Twist direction and degree of interference fringes collected in double-slit interference experiment can be used to determine the topological properties of vortex beams, and the modulation depth of interference fringes indicates the spatial coherence of vortex beams. The effects of distance between double slits and the initial coherence length of the beam on the visibility and the twist degree of interference fringes in the partially coherent vortex beam topological charge interference measurement are analysed by adjusting the double-slit distance and the coherence length of the partially coherent vortex beam under near-field conditions. The study found that the interference fringes are distorted with the increase of the double slit distance, but the interference fringes and their distortion are not obvious when the initial coherence length of the partially coherent vortex beam is shorter than the beam width. Visible interference fringes will appear when the initial coherence length is longer than the beam width and the twist degree of interference fringes also increase. The coherence length has little effect on the visibility and distortion of the interference fringes when the initial coherence is greater than the size of the light spot on cross-section where the double slits are located, but the distance between the double slits still affects the observation to the visibility of interference fringes and distortion phenomenon. The research results may be useful for the measurement of orbital angular momentum of partially coherent vortex beams.
Spatial coherence of optical fields determines the propagation property of a light beam. In this paper, we study the atmospheric turbulence effect on ultraviolet(UV) beams. The degree of coherence of the UV beam is modelled as a fractional multi-Gaussian correlated Schell function with rotational symmetry. Results show that the spectral density of the UV beam can retain its shape over 1 km in weak and moderate atmosphere turbulence. The beam spread and slow decay of spectral density distribution of the UV would be beneficial for non-line-of-sight UV communications.