust against beam truncation and misalignment. This technique is well suited for compact optical receivers and diagnostics in free-space and integrated platforms, with potential applications spanning high-capacity classical links, quantum technologies, and optical sensing systems.