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.

High-Accuracy Surface Profile Measurement Based on the Vortex Phase-Shifting Interferometry

According to the principle of phase-shifting interferometry and spiral phase characteristics of the vortex beam, this article proposes a method for detecting the surface profile of a transparent object, in which the +1 order vortex beam is generated by a spatial light modulator and is taken as the reference light. The influence of the nonlinear phase modulation characteristics of the spatial light modulator on the measurement precision is studied. The results show that nonlinear phase modulation has a great impact on the measurement. Then, the vortex lights with initial phases of 0, π/2, π, and 3π/2 are used to measure the H-type thin film sample based on the Twyman-Green interference system after correcting the nonlinear phase modulation characteristics. The experimental results show that the measurement error of the surface profile to an object with the theoretical value of 20 nm is 1.146 nm, and the feasibility of the optical vortex phase-shifting technique used to measure the surface profile of an object is verified.