Analyzing Orbital Angular Momentum (OAM) mode via localized beam sampling

Light beams carrying Orbital Angular Momentum (OAM) are of broad interest in fundamental optics and have important applications in communication, sensing, and metrology. Conventional techniques require capturing full-beam spot with large optics, restricting scalability and integration. In this work, we demonstrate that both the order and sign of OAM modes can be identified by sampling only a small part of the beam. Reliable mode discrimination was achieved using only 15-20 % of the beam cross-section. Experiments with l=± 2 agreed with analytical predictions within 1-10 % error, and simulations up to |l|≤ 10 confirmed systematic order-dependent shifts. This localized sampling approach provides a hardware-efficient and scalable pathway for OAM detection in optical communication, metrology, and integrated photonics.