Study on improvements of near-eye holography: form factor, field of view, and speckle noise reduction

Abstract

This dissertation presents solutions targeting on the three fundamental issues on near-eye holography: 1. Form factor, 2. Field of view (FOV), and 3. Speckle noise. For the form factor reduction of the holographic display, a multifunctional holographic optical element (HOE) is developed to replace the beam splitter and the eyepiece lens of the conventional holographic display. As a result, it enables us to build a compact see-through holographic display for augmented reality (AR). The feasibility of using HOE in the holographic display is verified by analyzing some optical characteristics of the HOE, based on the Kogelniks coupled-wave theory. In addition, a compensation method is introduced to compensate the wavefront aberration caused by use of the HOE. To build a wide FOV holographic head mounted display, I proposed two methods to increase the space bandwidth product (SBP) which can widen the FOV without losing eyebox size. In each optical path of +1st, 0th, -1st diffraction order of a spatial light modulator (SLM), I place a shutter synchronized with the SLM to display different holograms by temporally dividing the frames of the SLM. The second proposed method is called polarization selective holography which is a brand new hologram calculation and encoding method. In this work, an SLM can modulate waves in different modes through different states of wave retardation and polarization optical elements. Two sets of wave plates and polarizers with optimized states are placed in the path of +1st and -1st different diffraction orders of an SLM, so that the SLM reconstructs two different holograms via each path. As a result, the system achieves a two-fold increase in terms of SBP for a single SLM. Furthermore, a curved hologram can provide wider FOV comparing to a planar one, when they have the same SBP. A curved surface hologram can provide wider FOV. Thus, curved holographic display has a great potential to the near-eye holography. Since the curved surface SLM has not been developed yet, as an example, I also proposed a numerical method of spherical hologram computation of real objects in this chapter. For the speckle noise issue, some analyses are carried out to compare the quality of holographic reconstruction images using different light source such as a light-emitting diode (LED) and a laser diode (LD). Both of them have pros and cons. The main advantage of use of the LED in the holographic display is it can reconstruct the holographic image with significantly less speckle noise compared to laser or LD. However, an inherent problem caused by use of the LED is that it is difficult to reconstruct images located far away from the SLM. This problem is mitigated using a magnifier optical system. As a result, I demonstrate an LED based near-eye holographic display system not only to reduce the speckle noise but also to provide sufficient expressible depth range.