Real-time, broadband beam shaping with programmable freeform topologies

Abstract Phase-only spatial light modulators (SLMs) offer versatile beam shaping, but are limited to monochromatic operation due to diffractive dispersion. A potential solution is to implement the smooth wavefront transformation of refractive freeform optics as a phase shift pattern on the SLM, called programmable freeform optics. Unfortunately, current phase-only SLMs can only inflict phase shifts of a few times 2 π . This requires the use of a wrapping procedure that results in discontinuous phase jumps, reintroducing diffractive dispersion. This paper presents a differentiable design method for high-precision, shallow, refractive freeform topologies. These low-relief topologies can be implemented on an SLM, as a phase shift pattern with limited discontinuous phase jumps, allowing full-color beam shaping. This full-color performance is verified in a simulation setting, and is further experimentally analyzed in a setting with a commercial SLM. Differentiable raytracing enables the optimization of depth-constrained freeform surfaces for various source-target settings, but is limited by its computational speed. To address this limitation, an unsupervised learning method is furthermore introduced that predicts freeform topologies with a point source. By wrapping the predicted, resulting phase shifts, this approach enables full-color, real-time beam shaping with a single SLM.