We present highly transparent, wave front printed volume holographic optical elements (vHOEs), realized with a new recording method based on the pre-illumination of incoherent light patterns. The introduced amplitudemodulated pattern illuminates a distinct area on the unexposed, photopolymer-based holographic recording material prior to the hologram recording sequence. The incoherent pre-illumination scheme enables a precise tuning of the material’s local photosensitivity without the formation of a holographic volume diffraction grating. As a consequence, the pre-illumination exposure significantly suppresses the formation of transparency diminishing structures in the material that are formed concurrently with the volume diffraction grating during the hologram recording sequence. The pre-illumination component is integrated in an extended immersion-based wave front printing setup, which realizes vHOEs by sequentially recording single holographic elements in an array-like structure. A wide range of different recording configurations is enabled by our recording setup due to independent modulation of both wave fronts and the possibility to realize large off-axis recording angles. We introduce two hologram characterization methods, based on a diffraction efficiency and a slanted-edge method analysis, which are used to evaluate the implemented pre-illumination method and demonstrate significant improvements to the see-through quality of the presented wave front recorded vHOEs.
Recently, holographic optics such as volume holographic optical elements (vHOEs) receive increasing attention as optical combiners in augmented reality (AR) applications. Especially vHOEs fabricated by means of wave front printing have the potential to realize complex optical functions with high diffraction efficiency while maintaining excellent transmittance. We present the recording of a holographic combiner for AR applications fabricated by means of individually modulated recording wave fronts in our extended immersion-based holographic wave front printer setup. Holographic elements from our setup are made up of individual sub-holograms, so called Hogels. The implementation of two phase-only reflective spatial light modulators (SLMs) allows for the recording of Hogels and consequently vHOEs in a wide range of different configurations. Large-area vHOEs are achieved by adjacent recording of multiple Hogels in a step-wise fashion. Our immersion-based printer setup ensures a high numerical aperture for the recording configuration, which is directly linked to a wide angular range of possible replay configurations for wave front propagation in air. We present a reflective vHOE realizing a large off-axis to on-axis wave front transformation suitable as holographic combiner for retinal scanning displays. The vHOE is characterized by evaluating the diffractive properties of the hologram’s volume gratings, as well as investigating the vHOE’s combiner characteristics by means of field of view (FoV) and eye box size evaluation.
The recording of computer-generated holographic optical elements (HOEs) via the concept of holographic wave front printing has been a topic of rising interest in many research groups over the last years. Especially for applications in augmented reality (AR), holographic wave front printing has the potential to realize HOEs with complex optical transformations and high diffraction efficiencies while maintaining excellent transmittance. Here, we present a novel immersion-based holographic wave front printer setup, which allows the recording of reflection volume holographic optical elements (vHOEs) in both on-axis and off-axis configurations. HOEs fabricated via our wave front printing process are made up of individual sub-holograms, so called Hogels. Each sub-hologram is recorded via two phase-only reflective spatial light modulators (SLMs). Large-area vHOEs are achieved by adjacent recording of multiple Hogels in a step-wise fashion. Our immersion-based holographic printer setup ensures a high numerical aperture for the recording configuration, that is directly linked to a wide angular range in which recorded wave fronts can be replayed in air configuration. As a possible AR application, we demonstrate the recording of a holographic combiner for retinal projection. A single eye box is projected in the user's field of view (FOV) by means of a scanned laser projector source. Each Hogel of the holographic combiner performs an individual wave front transformation of large off-axis to on-axis angles, which contributes to the global holographic transfer function of the vHOE. Haze and clarity analysis of the recorded vHOE confirm high transmittance, which is crucial for AR applications.
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