Misalignment Measurement of Orbital Angular Momentum Signal Based on Spectrum Analysis and Image Processing

Misalignment measurement is important in radio frequency orbital angular momentum (RF-OAM)-related communication and radar applications. Misalignment causes distorted vortex wavefront and OAM spectra dispersion, therefore a conventional gradient method cannot be used directly. We propose a misaligned measurement method for RF-vortex beams based on image processing. Theoretical analysis and numerical simulations of the misaligned OAM wavefront demonstrate that the misalignment information can be measured by extracting the features of the OAM mean value distribution and the OAM variance value distribution. It is found in this paper that the extrema of mean OAM distribution and OAM variance distribution correspond to the tilted singularity point and original pivot point, separately. These extrema points can be found by image processing algorithms that recognize round speckles. A proof-of-concept experiment is presented, which employs a 2-D scanning probe to measure a 10°-tilted OAM beam with mode +1. The tilt measurement error is only 0.39°.