Discrimination of vortex and pseudovortex beams with a triangular optical cavity

A triangular optical cavity can be used to distinguish between two beams with the same intensity profile but different wavefronts. This is what we show in this paper, both theoretically and experimentally, in the case of beams with a doughnut-like intensity profile: one of them having a helical wavefront (vortex beam with orbital angular momentum) and the other with no orbital angular momentum at all (which we call pseudovortex beam). We write the mode decomposition of such beams in the Hermite-Gaussian basis and in the Laguerre-Gauss basis, respectively, and study how they interact with a triangular cavity in terms of their resonance peaks. The experimental results corroborate the theoretical predictions, demonstrating that each beam exhibits a distinct resonance pattern. This suggests that such a cavity can be used to identify beams carrying orbital angular momentum, effectively distinguishing them from pseudovortices. Moreover, we propose an experiment where three cavities may be used to filter out the pseudovortex from a superposition of vortex and pseudovortex.