How Does Diffractive Waveguide Technology Work in AR Glasses Display

In our previous article, "What is AR Diffractive Waveguide?", we explained the fundamental principles of diffractive waveguides and highlighted the differences between surface relief grating waveguides and volume holographic grating waveguides. Today, we will delve deeper into the core functions and optimization directions of diffractive waveguides, discussing why diffractive waveguides based on surface relief gratings are emerging as the mainstream display technology for AR glasses.

01 Core Functions of Diffractive Waveguides

1. Image Isometric Transfer

From our earlier articles, we know that for a diffractive waveguide to direct light emitted from a micro-projection system (optical engine) into the human eye, it must undergo coupling in and coupling out processes. Specifically, light emitted by the optical engine enters the flat waveguide through the coupling grating, propagates within it by total internal reflection, and is finally transmitted to the eye by the coupling out grating.

The most crucial aspect of this process is total internal reflection. But what exactly is total internal reflection?

Total internal reflection occurs when light travels from a medium with a higher refractive index to one with a lower refractive index, and the angle of incidence is greater than or equal to the critical angle. When the conditions for total internal reflection are met, light will continuously propagate through the flat waveguide by reflection without being transmitted out, thereby allowing the direction of light to be altered. A well-known natural phenomenon resulting from total internal reflection is the mirage.

Typically, AR glasses output images using an optical engine. However, placing the optical engine directly on the lens would obstruct the user's view and be visually unappealing. Moreover, relying solely on the optical engine would not achieve the desired effect of merging virtual and real images.

Leveraging the principle of total internal reflection, diffractive waveguides can perform isometric transfer of the images projected by the optical engine, allowing the optical engine to be positioned at the top or side of the glasses. This approach not only avoids obstructing the user’s line of sight but also, due to the high light transmission rate and thin profile of the diffractive waveguide, brings AR glasses closer in appearance to regular eyewear while achieving the desired effect of virtual-real integration.

It’s important to note that the diffractive waveguide is responsible solely for transferring the image to the eye and does not affect the content of the image itself, which means it does not have the capability to magnify or reduce the image size.

2. Two-Dimensional Pupil Expansion

Standard optical display solutions typically lack pupil expansion capabilities, limiting the viewer to see images only within the range of the optical engine's exit pupil size (i.e., eye movement range). For instance, if the exit pupil of the optical engine measures φ5mm, the user can only view the image within a φ5mm range. This is akin to looking at the world through a peephole, which significantly diminishes immersion and visual experience.

To address this issue, diffractive waveguides can achieve two-dimensional pupil expansion, enlarging the exit pupil while maintaining a compact size and a wide field of view. This effectively increases the eye movement range in both directions, providing a heightened sense of immersion and an improved visual experience, while also accommodating different interpupillary distances. This represents the second core function of diffractive waveguides.

There are generally two approaches to implement two-dimensional pupil expansion. The first involves using three one-dimensional gratings (i.e., coupling grating, bending grating, and coupling out grating). The second approach employs one one-dimensional grating (coupling grating) and one two-dimensional grating (coupling out grating).

In the first approach, light emitted from the optical engine is coupled into the waveguide through the coupling grating. The light then undergoes total internal reflection and strikes the bending grating, where a portion of the light is redirected to the coupling out grating, while the remaining light continues to propagate forward through reflection. This light will again hit the bending grating, and another portion will be redirected to the coupling out grating. This process is repeated to achieve one-dimensional pupil expansion.

Finally, the light reaching the coupling out grating will have some of it diffracted into the eye, while the remaining light continues to propagate forward through reflection, again interacting with the coupling out grating. This process results in another direction of one-dimensional pupil expansion. When these two one-dimensional expansions are combined, they create a two-dimensional pupil expansion.

In the second approach, the process also begins by coupling the light emitted from the optical engine into the waveguide using the coupling grating. The light then undergoes total internal reflection and strikes the two-dimensional coupling out grating. At this point, a portion of the light is diffracted into the eye, while the remaining light is divided and continues to propagate forward through reflection in both horizontal and vertical directions.

The light will then again interact with the coupling out grating, where another portion is diffracted into the eye. This process is repeated, effectively achieving two-dimensional pupil expansion.

The above describes the physical processes of the two-dimensional pupil expansion schemes. In comparison, the first scheme is relatively simpler in terms of the design and fabrication of the diffractive waveguide, but it occupies more overall lens area. The second scheme, on the other hand, requires the use of two-dimensional gratings, which are more complex to design and manufacture, making it more challenging to implement. However, this approach results in a more compact overall structure, allowing for a reduction in lens area.

By employing two-dimensional pupil expansion, we can not only increase the eye movement range and enhance user immersion but also reduce the weight and dimensions of the optical engine in both horizontal and vertical directions, making AR glasses lighter and more adaptable.

It is important to note that while two-dimensional pupil expansion replicates the image multiple times, we actually perceive only one image, not multiple images. This is because the image transmitted by the coupling out grating is not a real image but a virtual one. Additionally, the human brain tends to deceive itself by following the extended line of sight of the light beams it sees. The light beams generated by the pupil expansion correspond to different angles of the same virtual image, so regardless of how many different positions of expanded light beams the eye perceives, they will trace back to the same image based on the extended line of sight.

For example, it is similar to observing a candle through a plane mirror. The light from the candle reflects off the mirror and enters the eye, which then seeks the virtual image based on the extended line of the light rays. The three light rays depicted in the diagram can be understood as the expanded light beams at three different positions in the diffractive waveguide. As shown in the diagram, when we see these three light beams simultaneously, they all point to the same image.

Additionally, there is a common misconception that diffractive waveguides have low energy efficiency. In reality, this perception arises during the process of achieving two-dimensional pupil expansion, where the diffractive waveguide needs to divide the light energy into many parts and distribute it evenly across each exit pupil position. As a result, the energy per unit area is naturally reduced. However, if we were to collect all the light rays from the diffractive waveguide into the eye, we would find that its energy efficiency is actually not low.

Thus, the primary reason for the perceived low energy efficiency of diffractive waveguides is the pupil expansion. However, expansion is a significant feature of diffractive waveguides, and as previously mentioned, it offers numerous advantages. Therefore, it is essential to maximize energy efficiency while maintaining a certain level of pupil expansion.

02 Three Major Optimization Directions for Diffractive Waveguides

1. Optimizing Light Diffraction Efficiency

As mentioned earlier, many people perceive that diffractive waveguides have low energy efficiency. To address this issue, it is necessary to improve energy efficiency by optimizing the diffraction efficiency of light while maintaining a certain level of pupil expansion.

Since the feature size of nanoscale gratings is comparable to the wavelength of light, it is essential to treat light not as ordinary rays but as electromagnetic waves during propagation. When light strikes the grating, it is split into several different directions (diffraction orders), and inevitably, some of the light energy is lost in the process.

To ensure that the majority of light energy is coupled into the diffractive waveguide, we typically select a specific non-zero diffraction order (that meets total internal reflection conditions) as the working order of the diffractive waveguide. By precisely controlling parameters such as the grating period, duty cycle, groove depth, and sidewall angle, we can optimize the diffraction efficiency of light, concentrating most of the light energy into this working order. This, in turn, enhances energy efficiency and allows for increased image brightness.

2. Optimizing Diffraction Efficiency for Different Incident Angles

Another important factor to consider when optimizing the grating is the impact of the light's incident angle on diffraction efficiency.

Since the image projected by the optical engine forms a light surface, light from different positions on this surface enters the diffractive waveguide at varying angles. For diffractive waveguides, different incident angles result in different diffraction efficiencies, leading to inconsistencies in the overall brightness of the image.

Therefore, in addition to optimizing the diffraction efficiency for a specific diffraction order, it is also essential to optimize the diffraction efficiency for light at various incident angles to ensure uniform brightness.

3. Optimizing Diffraction Efficiency for Different Wavelengths

Different colors of light have varying wavelengths, which affects their diffraction efficiency. Additionally, the different wavelengths result in different diffraction angles, meaning that during the pupil expansion process, the interaction frequency of different colors of light with the coupling out grating will also vary. These two factors make it challenging for each color of light to enter the eye with equal energy proportions, resulting in issues with color uniformity. Thus, achieving good color uniformity in images using a single-layer diffractive waveguide is difficult.

To ensure that light of different wavelengths exits with equal energy proportions, a multi-layer (two layers or more) stacking of diffractive waveguides is typically employed. Each layer of the diffractive waveguide is optimized to control and enhance the energy for a specific wavelength range while also suppressing cross-talk between the layers. This approach ensures that light of different wavelengths ultimately enters the eye with equal energy proportions, improving color uniformity and displaying normal, vibrant images.

03 Summary

On one hand, diffractive waveguides have two core functions: image isometric transfer and two-dimensional pupil expansion. Based on these functions, they enable AR glasses to be lightweight and slim while accommodating a broader range of users, providing a strong sense of immersion and an excellent visual experience. Additionally, the integration of semiconductor processes enhances the manufacturability of diffractive waveguides, laying a solid foundation for AR glasses to enter the consumer market.

On the other hand, as the mainstream display technology for AR glasses, diffractive waveguides offer great potential but also present significant complexity. Optimization of diffraction efficiency must be considered from multiple aspects, including diffraction orders, incident angles, and wavelengths.

With continuous advancements in technology and further performance optimization, AR diffractive waveguides are poised to bring AR glasses into households, shining brightly in the era of the metaverse.