From the perspective of consumer demand, AR glasses have three key points and development trends: thin and light, strong integration of reality and reality, and reasonable price. However, neither the semi-transparent and semi-reflective display solution nor the arrayed optical waveguide display solution based on traditional geometric optics can take into account these three key points at the same time. To this end, AR diffraction optical waveguides emerged. It also has the advantages of being thin and light, having a large field of view, a large eye movement range, and low mass production costs. Therefore, it is generally considered to be the mainstream display technology route for the future AR consumer market.


FigureAR smart glasses using optical waveguide technology solution

What is a diffractive optical waveguide?

Diffraction light waveguide uses the diffraction characteristics of gratings to design the "light path", allowing light to propagate along the designed path, and guiding the image emitted by the projection system into the human eye. According to different grating types, diffractive light waveguides can be divided into two categories: surface relief grating waveguides and volume holographic grating waveguides.

Surface relief grating "carves" peaks and valleys on the surface of the material through processes such as photolithography and etching to achieve a periodic structure that can meet the required optical performance.

Figure Raster scanning electron microscope photo of surface relief

Surface relief gratings control the propagation path of light through fine periodic structures. When light is incident on the diffraction grating, it will be divided into multiple directions and propagated in multiple directions. Through the total internal reflection effect of the waveguide material, it will be reflected multiple times in the waveguide until it reaches the exit grating.


Figure Diffraction light waveguide path

The incident grating is responsible for introducing the beam from an external light source (such as a microdisplay) into the waveguide. The reflective grating guides and propagates the beam within the waveguide, and finally guides the beam to the user's eyes through the exit grating, thereby forming a clear virtual image.

The common wooden fences and iron fences in our lives are similar to the specific periodic structure of surface relief gratings.


Figure Zhige Technology AR Diffraction Optical Waveguide

Grating master processing and nanoimprinting

The mature process flow of surface relief grating is mainly divided into three major links:
1. Grating design
2. Grating master processing
3. Nanoimprint production
Grating masters and nanoimprinting are used to mass-produce gratings with optimized parameters (such as period, duty cycle, groove depth, side wall inclination, etc.) to achieve mass production of surface relief grating optical waveguides.
To make a vivid metaphor, the production process of diffractive optical waveguides is like "pressing" stamps in batches. First, the required pattern needs to be designed (grating design), then the stamps must be accurately produced according to the design (grating master processing), and finally Use this mold to "press" our designed patterns in batches on a specific substrate (nano imprint production).
Of course, in the production of real diffractive optical waveguides, this "stamp" and "press" have extremely complex structures and extremely high precision, and also have extremely high requirements for the process.

The characteristic size of the grating is at the micro-nano level, which cannot be achieved by ordinary optical processing and production processes. It needs to be processed by the method of producing chips.


Figure Flowchart Source: Microray Optics

It seems easy from the process, but it is very difficult to implement. For example: the accuracy and conformality of photolithographic line width in E-Beam, the consistency of trench depth in deep silicon etching, the consistency and steepness of sidewall inclination angles, rounding and other issues. We can compare this process to a chip manufacturing process of about 140nm. This not only requires strong micro-nano optical design capabilities and long-term experience in grating production, but also requires a complete set of expensive semiconductor equipment to make an ideal diffractive optical waveguide. This is also the reason why many optical manufacturers who want to enter the field of diffractive optical waveguides are actually making very slow progress.

The original article was first published on the WeChat official account (Aibang VR Industry News):AR diffraction light waveguide and nanoimprint technology

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