Design and optimization of compact LCoS projection optics using a freeform PBS prism

The miniaturization of waveguide-based projection engines has emerged as a key goal in the development of augmented reality (AR) optical systems and wearable display technologies. Among the various architectures, Liquid Crystal on Silicon (LCoS) has been widely adopted due to its high resolution and technological maturity. Nonetheless, further reduction of system volume remains challenging, primarily owing to the structural complexity of the illumination and imaging subsystems. In this work, a quantitative étendue analysis was performed based on the Köhler illumination scheme, which is commonly employed in LCoS systems. To overcome the limitations of the conventional polarizing beam splitter (PBS) prism, a freeform PBS prism incorporating multiple optically active surfaces was proposed. Guided by the étendue analysis, a compact initial optical configuration was constructed using the proposed prism, followed by stepwise optimization of the projection optics. In addition, the stray light characteristics of the system were analyzed to evaluate potential optical losses. The resulting projection engine achieves a total volume of 1.37 cm³, provides a 32° field of view and an exit pupil diameter of 4 mm, demonstrating significant potential for integration into compact waveguide-based AR displays.