Research Progress of Waveguide Micro Projection Light Engines in Augmented Reality Applications (Invited)

Significance With the rapid advancement of augmented reality (AR) technologies, human-computer interaction has undergone a profound transformation. Near-eye display systems have gradually become the core interface for immersive digital experiences, placing unprecedented demands on their optical design. A central challenge lies in how virtual imagery can be accurately and comfortably superimposed onto the real world. Among the many optical approaches explored in recent years, waveguide-based display systems have gained broad acceptance in portable and consumer-oriented AR devices. This trend is largely attributed to their slim profile, high see-through capability, and relatively large eye-box, all of which are essential for creating lightweight, socially acceptable smart glasses. Nevertheless, the performance of an AR waveguide system is determined not only by the waveguide itself, but also by the overall optimization of the micro-projection engine that feeds it. Acting as the interface between digital image generation and optical coupling, the projection engine plays a decisive role in system efficiency. A fundamental limitation of waveguides is their low light throughput: due to repeated expansion and out-coupling processes, less than 1% of the injected light typically reaches the viewer’s eye. As a result, the projection engine must generate extremely high source luminance—often on the order of hundreds of thousands to millions of nits—to ensure adequate visibility, particularly in outdoor environments. In addition to brightness, the projection engine must satisfy stringent constraints on size and power consumption. It must be compact enough to be integrated into the temple of eyeglass-like devices, while also operating efficiently to support extended, all-day use. These competing requirements make the micro-projection engine one of the most critical bottlenecks in advancing AR systems from experimental prototypes toward truly consumer-ready products. Progress As of 2025, the development of micro-projection engines is marked by parallel progress across several competing display technologies, each advancing through incremental but meaningful innovations. Silicon-based organic light emitting diode (Micro-OLED) displays have remained attractive due to their self-emissive operation, offering high contrast ratios, wide color gamuts, and fast response time. Historically, their adoption in waveguide systems was constrained by limited peak brightness. Recent work, however, has mitigated this issue. For example, Sony’s integration of microlens arrays has increased peak luminance from approximately 1600 cd/m² to around 5000 cd/m², while simultaneously tailoring the emission angular distribution to better match waveguide coupling requirements. Commercial implementations by companies such as INMO and NED+ further demonstrate that, when combined with efficient one-dimensional geometric waveguides, Micro-OLED engines can deliver high-quality imagery in extremely lightweight modules, some weighing as little as 1.7 g. Liquid crystal on silicon (LCoS) technology continues to play a central role in AR displays, largely due to its high achievable resolution—from VGA up to 4K—and its mature manufacturing ecosystem. Because LCoS devices rely on external illumination, recent research has focused on reducing the size and complexity of the illumination subsystem. Front-lit LCoS designs employing bidirectional side-mounted LED arrays and micro-mirror structures have achieved luminance levels exceeding 350000 cd/m² within volumes smaller than 1 cm³. In parallel, photonic integrated circuits (PICs) have enabled flat-panel laser display architectures that replace traditional free-space optics with chip-scale solutions, reducing engine volume by more than 80% while improving color performance. To address the well-known “gray box” artifact caused by stray light in LCoS systems, zonal illumination strategies have been introduced, selectively activating LEDs only in image-bearing regions. This approach not only improves contrast but also lowers overall power consumption. Micro-LED displays are widely viewed as a long-term solution for AR projection, primarily due to their exceptional brightness and inherent durability. Progress in this field has focused on efficient color generation and optical shaping. One example is the X-cube polychrome architecture adopted in JBD’s “Hummingbird II” engine, which delivers a full-color output of approximately 3 lm in a package measuring only 0.2 cm³. Other studies explore quantum-dot-based color conversion as well as metasurface integration to reshape the native Lambertian emission of Micro-LEDs into more collimated beams, thereby increasing usable on-axis intensity. Notably, research from Fuzhou University has demonstrated a retinal projection approach that combines Micro-LED arrays with flexible fiber bundles, enabling waterproof operation and an extended depth of field while maintaining high resolution. Beyond these technologies, digital light processing (DLP) and laser beam scanning (LBS) offer alternative high-performance solutions. DLP systems, based on digital micromirror devices (DMDs), are known for their high optical efficiency and fast refresh rates. Recent reference designs have incorporated freeform optics to achieve wide fields of view—up to 50 degrees—without sacrificing compactness. LBS systems, as seen in devices such as HoloLens 2 and emerging modules from TriLite and OQmented, employ MEMS scanning mirrors to raster laser beams directly onto the retina. This method is inherently focus-free and provides some of the widest color gamuts available in current AR displays. Conclusions and Prospects Overall, micro-projection engines for AR have entered a stage where multiple technological pathways coexist, each addressing different performance priorities. LCoS remains a practical choice for many consumer products due to its resolution advantages and cost efficiency, while Micro-LED technology is expected to dominate high-end applications once full-color manufacturing challenges are resolved. DLP and LBS systems will likely continue to serve specialized use cases that demand large dynamic range, compactness, or focus-free operation. Looking ahead, progress in this field will increasingly depend on the integration of device-level advances with system-level optical design. Technologies such as metasurface for wavefront manipulation and freeform optics for folded optical paths are expected to play a key role in overcoming current limitations in resolution, field of view, and efficiency. As AR moves toward large-scale commercialization, future research must balance performance improvements with manufacturability, cost, and thermal management. Stronger collaboration between academic research and industrial development will be essential in accelerating the transition from laboratory demonstrations to practical, all-day wearable AR glasses, ultimately shaping the next generation of digital visual interaction.