Measurement of the orbital angular momentum spectrum of light through turbid water using modal decomposition

Laser beams carrying orbital angular momentum (OAM) in underwater environments have been a topic of research for underwater communications and remote sensing applications. When a laser beam propagates through turbid water, the dominant form of attenuation and spatial dispersion is scattering due to small particles. The goal of this experimental study is to measure the transmitted OAM mode and its intermodal crosstalk via measurements of the OAM spectrum after propagation through turbid water. An initial beam is encoded with a single OAM state using a spiral phase hologram displayed on a high-resolution spatial light modulator. The optical receiver performs a phase cancellation measurement to decode the OAM on the incident beam. After recording images of the phase canceled beam, the OAM spectrum is found in post-processing. Three methods of post-processing are presented and compared to account for beam wander and an astigmatism in the experiment. After determining which method of post-processing gives the most accurate results, our results are compared to those in the literature. Our results show that an OAM beam maintains mode purity up to an optical depth (OD) of 12, whereas previous literature saw a loss of mode purity at an OD of 6. This is attributed to differences in receiver field of view, scattering volume, scattering length, and beam size.