AR silicon carbide waveguide analysis, from the perspective of waveguide design
01
Breakthroughs in materials often bring an industry to new heights and even open up new scientific and technological space for mankind.
The birth of silicon launched the entire era of semiconductors and computing, becoming the basis for silicon-based life.
So, will the emergence of silicon carbide bring AR waveguides to new heights?
Let's first look at the design of the waveguide.
Only by understanding the requirements at the system level can we clarify the direction of material optimization.
The most classic architecture of AR waveguides comes from the former Hololens Dr. Tapani Levola of Finland, and the waveguides are divided into three regions: the entry pupil region, the dilated pupil region, and the exit pupil region.
AR waveguide this piece, the Finns are the absolute core driving force.
From the earliest Nokia, to the Hololens, to the later Dispelix and so on.
(Tapani's classic patent for AR diffracted waveguide, filed at Nokia in 2002, is 23 years old)
02
The entry pupil region of the waveguide couples the entire FOV from the optical machine through the grating into the substrate, which can be glass, silicon carbide material, or even resin material.
Its working principle is similar to optical fiber transmission, when the incidence Angle meets the condition of total reflection, the light will be bound in the base and transmitted to the pupil enlargement area through total reflection.
In the dilated pupil region, light is replicated in the X direction and continues to the exit pupil region.
In the exit pupil region, light is copied in the Y direction and eventually coupled out to the human eye.
If the exit pupil of the optical machine (that is, the entry pupil of the waveguide) is compared to a "round cake", then the essence of the AR waveguide is to copy this "cake" from the optical machine into multiple, such as 4x4, in the exit pupil region.
Ideally, these "cakes" are expected to overlap each other to form a smooth, uniform brightness and color surface, so that the user sees the same picture anywhere on this surface (high uniformity).
AR waveguide design must first consider the requirements of FOV, which determines the size of the picture the user sees, and also affects the design requirements of the optical machine.
The second is the requirements of the Eyebox, which determines whether the user can see the full picture within the eye movement range, affecting comfort.
Finally, there are other indicators, such as brightness uniformity, color uniformity and MTF.
Summarize the flow of AR waveguide design:
Determine the FOV and Eyebox, select the waveguide architecture, set optimization variables and objective functions, and then make continuous optimization adjustments.
So, what does this have to do with silicon carbide? It matters.
The most important diagram in waveguide design is the k-vector wave vector diagram.
In simple terms, incident light (at a specific wavelength and Angle) can be represented as a vector.
The square box in the center represents the FOV size of the incident picture, and the ring area represents the FOV range that the waveguide material of that refractive index can support, beyond which light cannot exist in the waveguide.
The higher the refractive index of the base material, the larger the circle of the outermost ring, and the larger the FOV that can be supported.
Each time the grating is touched, an additional vector is superimposed on the incoming light. The magnitude of the superimposed vector of the grating is related to the wavelength of the incident light.
Therefore, light of different colors coupled into the grating will jump to different positions in the ring (inside the waveguide) due to different raster vectors.
Therefore, a single chip to achieve RGB three colors, can support a lot less FOV than monochrome.
03
To achieve large FOV, there is not only one way to increase the refractive index of the base, there are at least two ways to choose.
For example, it can be done through the splicing of FOV, such as the Hololens classic Butterfly architecture.
The grating in the input region cuts the incident FOV in half, transmits it from the left and right sides to the dilated pupil region, and splice it in the exit pupil region.
In this way, even with low refractive index materials, large FOV can be achieved.
With this architecture, Hololens 2 achieves FOV of more than 50 degrees based on a glass substrate with a refractive index of less than 1.8.
(FOV Spliced waveguide Classic patent filed by Microsoft Hololens2 in 2016)
It is also possible to achieve very large FOV through some architectural design of two-dimensional raster, which involves many details and is inconvenient to expand.
From the FOV point of view, the higher the refractive index of the base, the higher the upper limit of the system.
From this point of view, silicon carbide does provide a higher ceiling for the system.
As a waveguide designer, I certainly like silicon carbide because it gives me enough freedom to design.
But from the user's perspective, it doesn't really matter what base to use.
As long as it can meet the demand, good performance, low price, and light machine, it is a good choice.
Therefore, the choice of silicon carbide or other substrates should be considered comprehensively by the product team.
Need to be considered according to the application scenario, price positioning, design specifications, industrial chain maturity and other aspects.
04
To summarize:
1. If purely from the perspective of FOV, the current high refractive index glass achieves FOV of 50 degrees without pressure.
2. but if you want to achieve more than 60 degrees of FOV, silicon carbide is indeed a good choice.
Materials are a choice at the component and architecture level, and architecture in turn serves the function of the system, and ultimately through the product, to serve the user.
This is a tradeoff process, we need to choose from multiple dimensions such as scene experience, product form, system architecture, components and materials.
ZMSH SIC Substrate 4H/6H-N/Semi/3C/4H/6H-P Type Display
* Please contact us for any copyright concerns, and we will promptly address them.
