Theta-modulated generation of chromatic orbital angular momentum beams from a white-light source

We demonstrate a simple experiment to produce chromatic orbital angular momentum (OAM) beams from a white-light source based on theta-modulation. The array of pixels of a single spatial light modulator (SLM) is divided into four sub-domains, each of which individually displays a holographic grating both for producing the desired OAM beam and for modulating the orientation of the blazed grating. In the Fourier plane of a 4f optical system, a tailor-made spatial filter is employed for the coloration of various OAM beams with different colors. In the imaging plane, we record the generated chromatic OAM beams of different topological charges as well as their superpositions, which appear as mixtures of red, green and blue (RGB) in different proportions. Our work may find direct application in optical information processing and in free-space optical communications.