Focal spot of circular coherence source with high azimuthal contrast

Circular coherence sources, which are perfectly coherent along any ring that is concentric to the beam center, have the potential to preserve the spiral phase structures of optical vortices on propagation. Circular coherence sources also demonstrate self-focusing effect, which leads to potential applications, such as lens-less imaging.

In this study, we compare the propagation profile of a coherent beam focused by a lens and that of a circular coherence beam with the same component coherent beam. The intensity distribution of the circular coherent beam appears smoother in the focal area, which shows a lower contrast but wider spatial frequency range. Furthermore, we investigated the propagation profiles of a superposition of LG+4 and LG-4 beams with a beam pattern consisting of petal- shaped beamlets in azimuthal direction. For the coherent beam goes through a lens with a uniform transmittance, the focal spot keeps its original beam pattern. For the circular coherence beam, the focal spots show smoother light distribution in radial direction, however, in azimuthal direction, the contrast is comparable to the coherent beam. The petals of the beam-lets are clear without reduced contrast in between because of the perfect coherence in azimuthal direction. This shows the circular coherence beam can be applied for lens-less imaging without significant reduction of contrast in some cases.