August 28, 2020 Light Engines for XR Smartglasses By Jonathan Waldern, Ph.D. The near-term obstacle to meeting an elegant form factor for Extended Reality1 (XR) glasses is the size of the light engine2 that projects an image into the waveguide, providing a daylight-bright, wide field-of-view mobile display For original equipment manufacturers (OEMs) developing XR smartglasses that employ diffractive wave- guide lenses, there are several light engine architectures contending for the throne. Highly transmissive daylight-bright glasses demanded by early adopting customers translate to a level of display efficiency, 2k-by-2k and up resolution plus high contrast, simply do not exist today in the required less than ~2cc (cubic centimeter) package size. This thought piece examines both Laser and LED contenders. It becomes clear that even if MicroLED (µLED) solutions do actually emerge as forecast in the next five years, fundamentally, diffractive wave- guides are not ideally paired to broadband LED illumination and so only laser based light engines, are the realistic option over the next 5+ years. Bottom Line Up Front • µLED, a new emissive panel technology causing considerable excitement in the XR community, does dispense with some bulky refractive illumination optics and beam splitters, but still re- quires a bulky projection lens. Yet an even greater fundamental problem of µLEDs is that while bright compared with OLED, the technology falls short of the maximum & focused brightness needed for diffractive and holographic waveguides due to the fundamental inefficiencies of LED divergence. • A laser diode (LD) based light engine has a pencil like beam of light which permits higher effi- ciency at a higher F#. This in turn results in a much improved Liquid Crystal on Silicon (LCoS) microdisplay panel contrast when compared with traditional LED LCoS designs. • Looking to the future and miniaturizing still further, the projection lens itself may be integrat- ed into the waveguide using diffractive metasurfaces containing sub-wavelength structures or “metalenses,” which are better suited to laser illumination compared to LED illumination. • By leveraging the benefits of lasers and its holographic switchable optics platform, DigiLens is using its extensive experience with diffractive optical waveguides to create a new class of light engines referred to as Waveguide Integrated Laser Display (WILD). The WILD category of light engines offer OEMs more immediate time to market and is a lower-risk alternative to µLED based light engines. • Laser coherence and phase also provide added efficiency gains in holographic thin film wave- guides. 1 Extended Reality (XR) is a term that encompasses Augmented Reality (AR), Mixed Reality (MR), and Virtual Reality (VR) 2 Also referred to as a Picture Generation Unit or “PGU” © 2020 DIGILENS INC. | ALL RIGHTS RESERVED | TWITTER: @DIGILENSINC The Need for a Compact Light Engine for XR Glasses Delivering a wide field-of-view (FoV), color, day- light-bright device in a spectacle-like form factor at an acceptable price continues to prove to be a major development hurdle for consumer XR displays. The form factor challenge is only par- tially met using thin diffractive and holographic waveguides. Current light engines comprising of the microdisplay beam splitter and projection DigiLens’ Visualize Design v1 lens are increasingly seen as too bulky to satisfy the aesthetic requirements of glasses. speckle and other illumination non-uniformities. In addition, misunderstanding also prevails with Let’s dig deeper into the leading light engine respect to outdoor contrast requirements which options on the market today illuminated by ei- has an oversized impact on brightness and bat- ther Light Emitting Diodes (LED) or Laser Diode tery life. Here, ambient light can wash out the (LD). content and render the image unrecognizable. Under generally accepted guidelines, an Am- LIGHT EMITTING DIODES bient Contrast Ratio (ACR) of 3:1 is required for recognizable images, 5:1 for adequate readabil- µLEDs Engines ity, and 10:1 for appealing quality.3 µLED is a thin emissive panel technology that Given a 90% transparent waveguide and in- dispenses with beam splitting and illumination door office lighting condition (150nits), the XR optics, thereby offering higher brightness and display target luminance level should have at much smaller size. There is consequently great least 550nits for ACR 5:1 – which is considerably interest in this technology. Yole, a leading indus- less brightness than that required to overcome try analyst, believes that µLED technology could outdoor lighting condition of 300-3000nits (so match or exceed OLEDs for most key display 4 2-20x). It is notable that the HoloLens 2 display attributes, especially brightness and efficiency. is 1000nits and that all current XR displays use However, as Yole points out, there is no sign of a sunshades, which longterm are unwelcome for color µLED light engine for XR applications. The consumers. Ultimately, even assuming a dan- reasons for this can be attributed to both basic gerously darkened room, the waveguide display device physics and technological obstacles to contrast is likely perfectly acceptable at 30-40:1, achieving process maturity. so well short of and not to be confused with the LCD, µLED or µOLED native display panel con- As µLEDs get small enough to satisfy the trast, irrelevant for an XR display application. 2k-by-2k resolution ideal for XR, the pixels suffer from surface recombination, etching- The low etendue and the narrow band of laser related defects on the side walls of the chip, emitters makes them the ideal choice for deliv- current crowding, and other thermal effects ering excellent color gamut and high brightness which all impact the external quantum effi- with high transmissivity that can meet the large ciency. number of outdoor use cases requiring compact form factors. However, laser emitters have tradi- Driving the LED pixels in the panel brighter to tionally brought new problems including laser overcome low efficiency is limited by higher 3 Dobrowolski J A, Sullivan B T, Bajcar R C. Optical interference, contrast-enhanced electroluminescent device. Applied, Optics, 1992, 31(28): 5988–5996 DOI:10.1364/AO.31.005988; and Chen H, Tan G, Wu S T. Ambient contrast ratio of LCDs and OLED displays. Optics Express, 2017, 25(26): 33643–33656 4 µLED Displays 2018, Yole Development, July 2018 © 2020 DIGILENS INC. | ALL RIGHTS RESERVED | TWITTER: @DIGILENSINC PAGE 2 power consumption, current drop and heat dis- MOCVD reactor suppliers also have credible sipation requirements. Solutions to the efficiency roadmaps to deliver cost-effective tooling. De- problem have been identified (e.g. thermal an- spite these advances, there are still significant nealing, side wall passivation and improvements challenges to developing a consumer µLED to LED structures and materials). Reducing the based light engine to match diffractive wave- sheet resistance of the current-spreading layer guide requirements. of the µLED chip has also been shown to give a uniform current distribution and alleviate current Current Commercial Activity and Status of crowding. µLED Displays Another effect that appears as the pixel size is In recent years the market has seen significant reduced, is a gradual increase in the light emis- µLED business development, with Facebook sion through the side wall leading to a far field (Oculus) taking a lead by acquiring two Univer- deviation from the normal Lambertian character- sity spinoffs: Irish µLED developer InfiLED (2016) istic of the µLED. This side emission depends on and a UK company mLED (2017). Apple is also the semiconductor material index and the de- reportedly considering investing a further $330 vice structure, but can certainly be a problem in million into a Taiwanese factory to manufacture color displays as different materials are used for both LED and µLED displays for future iPhones, different colors. The combined side emissions iPads, MacBooks, and other devices, so µLED can lead to an angular color shift that can also display technology must be seen in the context distort the color balance of the final image, re- as being enabling to a broad range of display sulting in a subpar experience for enterprise and product categories. consumer use cases. Additionally, Facebook has demonstrated a green (520 nm) nHD (640x360) µLED array with 6-micron pixels on a 20-micron pitch.6 Pixel siz- es down to 2-4 microns may be in the cards. Improvements to circuit complexity, efficiency drop at low pixel sizes and wall plug efficiency resulting from reduced specific contact resistivity (<10-4 ohm/cm2) and sheet resistance of ~100k Plessey and Compound Photonics’ ohms square are also claimed. 0.26”diagonal display module Facebook and Plessey Efficiency is not the only challenge for µLEDs. An integration technology for active matrix driving, A partnership between Facebook and Plessey technologies for full-color realization, and defec- announced in March 2020 will leverage Plessey’s tive pixel control all still remain major develop- LED manufacturing expertise to help Facebook ment hurdles. Driving µLEDs is more complex prototype and develop new technologies. than OLEDs and using standard low tempera- ture polysilicon or oxide TFT backplanes might Plessey is increasing investment in their fab to not be as straightforward as expected. Full color improve volume and yield. It is notable that with also requires integration of phosphors or QDs to Oculus, Facebook can use µLED panels in both the µLEDs, a process that is in its infancy. VR headsets and AR transparent glasses, so they are likely taking a long term view into several On a positive note, Yole reports that leaders are sub-categories of XR displays, both handheld achieving 99% yields, and small die efficiency and head mounted, with VR likely being earlier is approaching or exceeding that of OLEDs.5 to market. 5 µLED Displays 2018, Yole Development, July 2018 6 www.mled-ltd.com © 2020 DIGILENS INC.