Optical Waveguides: The "Path of Light" from Photonic Chips to AR Glasses

In AR glasses, the optical waveguide is a core optical component that determines the display experience. It channels the image generated by the optical engine into the lens and directs it toward the user's eye via a specific structure, while maintaining lens transparency to allow the user to view both the real world and virtual information simultaneously.

Optical waveguides did not originate with the advent of AR. In fields such as photonic chips and optical communications, devices like modulators, detectors, and beam splitters also rely on waveguides to transmit optical signals. If metal interconnects serve as the pathways for electrical signals, optical waveguides act as the channels for optical signals—the distinction being that light relies on refractive index differences and geometric boundaries to constrain its propagation path.

Regardless of whether they are geometric, surface-relief grating, or volume holographic waveguides, the objective of the optical waveguides used in AR glasses is to direct light into the human eye along a designed path.

The essence of a waveguide: designing a controllable path for light

The core function of an optical waveguide is to confine light propagation to a specific region.

In photonic chips, waveguides can be made from materials such as silicon, silicon nitride, InP, or lithium niobate; in AR glasses, optical waveguides are typically composed of high-refractive-index glass or resin, though in recent years, silicon carbide (SiC) has also begun to be used as a substrate material for diffractive optical waveguides, utilizing gratings or reflective structures to achieve light in-coupling and out-coupling.

A waveguide controls not only the optical path but also the light field distribution—determining where light propagates, how it is redirected and split, and how it ultimately enters the human eye.

An excellent waveguide must simultaneously achieve low loss, high coupling efficiency, and good uniformity, while also being suitable for mass production.

How Waveguides Confine Light: Total Internal Reflection and Modes

Optical waveguides confine the propagation of light by means of total internal reflection.

When light travels from a high-refractive-index material toward a low-refractive-index material and the angle of incidence exceeds the critical angle, the light is totally reflected back into the high-refractive-index material. This phenomenon forms the basis of waveguides.

A larger refractive index contrast results in stronger optical field confinement, allowing for smaller waveguides. However, excessively strong confinement also makes the waveguide more sensitive to fabrication imperfections and increases scattering losses.

It is worth noting that the refractive index directly determines the upper limit of the field of view (FOV) for AR glasses. Increasing the refractive index of the waveguide material expands the range of supported light propagation angles, thereby creating the potential for designs with a wider field of view. For instance, high-refractive-index materials are generally more conducive to achieving waveguide systems with a large FOV; this is a key reason why silicon carbide (SiC, with a refractive index of approximately 2.6) has attracted significant attention.

Light does not propagate arbitrarily within a waveguide. Simply put, not every light beam entering the waveguide can propagate stably; only electromagnetic field distributions that satisfy specific boundary conditions can travel over long distances. These stable states are known as "modes." Different propagation modes possess distinct electric field distributions and propagation constants. Since crosstalk or uniformity issues can arise between multiple modes, mode control is a critical factor in AR waveguide design.

In AR displays, mode control directly affects brightness uniformity and color performance.

Where do optical losses occur in a waveguide

Real waveguides inevitably exhibit losses, primarily of four types:

1. Material absorption: Light energy loss caused by the material's inherent absorption characteristics and impurities.

2. Scattering loss: Light scattering caused by sidewall roughness and dimensional deviations during the manufacturing process. Micro- and nano-fabrication parameters—such as grating period, etching depth, and sidewall roughness in AR waveguides—directly affect diffraction efficiency, uniformity, and color performance.

3. Bending loss: When the bending radius is too small, some light radiates outward.

4. Coupling loss: Losses occur due to mismatches in mode size, angle, or polarization state when light transitions from one structure to another (e.g., optical engine to waveguide, or waveguide to out-coupling structure).

One of the keys to improving the optical efficiency of AR glasses is minimizing losses throughout the entire optical path: from the light engine through incoupling, propagation, and outcoupling, to the human eye.

Facilitating the smooth entry and exit of light into and out of the waveguide: incoupling and outcoupling

In AR optical waveguides, light coupling-in and coupling-out are core technologies:

1. Diffractive optical waveguides use gratings to alter the direction of light.

2. Geometric optical waveguides use reflective structures to redirect the optical path.

3. Volume holographic optical waveguides use holographic elements to control light propagation.

The essence of these technologies lies in enabling highly efficient mode conversion for the light.

Waveguide Engineering Evaluation Criteria

Assessing the viability of a waveguide requires a comprehensive evaluation of the following factors:

1. Propagation loss: Optical power attenuation per unit length.

2. Coupling efficiency: The efficiency of light entering and exiting the system, which directly impacts the brightness perceived by the eye.

3. Bending loss: An assessment of the waveguide structure's optical stability in designs featuring thin profiles or curved surfaces.

4. Mode and polarization control: Factors influencing display uniformity, color performance, and system stability, which must be considered in conjunction with the waveguide design.

5. Manufacturing tolerances: The controllability of factors such as lithography errors, etching deviations, and variations in film thickness.

In conclusion

Whether in photonic chips or AR displays, the function of optical waveguides is to control the propagation path of light.

With the continuous evolution of various technical approaches—such as geometric optical waveguides, surface-relief gratings, and volume holographic waveguides—and the transition of new materials like silicon carbide from the laboratory to engineering applications, optical waveguide technology is entering a new phase of competition. This phase moves beyond the sole pursuit of optical performance to a comprehensive contest involving performance, cost, yield, and mass manufacturing capabilities.

Source: Compiled by Pingsheng Semiconductor and Aibang Zhizao