Design and optimization of compact freeform lens array for laser beam splitting: a case study in optimal surface representation

Abstract

We present a theoretical design study concerning compact freeform lens array for laser beam splitting. In our approach beam splitting optical element is designed as a compact element combining both functions of collimation and beam splitting into one component. Main emphasis of our paper is on the design methodology and subsequent theoretical analysis of design approach for such elements using examples of regular and irregular beam splitting elements. We use multi-parameter optimization method in conjunction with custom merit function implemented in ray-tracing software to implement our designs. In our design approach, we introduce a novel pupil sampling method based on Fibonacci grid. First, we deal with examples using regular lens arrays designed independently. Next, we evaluate some of standard and readily available freefrom surface representations that can replace usual description of lens array by compact continuous freeform surface. As a final example, we consider example design of irregular beam splitter with potential for control of power distribution between spots in focal plane. Some of designed structures are initially produced by optimization with some discontinuities that need to be removed to enforce smoothness of final design. We demonstrate application of local filtering techniques to produce smoothed versions of surfaces with discontinuities. Possible applications of similar designs are in laser fiber coupling and off-axis multi-spot generation where power splitting ratio can be arbitrarily predefined.