Radiation force alignment for accurate orbital angular momentum measurement in acoustic vortex beams

We introduce a hologram-based method to determine the physically optimal axis for measuring the orbital angular momentum (OAM) transferred by acoustic vortex beams to an absorbing target. The axis is aligned with the net radiation force vector obtained from holographically measured pressure field. This alignment eliminates the transverse force component, making the measured torque independent of lateral position and equal to the OAM flux. The method is validated with vortex beams generated by multi-sector phase plates on a planar transducer and significantly improves upon conventional geometric alignment for nonparaxial and aberrated beams.