Diffractive waveguides have become an important technology for augmented reality (AR) displays, helping deliver digital imagery through thin, transparent optics. Within this broad category, there is more than one way to control light.
Surface relief grating (SRG) waveguides and holographic waveguides both rely on diffraction, but the structures they use to create that diffraction are very different. Magic Leap focuses on SRG waveguides, an approach that offers extensive control over optical design and a path to scalable manufacturing.
The differences between SRG and holographic waveguides can influence optical performance, material choices, design flexibility, manufacturing processes, and the ability to produce waveguides consistently at scale. Both architectures have valuable applications, but for developers balancing the many demands of advanced AR glasses, SRGs offer a particularly versatile combination of optical control and manufacturability.
Two Approaches to Diffraction
In an SRG waveguide, diffraction is controlled by physical structures patterned onto the surface of the waveguide. These features can be only nanometers in size, with their pitch, depth, angle, and other geometric features carefully designed to control how light enters the waveguide, travels through it, and exits toward the eye.
Holographic waveguides accomplish a similar task using a different type of structure. Volume holographic gratings (VHGs) are typically created by exposing photosensitive material to interfering laser beams. The resulting interference pattern is recorded within the material and acts as a grating that can redirect specific wavelengths and angles of light.
This approach has some attractive characteristics. VHGs can support thin waveguides with excellent transparency and minimal eyeglow. Their optical selectivity can also be useful when a system has tightly defined wavelength and angular requirements.
SRGs give designers a unique set of tools. Because diffraction is controlled through physical grating geometry, engineers can adjust individual features to change how the waveguide interacts with light. For SRGs, specifically, pitch, depth, slant, and feature shape become design variables that can be tuned for the needs of a particular system.
That level of control becomes especially valuable when an AR display has to satisfy several optical requirements at the same time.
Different Strengths, Different Tradeoffs
Brightness, efficiency, color, uniformity, field-of-view, and image quality are interconnected in a waveguide. Improving one can influence another, so successful designs depend on finding a balance that works for the intended application.
Holographic gratings can achieve high diffraction efficiency within the wavelengths and angular ranges for which they are designed. Their selectivity, however, requires careful consideration when designing broadband or full-color systems. Red, green, and blue light interact differently with the grating, adding complexity as designers work to deliver consistent performance across the visible spectrum. If those differences are not adequately managed, a white virtual object within an image might appear warmer or cooler, like different temperature lightbulbs do, or the image's brightness may vary as the viewer's eye scans across the display.
The physical structures of SRGs provide considerable freedom to manage these competing requirements. Engineers can modify grating geometry across different regions of the waveguide, tuning how light is distributed and how efficiently it reaches the eye. This gives designers more options when balancing efficiency, uniformity, color performance, and field-of-view.
That flexibility matters because AR glasses rarely have a single optical priority. A wider field-of-view may need to coexist with a large eyebox. Bright imagery has to be balanced against power consumption. Color and image uniformity must remain consistent as the eye moves across the viewing area. The waveguide also has to work with the selected light engine and fit within the broader optical and mechanical design.
SRGs give engineers a larger design space for working through those constraints. That flexibility is one of the key reasons Magic Leap chose to focus on SRGs, giving our engineers more ways to tune waveguide performance for advanced AR displays.
Why Manufacturing Matters
Optical performance is only part of the equation. For a waveguide design to move beyond individual demonstrations and prototypes, developers also have to consider the materials and processes required to manufacture it repeatedly and economically.
Holographic waveguides depend on photosensitive recording materials and carefully controlled optical exposure processes. Material properties, exposure conditions, and other variables can affect the resulting grating. Maintaining consistent performance can become more difficult as designs grow more complex and production volumes increase.
SRGs follow a manufacturing path that shares important characteristics with processes already used throughout advanced electronics. A precise master template containing the nanoscale grating structures is created first. That pattern can then be replicated across waveguides using processes such as nanoimprint lithography.
This creates an important advantage for scaling production. Once a successful master and manufacturing process have been established, the same precisely defined structures can be reproduced across subsequent waveguides. The approach also allows manufacturers to work with materials and wafer-based processes that can be selected and optimized around performance, cost, and production requirements.
Creating the master template is demanding. Small variations in nanoscale features can change how light diffracts, and those changes can carry through to every waveguide produced from it. Mastering, replication, metrology, and process control therefore have to work closely together.
Magic Leap has spent years building these capabilities. Advanced mastering processes, custom manufacturing equipment, metrology, and wafer-based fabrication allow optical designs to move from simulation into physical waveguides with tight control over the structures that determine performance.
This interdisciplinary approach brings optical design and manufacturing together throughout development. Engineers can fabricate a waveguide, measure its performance, compare those results with simulations, and use the findings to refine the design or manufacturing process. For partners, that provides a development path that considers future manufacturing requirements throughout the waveguide optimization process.
Finding the Best Fit
Holographic waveguides remain an interesting option for applications that can benefit from their transparency, low eyeglow, and ability to efficiently diffract light within specific wavelength and angular ranges. As requirements expand across larger fields-of-view, full-color performance, optical efficiency, system integration, cost, and manufacturing scale, the advantages of SRGs become increasingly compelling.
The physical grating structures used in SRGs give optical engineers extensive control over how light behaves throughout the waveguide. SRG manufacturing also provides a repeatable path from a carefully engineered master to larger-scale replication. Together, those characteristics allow SRGs to address optical performance, material and production considerations, and design flexibility within the same architecture.
Realizing that potential requires expertise across the entire waveguide development process. Optical modeling has to connect with mastering, fabrication, metrology, and system-level design decisions. Magic Leap brings those disciplines together to help partners develop and prototype SRG waveguides around the specific performance, cost, and manufacturing requirements of their AR device programs.
Explore how Magic Leap's waveguide design and manufacturing expertise can support the development of advanced AR waveguides.