Abstract
In this study we investigated the undesired but possible fringe formation during the recording of large size holographic optical elements (HOE) using a dry photopolymer. We identified the deformation of the recording element during hologram exposure as the main source for this fringe formation. This deformation is caused mainly by the one-sided heating of the recording element, namely, the dry photopolymer–recording plate stack. It turned out that the main source for this heating was the heat of polymerization in the dry photopolymer released during the exposure interval. These insights were translated into a physical model with which quantitative predictions about thermal fringe formation can be made depending on the actual HOE recording geometry, recording conditions and characteristics of the dry photopolymer. Using this model, different types of large size HOEs, used as components to generate a steerable confined view box for a 23” diagonal size display demonstrator, could be recorded successfully without thermal fringe formation. Key strategies to avoid thermal fringe formation deduced from this model include balancing the ratio of lateral recording plate dimension R to its thickness h, recording the power density P or equivalently the exposure time texp at a fixed recording dosage E, and most importantly recording the the linear coefficient of thermal expansion (CTE) of the recording plate material. Suitable glass plates with extremely low CTE were identified and used for recording of the above-mentioned HOEs.
Highlights
A holographic optical element (HOE) utilizes diffractive optics to redirect light and manage its spectral and polarization behavior
Thin film HOEs are of upmost importance for slim optical devices which have to facilitate very complex optical functions, for example augmented reality head up displays (AR-HUD) and transparent displays (TD) [1,2] in automotive applications, augmented reality transparent screens (TS) [3], head mounted displays (HMD) [4,5,6], diffractive-based virtual reality (VR) glasses [7], eye tracking systems [8], and autostereoscopic 3D displays (ASD) [9]
Regarding the increasing use of light emitting diodes (LED) and laser diodes (LD) as light sources, the advantages of HOEs realized as Volume Bragg Gratings (VBG) recorded in a dry photopolymer can be fully utilized
Summary
A holographic optical element (HOE) utilizes diffractive optics to redirect light and manage its spectral and polarization behavior. By increasing the ratio of W/h and/or H/h we observed that the chance that the hologram shows macroscopic dark and bright fringes instead of a homogeneous diffraction efficiency or homogeneous brightness increased dramatically This fringe formation spoils the function and the quality of the HOE. It should be noted that this fringe formation is different from thickness fringes— often named wood grains—caused by interferences from Fresnel reflected beams at the recording plate surfaces This fringe formation is different from dark fringes caused by birefringence either in the recording support or the substrate of the photopolymer film. The seventh section describes a model for the thermal fringe formation as a local diffraction efficiency modulation, using the estimated recording element deformation in conjunction to the recording geometry. Detailed mathematical matter to derive the quantitative physical model are given in the appendices
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