For years, movies have teased us with the possibilities of augmented reality (AR) and mixed reality (MR), from Minority Report to Iron Man. But unlike robotic insects and flying suits, AR and MR smart glasses are much closer to reality for everyday consumers. One of the major barriers has been consumers’ desire for lightweight smart glasses that resemble standard eyeglasses and can be worn all day, every day. Compact, chic designs require projection technology and batteries that are as small and power-efficient as possible, which has been difficult to achieve. The internet hype that AR glasses are just around the corner has given into the reality that there are a number of large problems to solve before mass deployment in the consumer space can begin.
The “hype” has been replaced with a gradual iterative evolution. On one side, we see virtual reality (VR) headsets migrating into the VR/VPT (video pass-through) headsets. VPT is more in the realm of VR, but enables limited interaction with reality (a tenet of the AR promise). Set for release in 2023, these VR/VPT headsets are primarily for indoor use. So, when you do not have to battle the largest light bulb in the universe (the sun), most of these VR/VPT headsets tend to use microOLED displays.
Migrating into full AR glasses with all-day wear means the display will require a high dynamic range to work equally well in dimly lit indoor spaces, as well as mid-day sun outdoor spaces.
MicroOLEDs simply do not have the power (measured in nits) to compete against the sun in outdoor applications. So, AR glasses require a different light source with contenders including conventional LEDs, microLEDs, and edge-emitting lasers with vertical cavity surface-emitting lasers (VCSELs) being potential future sources.
All of these light sources come with their different attributes:
Conventional LEDs are a mature technology and relatively cheap. However, they are also a Lambertian emitter (multidirectional light beams) and have an etendue topic that must be addressed if the AR glasses are to be small and stylish.
MicroLEDs offer an amazing solution for AR glasses. A major drawback is that the technology is not commercially available, and the fully integrated monolithic module is several years away. And with a new technology introduction that is yet to slide down the cost curve, it’s not a simple solution.
Edge-emitting lasers have been around some years and have a great power density. The material systems for these lasers are sensitive to the environment, which means the parts need hermetically sealed packages. Historically, this hermetic seal has been provided in the shape of industrial-style transistor outline (TO) cans. While the TO38 can is small (3.8 mm in diameter), it contains only one laser; for RGB (three lasers), the light source size increases. This is the exact opposite of the design requirement.
Visible VCSELs are some years away from commercial availability.
Laser beam scanning offers high power in a small package
One of the leading lighting solutions is laser beam scanning (LBS) because it features small size-to-power density ratios and weight-to-performance ratios that enable excellent-quality image resolution. Smaller battery size can also be achieved by efficient light coupling from the source into the optics. To do this, the light source must be compact and have an optical power density high enough to transmit light in a narrow, solid angle. Lasers are known to have the highest optical power density, but the existing TO can packaging technology is not small enough or efficient enough to achieve these goals. There is potential to combine all of the individual red, green, and blue lasers into one surface-mountable, hermetically sealed package.
A major breakthrough in miniature laser modules is the recent launch of ams OSRAM’s Vegalas RGB laser module prototype (see figure). Vegalas takes one step closer to making AR smart glasses a reality for consumers by cutting the size of projection light engine in half. It is just 7 × 4.6 × 1.5 mm in size, allowing it to fit in standard, lightweight eyeglass frames. The Vegalas module combines three powerful lasers at red (640 nm), green (520 nm), and blue (450 nm) wavelengths in a robust, surface-mount package. The package is hermetically sealed to prevent contamination of the module’s laser chip-on-submount emitters.
Near-to-eye waveguides key to consumer AR smart glasses
Along with lightweight, compact designs, AR smart glasses must not interfere with peripheral vision or eye contact, so users can interact with the real world as much as possible. To accomplish this, near-to-eye (NTE) display waveguides are often needed. Other solutions such as back-illuminated LCD or OLED displays use a front partial mirror, which requires reflection at an angle between the illuminating matrix and the users’ eyes (sometimes referred to as bird bath). This pushes the glasses far from the face and limits light. To compensate for the loss of light, the lenses must be dark like sunglasses.
Narrow-beam lasers, such as in the Vegalas module, are the ideal choice for waveguides of an NTE system. Narrow laser beams are extremely efficient, and by driving the lasers at very high pulse rates, the occurrence of speckle—the random, granular pattern that degrades the image quality of projection displays—is reduced. With LBS, light can be accurately uncoupled from a scanning mirror, through the relay optics and onto the waveguide entrance. Due to the laser’s focused beam, all light is used for illumination, significantly increasing efficiency.
Advanced waveguide combiner optics are another important component because they make it much easier to integrate with the glasses without requiring a great deal of customization. ams OSRAM has collaborated with partners in the LaSAR Alliance to develop a reference design of a projection light engine. The reference design is for an LBS AR glass system. The light engine measures just 10 × 11 × 6 mm (0.7 cm3), much smaller than other LBS light engines.
Thanks to advanced waveguides and laser technology, art may soon imitate reality with millions of consumers staring straight ahead through a pair of AR or MR smart glasses.