What is AR Diffractive Waveguide?

Diffractive waveguide is a mainstream optical display solution for AR glasses. Many AR devices adopt this technology. Why are major AR hardware manufacturers so fond of diffractive waveguides? What exactly is a diffractive waveguide?

01 Definition of Diffractive Waveguide

To gain a deeper understanding, we can break down the term "diffractive waveguide" into two parts: diffraction and waveguide.

Typically, we know that light can propagate in three ways: straight-line propagation, reflection, and refraction. For example, infrared sights, periscopes, and the way a straw looks bent when placed in water are all based on these three principles. Diffraction, on the other hand, is the fourth way that light can propagate.

In the 17th century, the Italian mathematics professor Francesco Grimaldi discovered and coined the term "diffraction," which comes from the Latin word "diffringere," meaning "to break into pieces." This refers to the original direction of wave propagation being "broken" and bent into different directions.

In his experiments, he passed a beam of light through two small openings and onto a screen in a dark room, observing a pattern of light and dark stripes at the edges of the projection. Therefore, diffraction refers to the physical phenomenon in which the direction of wave propagation changes when encountering obstacles or slits.

Due to the fact that noticeable diffraction effects can only be observed when the size of an obstacle or the width of a slit is comparable to or smaller than the wavelength of the wave, it is often difficult to see light diffraction in our daily lives. However, under certain special conditions, we can observe it. For example, the "glory" phenomenon seen in the sky, which appears as a colorful halo around shadows, is a result of sunlight diffracting through small water droplets and ice crystals in the clouds.

Having discussed diffraction, what exactly is a waveguide?

In our world, various types of waves exist, including light waves, sound waves, and electromagnetic waves.

A waveguide is a device that transmits these waves from one location to another. Thus, a light waveguide is a medium or device that guides light waves as they propagate.

With an understanding of both diffraction and waveguides, we can define a diffractive waveguide: simply put, it is a medium that utilizes the diffraction of light to guide light waves as they travel.

Diffractive waveguide

To explain further, a diffractive waveguide is designed to utilize the diffraction properties of gratings to create a "light path," allowing light to propagate along a predetermined route and guiding the light emitted from a micro-projection system into the human eye.

The diffractive grating, an optical element with a periodic structure, is the core component of the diffractive waveguide. Based on the type of grating used, diffractive waveguides can be categorized into two types: surface relief grating waveguides and volume holographic grating waveguides.

02 Surface Relief Grating Waveguide

Surface relief gratings are created by "sculpting" high peaks and low valleys on the surface of a material through processes such as photolithography and etching. This fabrication technique achieves a periodic structure that meets the required optical performance.

These gratings manipulate the light that interacts with them, allowing for effective diffraction and guiding of light within the waveguide. Surface relief gratings are widely used due to their simplicity in fabrication and their ability to be integrated into various optical systems.

SEM Image of Surface Relief Grating

Surface Relief Grating Waveguide to guide the light emitted from a micro-projection system (optical engine) into the human eye, the light must pass through the processes of coupling in and coupling out. Specifically, the light emitted from the optical engine enters the waveguide through the input grating, propagates through total internal reflection within the flat waveguide, and is finally transmitted to the human eye by the output grating. The input and output gratings used here are surface relief gratings.

Due to the nanoscale features of the grating being comparable to the wavelength of light, light should not be considered as ordinary rays but rather treated as electromagnetic waves. When light strikes the grating, it undergoes multi-order diffraction.

For instance, if the optical engine emits monochromatic light (such as green light), this light will be split into several beams traveling in different directions (diffraction orders) upon hitting the input grating. One of these non-zero diffraction orders (e.g., +1 order) will satisfy the total internal reflection condition of the flat waveguide, allowing it to enter and propagate through the waveguide via total internal reflection. This specific diffraction order is referred to as the working order of the diffractive waveguide. By precisely controlling parameters such as the period, duty cycle, groove depth, and sidewall angle of the grating, the majority of the light energy can be concentrated into the working order of the diffractive waveguide, effectively coupling most of the light energy into the waveguide. This process is known as the coupling-in process of the diffractive waveguide.

Correspondingly, when the light that is propagating through total internal reflection within the diffractive waveguide encounters the output grating, it will also produce several diffraction orders. One of these non-zero orders will exit the diffractive waveguide in a specific direction, subsequently entering the human eye. This is known as the coupling-out process of the diffractive waveguide.

If the optical engine emits colored light, in addition to the aforementioned processes, there will be other complexities involved. Due to the varying wavelengths of different colors of light, their diffraction efficiencies will differ as well. Consequently, during propagation, the energy of each color of light may be lost to varying degrees, resulting in dispersion. By optimizing various grating parameters, the grating can precisely control the energy of different wavelengths of light, thereby minimizing dispersion issues and ultimately allowing us to see images with accurate colors.

To address the complex issues of grating diffraction, the company has developed a comprehensive suite of calculation software for different types of gratings, based on the Fourier Modal Method (FMM), which can quickly and accurately compute diffraction problems related to gratings.

Moreover, the company possesses a fully equipped grating master processing center and a complete mass production system for diffractive waveguides, achieving close coordination between design and manufacturing. When designing gratings, considerations can be given to the processing capabilities of the master and the production techniques, allowing for timely adjustments and optimizations when issues arise, resulting in a rapid product iteration cycle.

The diffractive waveguide, which is highly favored by major AR hardware manufacturers, specifically refers to the surface relief grating waveguide. Its advantages include a slim design, large field of view, wide eye movement range, and low mass production costs, making it widely regarded as the mainstream display technology route in the AR industry.

03 Volume Holographic Grating Waveguide

The propagation process of light in a volume holographic grating waveguide is fundamentally similar to that in a surface relief grating waveguide.

The key difference lies in how the volume holographic grating is created. Instead of being "sculpted," the volume holographic grating is formed by exposing a photoresist film on a substrate to interference patterns created by two coherent beams of light. This process generates a periodic spatial distribution with varying refractive indices at the molecular level. Volume holographic gratings typically operate under Bragg diffraction conditions.

What are Bragg Diffraction Conditions?

In 1912, German scientist Max von Laue discovered the phenomenon of X-ray diffraction in crystals, laying the groundwork for the study of X-ray diffraction physics. That same year, Lawrence Bragg, through repeated studies at the Cavendish Laboratory, concluded that this phenomenon is a type of wave diffraction effect.

In 1913, Lawrence Bragg and his father, Henry Bragg, jointly proposed the Bragg form of X-ray diffraction (known as Bragg diffraction). They found that when subatomic particle waves enter a crystal, if the wavelength of the particle waves is close to the distance between atoms in the crystal, the particle waves will be scattered by the atoms in a mirror-like fashion. This scattering will result in constructive interference according to Bragg's law, forming concentrated peaks of waves (known as Bragg peaks). The Bragg conditions are the criteria that must be met for constructive interference to occur.

Scchematic diagram of Bragg diffraction

Based on the principle of Bragg diffraction, when light waves meet the Bragg conditions, volume holographic gratings can achieve very high diffraction efficiency. However, the Bragg conditions impose stringent requirements on the angle and wavelength of the incident light. If these conditions are not met, the diffraction efficiency can drop rapidly. This results in volume holographic grating waveguides struggling to achieve good color uniformity, which fails to meet market demands.

Currently, volume holographic grating waveguides exhibit significant gaps compared to surface relief grating waveguides in terms of display performance, product commercialization, and industrial support.

04 Summary

AR diffractive waveguides utilize the diffraction characteristics of gratings to design "light paths," allowing light emitted from micro-projection systems to be directed into the human eye. Based on the type of diffraction grating used, diffractive waveguides can be classified into surface relief grating waveguides and volume holographic grating waveguides.

Surface relief grating waveguides offer advantages such as being lightweight, having a large field of view, a wide eye movement range, and low mass production costs. Consequently, they are widely regarded as the mainstream display technology in the AR industry. While volume holographic grating waveguides exhibit very high diffraction efficiency, they struggle with color uniformity due to the stringent Bragg diffraction conditions and are still in the early stages of technology development, requiring significant advancements to achieve breakthroughs.

With ongoing technological advancements and improvements in processing, AR diffractive waveguides based on surface relief gratings are beginning to enter the consumer market. It is believed that in the future, they will provide exceptional AR display experiences for more AR hardware manufacturers.