Tapani Levola

Diffractive optics for virtual reality displays

Abstract— Near-to-eye (NTE) displays are generally systems where the imaging optics of a microdisplay is brought close to the eye, like a magnifying glass. In portable NTE terminals, the challenges are in achieving low power consumption as well as low weight and small size. Part of the problem is related to the microdisplay, but the optics also plays a major role. A study on how diffractive optical elements (DOEs) on planar waveguides can be used to miniaturize the optics of NTE displays is presented and the performance of the system is analyzed.

Replicated slanted gratings with a high refractive index material for in and outcoupling of light

Diffractive slanted gratings are manufactured onto plastic light guides using a high refractive index material and UV replication technology. We show that the manufacturing of such components is possible in large quantities. The applications of the slanted gratings are a high efficiency light in- and outcoupling with plastic light guides. We also show that it is possible to control which outcoupling diffraction order, reflective or transmissive, is dominating and hence to maximize the light power to one direction.

7.1: Invited Paper: Novel Diffractive Optical Components for Near to Eye Displays

Diffractive Optical Elements (DOEs) on planar waveguides can be used to miniaturize the optics of a Virtual Reality Display (VRD). A biocular VRD that uses the diffractive planar waveguide as an Exit Pupil Expander (EPE) is ergonomically excellent but suffers from low efficiency and from uneven light distribution in the perceived virtual image. Using novel diffractive structures the efficiency of the system can be dramatically improved and an excellent performance over a wide Field of View (FOV) can be achieved. In this paper it is shown that with deep slanted grating profiles more than 90% diffraction efficiency can be reached and that etendue of the system is improved. It is also shown that the binary sub-wavelength gratings with a proper groove shape are not polarization sensitive. The performance of the EPE plate can be optimized using the filling ratio, the grating depth and the slanting angle as tuning parameters. The volume production methods of the deep slanted grating structures are discussed and some experimental results are shown.

Exit pupil expander with a large field of view based on diffractive optics

Abstract— The field of view is an important parameter of a near-to-eye display. To achieve an immersive viewing experience, the field of view should be as high as possible. Presently, in most of the commercially available devices the field of view is between 15° and 30°. In this paper, a large-field-of-view exit pupil expander that is based on diffractive optics was demonstrated. Usually these types of diffractive expanders cannot have a field of view much more than 25°. Here, an exit pupil expander with an extended field of view, based on two stacked plates, was demonstrated. The expander is designed for green light and it achieves a field of view of more than 40°.

Display technologies for portable communication devices

Telecommunications networks are rapidly advancing toward adequate capacity to support multimedia content transmission between mobile terminals. This development opens opportunities for terminal manufacturers to realize functions on the devices that previously have been considered impractical. It is necessary for the terminals, however to exploit advances in new display technologies, such as reflective-color liquid-crystal displays, bistable displays, organic emissive displays, and microdisplays, to make use of the new functionality. The state of the art of mobile phone and personal digital assistant displays as well as some of the emerging technologies are reviewed based on their applicability in new uses in mobile terminals.

A novel diffractive backlight concept for mobile displays

efficiency demands on mobile communication device displays have become severe with the emergence of full video capable phones and mobile telephony services such as the third- generation networks (3G). One way of reducing the power dissipation of a mobile liquid-crystal display (LCD) is to efficiently distribute and outcouple the light available in the backlight unit (BLU) to direct the light in a spectrum-specific fashion through the respective color pixels. This paper describes a diffractive optics approach to realize a novel backlight unit. A model grating structure was fabricated and the outcoupled distribution of light was studied. The results verify that the new BLU concept based on an array of spectrum-specific gratings is possible.

Diffractive backlight grating array for mobile displays

— The display backlight unit (BLU) is the most power-consuming subunit in mobile liquid-crystal displays. The state-of-the-art BLUs utilize scattering, refractive, and reflective microstructures to generate a uniform distribution of white light through the display. More effective means of transmitting light through the display color filters could be obtained by using diffraction, but previously proposed diffractive backlights do not fully utilize all the possibilities to design gratings effectively for optimal color separation and outcoupling. This paper presents a new pixelated diffractive backlight grating array as an approach for overcoming these obstacles in BLU design. A model array was fabricated to couple out red, green, and blue primary colors from the respective subpixel locations. The results show that it is possible to manufacture such an array and that the light couples out as intended, giving a starting point to design mobile-display modules with low light-transmission losses.

Near‐to‐eye display with diffractive exit pupil expander having chevron design

— Near-to-eye displays (NEDs) provide a unique way to perceive a larger image than the device itself. The user acceptance of commercially available NEDs has not been high partly because of reported physiological symptoms. Devices also tend to be thick and heavy, and therefore uncomfortable to wear. To overcome these shortcomings and to make a very usable device, a new approach was needed. By using very thin plastic light guides with diffractive structures on the surfaces, many of the known obstacles can be notably reduced. These exit pupil expanders (EPEs) enable a light and thin design for see-through NEDs. The so-called chevron EPE was designed to further improve the design and usability aspects of NEDs. The diffractive EPE has typically one incoupling grating area that delivers light into the light guide symmetrically towards the left and right eyes. By using slanted or overhanging gratings, the incoupling is very asymmetric. If the incoupling area is divided into two parts, each having opposite slanting angles, the EPE plate can be cut in half, and the left and right parts can be separated. The plates can be further tilted to a chevron shape following more closely the human face and mimicking the conventional eyeglass design. The reflection of the light from the tilted plate is directed out from the image-forming optics, and therefore the contrast is improved. The privacy of the usage is improved because at the output the secondary beams are harder to observe. Because the device is brought closer to face, it enables better weight balance. The chevron geometry partially prevents the ambient light from the sides to enter the diffractive structures, and thus the unwanted reflections are also reduced. Furthermore, the half-size of the chevron EPE parts from the undivided EPE results in a better production yield.

55.3: Diffractive Backlight Light Guide Plates in Mobile Electrowetting Display Applications

Power efficiency demands on mobile displays are increasing rapidly, as new multimedia services and applications are being adopted by users. Diffractive backlights and electrowetting displays have been proposed as some of the solutions to solve the poor efficiency of current mobile display systems. In this study, an electrowetting display was coupled with a pixelated, diffractive becklight light guide plate. This is to our knowledge the first time when a pixelated, diffractive backlight has been demonstrated in conjunction with an actual display. The results of the study show that the backlight and display panel need to be optimized as a system in order to obtain an applicable module for mobile use.

Mobile display backlight light guide plates based on slanted grating arrays

Modern mobile communication devices have user interfaces that are dominated by high-quality displays. Increased multimedia use imposes high demands on the design of display modules, as the content available for mobile use becomes visually richer. Especially the power dissipation of the display can limit the amount of time available for multimedia consumption and interaction. In the mobile liquid-crystal display (LCD), the energy efficiency is determined by the backlight design. State-of-the-art backlights direct white light through a display subpixel array, with high uniformity and up to 90% efficiency in white light output. Therefore, it is difficult to obtain system-level energy savings by improving the backlight design alone. Diffractive backlights have recently been proposed to reduce the power dissipation of the display module, and slanted grating arrays are among the enabling optical features that allow for reduction in power dissipation beyond what is available in the state of the art. By the use of diffractive grating arrays, the required primary color (red, green, or blue) is directed through the LCD subpixel array with geometrical registration, instead of flooding the whole LCD with white light and filtering the primary colors through the subpixel color filter array. We present a study on grating structures based on slanted grating arrays fabricated in high refractive index materials. The grating design principles and grating outcoupling results are provided, and an outline of a new embedded system design is given. Emphasis is on grating array design aspects for future energy-efficient display system design. The results show that savings in power consumption can be expected with advanced display system design based on embedded slanted grating array backlight light guide plates.