Dissertation Defense: Ted Lee, "Multiplexing for DMD-Based Near-to-Eye Displays"

Abstract:

Augmented Reality (AR) devices optically superimpose information over the real world through an optical channel, which often involves an angular bandwidth limited image guide employing Total Internal Reflection (TIR) due to material refractive index constraints. This bottlenecks to packaging size and couples it to the Field of View (FOV) and resolution. Previous architectures have focused on physically increasing the pixel count while reducing pixel size to combat this trade off, while still obeying the inverse relationship between the two. This dissertation discusses and demonstrates two multiplexing methods to address the challenge.

By applying wavelength and time multiplexing similar to what is used in optical communication along with newly developed diffraction multiplexing increases the effective optical bandwidth for the AR display. Wavelength multiplexing with two wavelength sources are alternatively used in time sequential manner to in-couple the optical information through the bandwidth limited image guide, to effectively double the physically limited FOV of the image guide. At the output coupler of the image guide, a wavelength sensitive Volume Holographic Grating (VHG) angularly reflects and redistributes each part of the FOV to present a seamless full FOV image. Diffraction multiplexing in time is done by using the DMD in a diffractive manner that produces spatial modulated images at discrete angles, where each FOV per wavelength is further divided into sub-FOVs per each diffraction order. In the projection system, there is a FOV shifter then shifts the sub-FOVs side-by-side and creates a full FOV image. This multiplexing method reduces the display size while maintaining the image resolution, breaking the tradeoff between package size and FOV/resolution. The added benefit of this method is that the bulky optics required for the FOV shifter can be placed closer to the image guide, reducing the overall display package size near the temple of the glasses.

Demonstration systems of each method were built and shown with static images, with quantitative image metrics, contrast, uniformity, and crosstalk used to compare performance loss caused by the complex multiplexing. These architectures demonstrated a hybrid approach NED projection system that combines multiple domains of multiplexing, to increase the FOV of NED without the typical package volume, FOV, resolution, and energy consumption trade-offs. Showing the untapped potential of multiplexing on DMD-based near-to-eye displays.