Wavefront detection method for orbital angular momentum modes based on conformal mapping-spatial phase shifting interferometry

The orbital angular momentum (OAM) of light has garnered significant interest in recent years owing to its various applications, and extensive creative research has been conducted to generate OAM. However, the particular helical phase structure of an optical vortex leads to non-smooth and discontinuous phase profiles and hinders the accurate recovery of the phase distribution of the vortex beam. Significantly, the existence of a wavefront dislocation leads to the failure of the traditional phase unwrapping algorithm. At the same time, it is essential to detect the wavefront of OAM modes in real-time for free-space optical communication and optical precision measurement. Therefore, we designed conformal mapping-spatial phase-shifting interferometry and achieved rapid and high-precision wavefront measurements for the OAM modes. The wavefront of the OAM modes with a topological charge of 1,2,4 and 6 were measured, respectively. The results were significantly consistent with the anticipated results based on simulations. This study revealed the mathematical mechanism behind the forked fringe patterns and presented a method for demodulating the helical wavefront from the forked fringe patterns.