Measurement of the orbital angular momentum spectrum of light through artificial turbulence by interferometry

Abstract

The creation and detection of light carrying orbital angular momentum (OAM) has been of great interest for applications that require a beam to propagate through atmospheric turbulence such as for free-space optical communications and remote sensing. In this experiment, Laguerre-Gaussian (LG) beams are created using a high-resolution deformable micromirror device (DMD) and then propagated through two artificial turbulence strengths. To measure the OAM encoded on the LG beams, the wavefront is decomposed into an orthogonal basis set denoted as the OAM spectrum. The OAM spectrum is measured using two forms of Mach-Zehnder interferometers (MZI). The first interferometer is a modified MZI that is used to measures the OAM spectrum by first measuring the angular correlation function with a single interferogram. The second interferometer is a traditional MZI used to record an interferogram that can then be processed to extract the phase of the LG beam. The measured phase is then used to find the OAM spectrum by applying modal decomposition. To improve the OAM spectrum measurement, a singularity tracking algorithm is used to correct for the turbulence distortions. Each interferometric technique is compared with and without the presence of artificial turbulence. Both interferometric methods were significantly affected by turbulence, but the traditional MZI was able to measure the spreading of the OAM spectrum well with the support of a singularity tracking algorithm.