Self-interferometric helical phase resolving of 2 μm vortex beams

Optical vortex beams are characterized by helical phase fronts and phase singularities, whereas vector vortex beams additionally contain spatially varying polarization that can strongly affect interference visibility. This makes the phase topology of directly emitted vector vortex lasers difficult to identify by conventional self-referenced interferometry, especially when imperfect coherence obscures forked dislocations. Here, we demonstrate a Mach-Zehnder self-interferometric phase-retrieval method for 2 μm vector vortex beams. By using controllable lateral displacement, relative phase gradients, and calibrated lens-induced curvature, the inherent phase characteristics of orbital angular momentum (OAM) are revealed through interference fringes. Combined with inverse Fourier phase retrieval and a locally plane or calibrated spherical reference, the magnitude and sign of the OAM topological charge can be determined from the recovered helical phase. Interference simulations further show that transverse displacement redistributes visible phase singularities, while the contour-defined winding number of a specified scalar component remains invariant provided that the component is nonzero on the contour and no zero crosses it. To the best of our knowledge, this is the first experimental resolution of the helical phase of vector vortex beams in the 2 μm band. The method also separates OAM phase winding from polarization-dependent visibility and provides a practical diagnostic route for mid-infrared structured-light sources used in free-space optical links, lidar, and mode-resolved beam characterization.