Azimuth measurement based on OAM phase spectrum of optical vortices

The optical vortex is a new type of structured light field with a helical wavefront. Because of carrying orbital angular momentum (OAM), the optical vortex can be used to measure the shape and angular orientation of a static object in optical remote sensing. These measurements take advantage of variations in the intensity or phase of the OAM mode. In this paper, we propose a method of target azimuth measurement based on OAM phase spectrum of optical vortices. The OAM phase spectrum consists of helical phase terms in different OAM modes, where the helical phase is obtained by a four-step phase-shifting method. By using the approximate duality relationship between the OAM phase spectrum and the azimuth, we can accurately measure the azimuth of a single target or multiple targets. The resolution of the azimuth measurement is inversely proportional to the OAM scanning range. Experimentally, the target azimuth measurement by employing OAM phase spectrum analysis is realized and the measurement error is less than 2.5 degrees. On this basis, the inverse relationship between the OAM scanning range and azimuth resolution is verified, where the error between experimental and theoretical azimuth resolution is less than 0.3 degrees.