Digital Decomposition Technique for Analyzing the Purity in Orbital Angular Momentum (OAM) Modes

Light beams carrying Orbital Angular Momentum (OAM) are highly versatile and have many applications, especially in free-space optical communication. However, it is critical to accurately estimate the OAM spectrum to ensure optimal performance, i.e., the power distribution among the many OAM modes. When the modal decomposition technique fails due to a perturbed experimental setup, such as when the generated OAM mode and correlation filter are misaligned, a single-shot digital approach can be employed to derive the OAM spectrum. This method involves interfering with the perturbed light with a reference light, typically a plane wave, and then digitally processing and evaluating the resulting interference pattern to obtain the OAM spectrum. In our study, we utilized this single-shot digital technique to assess the purity of the generated OAM mode. Factors such as the optical aberrations e.g. astigmatism caused by the optical component, the misaligned fork grating generated on the SLM, and the laser-SLM alignment can all impact the purity of the generated OAM mode. Any perturbation will result in an OAM spectrum, which the single-shot approach can reliably detect.