Optical Waveguides for Augmented Reality Smart Glass

What Is an Optical Waveguide?

Optical waveguides are thin flat sheets of high refractive index glass or polymer, which superimpose digital images on the user’s field of view, thus augmenting the real world with timely information.

The optical glass or polymer is optimized for specific properties, such as refractive index, clarity, transmittance or weight, enabling applications such as:

• Smart glasses and head-mounted headsets for healthcare and manufacturing
• Heads-up displays for transportation and the built environment
• Industrial components for telecommunications and integrated optical circuits

Critical factors include:

• high field-of-view
• crisp images
• high transparency
• color neutrality, and 
• lightweight form factor.

Optical Waveguides and Augmented Reality

In augmented reality, the optical waveguide forms the backbone of the device, with the image being created by a micro-display and channeled by optical couplers into the user’s view, as shown below.

Image courtesy of Chinese Academy of Sciences

• The micro-display is based on LCD (Liquid Crystals), OLED (Organic LEDs) or LCoS (Liquid Crystal on Silicon), resulting in vibrant colors and high contrast. 
• The micro-display emits light into an in-coupler, an optical grating that channels light at a specific angle into the waveguide (by diffraction or reflection).
• The waveguide material is of a higher refractive index than the surrounding air, which results in a strong confinement of light. This allows light to ‘zig-zag’ its way down the waveguide (with minimal loss) by a process called Total Internal Reflection (TIR), until it reaches the out-coupler.
• The out-coupler, also an optical grating, then combines the virtual image (from the micro-display) into the real world view of the user.

In this way, digital information is superimposed onto the real world, bringing intelligent wayfinding, real-time translations and safety instructions to consumer and enterprise users.

Extended Reality (XR)

Extended Reality is an overall umbrella term, which covers the following technologies:

In this article, we focus on waveguides for augmented reality (AR), although many of the same principles apply to virtual reality (VR) and mixed reality (MR) devices.

Let’s take a step back for a moment. 

What Is A Waveguide?

Waveguides are structures that transmit energy waves in a specific direction.

The wave can take the form of a fluid (liquid or gas), an acoustic wave, or an electromagnetic wave, such as thermal energy, radio waves, or visible light.

Without the waveguide, wave energy would emanate in all directions equally, with an intensity that decreases according to the inverse square law (where intensity decreases with the square of the distance).

The waveguide is thus a way to channel energy from one point to another with minimal loss.

Where Can We Find WaveGuides?

Waveguides appear in everyday life as:

• Canals, pipes or ducts for fluids (e.g. water, oil, natural gas)
• Air ducts and horns which guide sound in musical instruments and loudspeakers
• Light pipes, used to channel sunlight from rooftop solar collectors to darker parts of the building interior
• Metal rods which conduct waves in ultrasonic machining
• Transmission lines in electrical circuits
• Transparent dielectric optical waveguides (including optical fibers) that transmit visible light

There are also some unusual applications of waveguides found in nature:

• The SOFAR (Sound Fixing And Ranging) channel, a horizontal layer of water in the ocean which acts as a waveguide for low-frequency sound. This is due to variations in temperature and pressure of undersea water which allows whale song to be carried across enormous distances.
• Gravitational waves (tiny distortions in spacetime) can act as waveguides for electromagnetic radiation, conveying information across the universe from cosmological events such as colliding black holes.

Waveguide Materials

Dielectric waveguides

Dielectric waveguides use insulating materials (such as glass or plastic) that form electrical dipoles when subjected to an electric field. They guide waves through their internal structure by having a higher refractive index compared to their immediate surroundings. Applications include fiber-optics, integrated optical chips and biomedical imaging.

Metallic waveguides

Metallic waveguides have conductive walls that confine electromagnetic fields. They often take the form of a hollow metal pipe, of rectangular or circular cross-section, and operate at microwave frequencies. Applications include microwave ovens, radar systems and satellite communications.

Nanoplasmonic waveguides

Nanoplasmonic waveguides use nanometer-scale metallic structures embedded onto dielectric materials to propagate light along the metal-dielectric interface. They enable strong light confinement, with applications in nanocircuits, nanophotonics and biosensing.

Meta waveguides

Meta waveguides use metamaterials to guide electromagnetic waves. These materials have subwavelength structures which manipulate the phase and polarization of light.

Optical Waveguide Structure

Optical waveguides are manufactured with transparent dielectric materials, and are composed of a light-transmitting core and sometimes a surrounding cladding (or air).

Image courtesy of RP Photonics

The first image shows a planar waveguide which restricts light in 1 dimension. The second image shows a channel waveguide which restricts light in 2 dimensions. 

An optical fiber is an example of a 2-dimensional waveguide, of circular cross-section, with a circular core and outer cladding material.

Image courtesy of CableWholesale

Optical Waveguide Technologies

Optical waveguides typically follow one of the following topologies:

• Diffractive waveguides
• Reflective waveguides
• Holographic waveguides

These are now explained in further detail.

Diffractive Waveguides

We have seen how waveguides combine computer-generated imagery onto the user’s field of view. This is achieved through the use of optical gratings.

A grating is a framework of parallel structures used to cover openings. Everyday examples include storm drains, air vents, or window security bars.

Optical gratings have the same structure but are measured in nanometers.

A diffraction grating causes light to ‘splay out’ after encountering a slit in the grating. The net effect is the same as a prism, whereupon all the colors in natural sunlight are revealed.

A surface-relief diffraction grating has periodic nano-sized structures, built directly on the glass surface, consisting of grooves or ridges which diffract light, separating it into wavebands, by altering its phase.

One of the main disadvantages of diffractive waveguides is ‘spectral dispersion’ – when the different colors in light diverge inside the waveguide, resulting in a possible color fringe effect.

Slanted surface relief diffraction grating – credits: URL

Reflective Waveguides

Reflective waveguides are based on an array of partially reflective mirrors, embedded into optical glass that guides light by total internal reflection, just as with diffractive waveguides.

The reflective kind however employ geometrical optic principles and are fabricated using glass processing techniques like grinding, polishing and thin-film coatings.

This waveguide technology offers larger fields of view, higher brightness, better color uniformity and minimal spectral dispersion, when compared to diffractive waveguides.

Holographic Waveguides

In comparison with the above-mentioned physical structures, a holographic grating creates a series of step-like changes in refractive index. 

This optical pattern is created through interference by superimposing lasers on photosensitive material, resulting in a sinusoidal groove profile. 

The net effect is to diffract light by changing the refractive index experienced by the wavefront.

AR Wearable Device Requirements

Some typical requirements for AR wearables, as specified by ST Microelectronics, are shown below:

Typical Images of AR Smart Glasses

Here are some images of AR smart glasses, showing informational messages overlaid on top of the user view:

Outlook

Optical waveguides play an important role in telecommunications and consumer electronics, with growth expected from USD 8 Billion in 2025 to USD 11 Billion in 2029. 

This growth assumes continued demand for telecommunications, healthcare and electric vehicles, primary drivers for optical waveguide technologies.

Another important use case is the augmented reality device sector, expected to grow to USD 42 Billion by 2029, with major tech companies investing in core R+D and acquisitions in this space.

All good news for smart glass in general, as this sector continues its strong trajectory in innovation, bringing disparate technologies and benefits to industrial, medical and consumer devices.