Speckle-reduced holographic projection with a piston-mode spatial light modulator

Abstract

Computer-generated holography, a disruptive technology for projection displays, enables variable projection distance by combining a variable lens hologram with an image hologram and encoding it onto a phase-modulating spatial light modulator. The basic structure of a holographic projector includes a spatial light modulator (SLM) to modulate the phase of an incident reconstruction light beam, i.e., a coherent light source (i.e., LASER). Speckle appears in the projected content due to the quasi-random interference of a coherent light source on an optically rough surface that serves as a projection screen. A partially coherent light source, such as a light emitting diode can be used in holographic projection, but image sharpness is sacrificed. Furthermore, due to the limitation of the phase retrieval algorithm, which calculates the hologram to be encoded on the spatial light modulator from a given graphic, the resulting projected content can look speckly due to phase retrieval algorithm imperfections. In this work, we demonstrate reduction in speckle arising from phase retrieval algorithm imperfections through the use of a piston-mode spatial light modulator. The SLM is a two-dimensional micromirror array with extraordinarily fast switching characteristics that enables the implementation of a temporal averaging method for speckle reduction without additional optical components and keeps the benefits of holographic projection.