From Total Internal Reflection to SiC Waveguides: How AR Glasses Break Through Display Optics Bottlenecks

One of the core technical barriers for AR glasses is not merely the micro-display itself, but rather how to use optical waveguides to achieve a high-quality overlay of virtual images onto the real world within transparent lenses.

Optical waveguides must transmit light from one end to the other within a thickness of just a few millimeters, while simultaneously ensuring uniform light output and precise delivery into the human eye. This is not simply an "extension of the screen," but a feat of optical engineering requiring precise control over light propagation—a process underpinned by ongoing exploration into materials, diffraction, polarization, and complex optical algorithms.

Total Internal Reflection: The fundamental principle of light transmission in waveguides.

Why can an ordinary, transparent flat sheet transport light from one end to the other without any leaking out? The answer lies in a principle found in high school physics textbooks: Total Internal Reflection (TIR).

When light travels from a medium with a higher refractive index to one with a lower refractive index and the angle of incidence exceeds the critical angle, the light does not refract out across the interface; instead, it undergoes total internal reflection and is reflected back into the high-refractive-index medium.

AR optical waveguides utilize this very principle: an image beam emitted by a micro-display is collimated by a projection system and enters the waveguide through an input coupling region at one end; it propagates within the waveguide via total internal reflection and is finally extracted through an output coupling structure at the other end, directing the light into the user's eye. The difference in refractive index between the waveguide substrate and the surrounding air confines the light, allowing it to propagate within the thin sheet of glass or resin.

However, total internal reflection imposes a significant constraint: light propagation within the waveguide is not arbitrary; only light meeting specific angular conditions can propagate stably. If light falls outside the supported range, leakage may occur. This propagation limitation directly affects the Field of View (FOV) of AR glasses—that is, the angular range over which the virtual image covers the user's field of vision.This is a physical limit that can be precisely described by a formula. Assuming a single micro-projector couples a light beam into a waveguide—where the refractive index of the waveguide medium is n and that of air is 1—the maximum supported field of view for the single micro-projector satisfies the following:

When the refractive index (n) is 1.5, the theoretical field of view (FOV) is around 30°; at n = 1.75, the theoretical range increases to approximately 48°; and as n approaches 2, it can theoretically expand further to about 60°. This is a key reason why the FOV of AR glasses has generally remained in the 30°–40° range for years: beyond optical architecture constraints, the material's refractive index is a critical factor influencing FOV expansion.

It should be noted, however, that the FOV in an actual AR system is not determined solely by the refractive index; it is also influenced by a combination of factors, including the numerical aperture (NA) of the micro-display, the design of the optical engine, the waveguide structure, the pupil expansion method, and optical uniformity.

The natural field of view (FOV) of a single human eye is approximately 160° × 130°, with a binocular horizontal overlap exceeding 120°. Current AR systems, with FOVs in the range of only a few tens of degrees, still fall significantly short of the human eye's natural field of view, leaving room for improvement in the coverage of virtual imagery. Consequently, exploring materials with higher refractive indices, optimizing waveguide structures, and enhancing system efficiency have become key directions in the development of AR optics.

Silicon Carbide: New Explorations into High-Refractive-Index Materials

In recent years, silicon carbide (SiC) has emerged as a high-refractive-index material of significant interest in the field of AR optics. With a refractive index reaching approximately 2.6–2.7, it offers distinct advantages over traditional optical glass and resin materials. Theoretically, a higher refractive index expands the angular range of light propagation supported by the waveguide, providing a new material option for AR displays with wide fields of view.Research and validation efforts regarding waveguide solutions based on high-refractive-index materials are also steadily advancing. For instance, the Meta Orion prototype demonstrated the potential for achieving wide fields of view using SiC waveguides, highlighting the promise of high-refractive-index materials in the AR display sector.

However, high-refractive-index materials are not the sole solution to every optical challenge. For diffractive optical waveguides, a long-standing issue is the appearance of rainbow artifacts: when ambient light (such as sunlight) strikes the diffraction grating on the waveguide surface, light of different wavelengths undergoes varying degrees of dispersion, resulting in colored fringes superimposed on the field of view. High-refractive-index materials can alter light propagation characteristics within the waveguide and facilitate the optimization of diffractive structure designs, thereby mitigating the impact of rainbow artifacts.

A team from Westlake University has demonstrated research findings on diffractive waveguides based on SiC materials, achieving progress in miniaturization, high optical efficiency, and the mitigation of rainbow artifacts, thereby opening up new avenues for the development of high-refractive-index waveguides.

Furthermore, SiC possesses high thermal conductivity. While the thermal conductivity of ideal single-crystal SiC can reach approximately 490 W/(m·K)—a figure typically reduced in fabricated waveguide devices due to factors such as lattice defects, processing techniques, and material structure—it remains significantly higher than that of conventional glass. This implies that SiC offers distinct thermal management advantages for high-brightness AR display systems, helping to enhance overall system stability.

However, high refractive index also presents manufacturing challenges. Due to the high hardness of SiC, traditional machining methods struggle to meet the requirements for fabricating complex nanostructures. Applications involving diffractive waveguides necessitate processes such as high-precision etching to create nanoscale structures on the material's surface. Currently, SiC waveguides are transitioning from experimental validation toward industrialization; issues regarding cost, processing efficiency, and mass production capabilities remain to be addressed.

From "Line" to "Surface": A Dimensional Leap in Exit Pupil Expansion

Once light enters the waveguide, the next challenge is to output it uniformly while covering the range of human eye movement. The exit pupil of the light beam generated by a micro-display is typically only a few millimeters in size; however, since the eye moves naturally during use, the limited exit pupil must be expanded to create an "eyebox" area suitable for viewing. This process is known as Exit Pupil Expansion (EPE).

Geometric Optical Waveguides: Precision Design of Mirror Arrays

Geometric optical waveguides utilize an array of embedded semi-reflective mirrors to progressively distribute light energy across multiple reflective surfaces. As light encounters each reflective surface, a portion of the energy is reflected out of the waveguide and into the user's eye, while the remainder continues to propagate within the waveguide. Precise control over the transmission-to-reflection ratios of these surfaces enables beam expansion along a single dimension.

More advanced two-dimensional array geometric waveguides incorporate two sets of beam-splitting surface arrays oriented in different directions to achieve light expansion in both horizontal and vertical dimensions; this expands the "eye box" in two dimensions, thereby increasing the tolerance for headset positioning. Certain 2D array waveguide designs can achieve a field of view spanning tens of degrees while maintaining a thickness in the millimeter range through structural optimization.

Geometric waveguides typically offer superior imaging quality and color performance, along with a lower risk of rainbow artifacts. However, their multi-layer semi-reflective/semi-transmissive structures impose stringent requirements regarding coating precision, structural alignment, and manufacturing consistency; consequently, process complexity and production costs remain areas for ongoing optimization.

Diffractive Optical Waveguides and Surface-Relief Gratings: Nanoscale Light-Control Technology

Diffractive optical waveguides employ a completely different approach: instead of embedding reflectors within the waveguide, nanoscale periodic structures—specifically, surface relief gratings (SRGs)—are fabricated on the waveguide surface or within its structure. SRGs are typically created using processes such as electron-beam lithography, nanoimprint lithography, or ion-beam etching to form physical grating patterns on the waveguide surface, with characteristic dimensions ranging from tens to hundreds of nanometers.

The core operating principle of the SRG can be described by the grating equation:

The figure below shows a schematic of the structure of a typical slanted grating in the OAS software; by designing the grating period, duty cycle, depth, and slant angle, the diffraction efficiency for different wavelengths and diffraction orders can be precisely controlled.

In AR waveguides, an SRG typically comprises three main functional regions:

· In-coupling grating: Responsible for coupling the collimated light beam emitted by the micro-projection engine into the waveguide, ensuring it meets the conditions for total internal reflection propagation.

· Turning grating (expansion grating): Changes the direction of light propagation—typically redirecting horizontally propagating light to a vertical direction—to achieve one-dimensional pupil expansion.

· Out-coupling grating: Uniformly extracts light from the waveguide and directs it into the user's eye to produce the display.

This "three-grating" architecture enables diffractive waveguides to achieve two-dimensional exit pupil expansion with a relatively simple structure while maintaining a slim optical profile, making it a key technological direction for consumer-grade AR glasses. Compared to geometric waveguides, diffractive waveguides offer advantages such as a lightweight, thin form factor and greater design flexibility; some designs can achieve thicknesses in the millimeter or even sub-millimeter range.

SRG Design and Optimization Tools: Because the dimensions of the grating structures are comparable to the wavelength of light, traditional geometric optics methods cannot accurately describe their optical behavior; consequently, electromagnetic simulation methods such as Rigorous Coupled-Wave Analysis (RCWA) are required. RCWA operates by dividing the grating region into multiple thin layers and using Fourier expansion to describe the electromagnetic field distribution; it then solves Maxwell's equations to calculate the efficiency and polarization response of various diffraction orders.

By integrating algorithms such as Particle Swarm Optimization (PSO), engineers can identify superior designs within a parameter space defined by variables such as grating period, depth, duty cycle, and slant angle. For instance, in the case of S-polarized light at a wavelength of 550 nm, parameter optimization can boost diffraction efficiency from approximately 56% to over 94.7%.

Key advantages of SRG:

• Ultra-simple structure with a thickness as low as under 0.5 mm;

• High design flexibility, allowing for the control of beam angle, uniformity, and polarization through grating morphology;

• Suitable for low-cost mass production using nano-imprint technology (e.g., for consumer-grade AR glasses).

Inherent challenges of SRGs:

• Rainbow artifacts: Dispersion occurs when ambient light (such as sunlight) strikes the surface grating, producing colored fringes. High-refractive-index substrates (e.g., SiC) can suppress this effect.

• Dispersion: Dispersion occurs when ambient light (such as sunlight) strikes the surface grating, producing colored fringes. High-refractive-index substrates (e.g., SiC) can suppress this effect.

• Low optical efficiency: Single-pass diffraction efficiency is typically 10%–30%; overall efficiency requires improvement through grating profile optimization or cascaded designs.

• Polarization sensitivity: SRGs exhibit significantly different responses to different polarization states, necessitating the use of polarized light sources or polarization conversion devices.

Therefore, the development of diffractive optical waveguides focuses not merely on achieving thinner structures, but on striking a better balance among optical efficiency, uniformity, field of view (FOV), and environmental adaptability.

The Physical Balance Between Efficiency and Uniformity

Diffractive optical waveguides have long faced a fundamental conflict: enhancing display brightness requires increasing grating coupling efficiency; however, higher efficiency can lead to increased parasitic diffraction and energy leakage—particularly with wide fields of view—thereby compromising display uniformity. Conversely, prioritizing uniformity to an excessive degree may come at the cost of overall optical efficiency. This conflict between efficiency and uniformity stems from the passive response of conventional gratings to polarization states and propagation paths.

In recent years, polarization volume holographic gratings (PVGs) have offered a new avenue for optimization. Unlike surface-relief gratings, PVGs utilize a periodic refractive-index structure formed within the material via holographic interference to modulate light propagation. Research has shown that, under specific conditions, PVGs can leverage polarization conversion mechanisms to enhance light propagation efficiency and reduce energy losses caused by certain parasitic diffraction effects. Among these, the anomalous polarization conversion (APC) mechanism has attracted significant attention.

By actively modulating the polarization state of light to direct more optical energy along the target path, this mechanism holds the potential to enhance coupling efficiency and brightness uniformity. However, technologies such as PVG and APC remain under active research; their large-scale practical application requires further validation regarding material systems, manufacturing processes, and cost-effectiveness.

Micro-display and System Integration: Overall Balance of the Optical System

Optical waveguides serve as "conduits," while the optical engine at the front end—the micro-display—determines brightness, contrast, and color. Currently, the primary micro-display technology pathways being explored in the field of AR near-eye displays include Micro-OLED, LCoS/DLP, and Micro LED.

Micro-OLED: A relatively mature near-eye display solution

It offers extremely high contrast (in the millions-to-one range) and the highest level of maturity. Typical luminance at the eye ranges from 1,000 to 1,250 nits, providing basic readability in moderately bright environments, though performance remains insufficient in strong outdoor light. While some samples can achieve peak brightness levels of tens of thousands of nits, power consumption and lifespan are limiting factors.

LCoS/DLP: High-brightness approach, but with challenges regarding miniaturization.

Capable of achieving brightness levels of tens of thousands of nits, these systems feature bulky optical engines (4–6 cc) and high thermal management requirements, making it difficult to achieve a slim, lightweight form factor.

Micro LED: A Key Candidate for Future AR Displays

With a theoretical brightness exceeding 100,000 nits, a volume of less than 0.5 cc, and a lifespan surpassing 100,000 hours—alongside superior compatibility with SiC waveguides—it is regarded as the ultimate solution. However, the yield of mass transfer for full-color displays remains the primary engineering bottleneck, and current capabilities do not yet meet the requirements for consumer-grade mass production.

Moreover, the trade-offs between field of view (FOV), eye box, waveguide thickness, coupling efficiency, and power consumption make AR optical design akin to solving an optimization problem within a high-dimensional parameter space. This explains why AR glasses that truly meet consumer-grade requirements have yet to see widespread adoption: the challenge lies not in the feasibility of any single technology, but in achieving a superior level of holistic balance across the entire system.

Synergistic advancement of AR optics through multiple technical pathways

AR optical waveguide technology is currently undergoing continuous evolution. On one hand, high-refractive-index materials offer new avenues for expanding the field of view and optimizing optical performance; on the other, technologies such as PVG and polarization control are addressing the trade-off between optical efficiency and uniformity in diffractive waveguides. Meanwhile, the development of emerging micro-display technologies like MicroLED will further drive the evolution of AR systems toward higher brightness and more compact form factors.

Different application scenarios impose varying requirements regarding field of view, weight, cost, battery life, and display quality; consequently, the future AR eyewear market is likely to see the coexistence of multiple technical solutions.

From total internal reflection to high-refractive-index materials, and from geometric reflection structures and diffractive gratings to polarization volume holographic technology, the evolution of optical waveguides is essentially a continuous quest to control light propagation with greater precision. Each technological advancement represents a breakthrough in the boundaries of material properties, optical design, and manufacturing processes.

In the future, as optical systems, micro-display technologies, and computing power further converge, AR glasses are poised to move closer to the goal of being more natural, lightweight, and suitable for all-day use. Optical waveguides will remain a critical core technology determining the display experience of AR glasses.

Source: Compiled by Wuhan Eryuan Technology and Aibang Zhizao