# Image Quality for Near-Eye Display Based on Holographic Waveguides
## 3 Results
### First DOE Configuration  
The first configuration under study is an analog of the scheme shown in Figure 2c and implements the principle of geometrical separation of the incident light into two parts to increase the size of the angular field of view. The difference between this configuration and the ones proposed in [20; 30] is that all three DOEs on the surface of the waveguide have the same period, while the referenced configurations have periods of the input and relay gratings that differ by \(\surd\)2 times. Figure 6 shows the holographic waveguide of the first configuration with the input DOE consisting of the superimposed diffraction gratings, as well as the different orientations of the direction of the diffraction gratings. Due to a symmetrical structure, a higher field of view is provided by dividing the input light into two symmetrical components propagating in the waveguide in opposite directions.  
In this implementation, the input DOE is a diffraction grating formed by two superimposed one-dimensional diffraction gratings with a \(60^{\circ}\) angle between the line directions, as the layout in Figure 6a shows. Figure 6b is a photo of a single waveguide with diffraction gratings, recorded in accordance with Figure 5a. When the radiation enters the DOE1 area the diffracted beams propagate towards the DOE2a and DOE2b areas providing the expansion of the exit pupil in the horizontal direction and then due to the diffraction unfolds downwards towards the output DOE3 area. Light diffracted by the output DOE3 exits the plate toward the observer's eye. At the same time, part of the radiation continues  
Figure 6: The first configuration of the holographic waveguide: (**a**) DOE configuration scheme; (**b**) photo of a single holographic waveguide; (**c**) resulting AR image.  
to propagate in the holographic waveguide along the DOE3 area, which provides the exit pupil expansion in the vertical direction.  
The resulting field of view is represented by Figure 6c. Two negative factors affecting the quality of the generated AR image were identified in this implementation. Firstly, there is a combinational parasitic periodicity in the horizontal direction within the entire superimposed DOE1 area. Secondly, a sufficiently accurate alignment of DOE1, DOE2a, and 2b is required. The presence of gaps or overlaps in these areas leads to a decrease in diffraction efficiency, resulting in visible dark bands in the output image, as Figure 6c shows. Recording of each diffraction grating on the holographic setup is carried out sequentially using masks (or diaphragms), but even 0.2 mm accuracy of their positioning relative to the plate does not allow getting rid of the above-described effect. It happens due to large angles of convergence of reference and object beams in the recording setup and the final thickness of the mask.  
It is experimentally shown that the main disadvantage of the holographic waveguide with the first DOE's configuration is a high brightness non-uniformity of the observed image. A bright vertical stripe is present in the image, caused by diffraction on the superimposed parasitic grating. Some part of the light goes from DOE1 right to DOE3, bypassing the DOE2 area. The width of the stripe in the image corresponds to the diameter of the input pupil. For the first configuration of the holographic waveguide the achieved level of brightness non-uniformity was about 65% because light and dark stripes are explicit in the image.  
It is not possible to reduce the diffraction efficiency of the <<parasitic>> diffraction grating to reduce the brightness of the central stripe in the image because it would as well determine the in-coupling efficiency. That will decrease the brightness of not only the central stripe but also the observed image.