Dennis Hönel

Holographic recording aspects of high-resolution Bayfol HX photopolymer

We have been developing a new class of recording materials for volume holography, offering the advantages of full color recording and depth tuning without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two-chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network gives the necessary mechanical stability to the material prior to recording. In this paper we describe several aspects of holographic recording into Bayfol® HX which are beneficial for its effective use and discuss them within a more elaborate reaction-diffusion model. Inhibition phenomena and the influence of precure are studied within this model and are investigated experimentally for single hologram recording and angular multiplexed hologram recordings. Also the dark reaction after the exposure period and the minimum allowable waiting time for full hologram formation are addressed. The proper understanding of these phenomena is important for the optimal usage of these new materials, in for example step-and-repeat mass production of holograms.

Reaction-diffusion model applied to high resolution Bayfol HX photopolymer

We have been developing a new class of recording materials for volume holography, offering the advantages for full color recording and depth tuning without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network gives the necessary mechanical stability to the material prior to recording. In this paper we show that the recording process in these materials can be successfully described within a reactiondiffusion model. For the first time the combination of plane-wave recording data in transmission and reflection geometry was used to extract the model parameters. This was achieved via a master curve construction of the respective power density response functions of the photopolymer at saturation recording conditions. Within that model, power density response, spatial frequency response, non-locality effects, beam ratio effects and even dosage response can be predicted and explained for a wide range of CW recording conditions which are important for various holographic applications of these new materials.

New recording materials for the holographic industry

This paper describes a new class of recording materials for volume holographic applications suitable to meet commercial manufacturing needs. These next-generation holographic photopolymers have the ability to satisfy the unmet demand for color and depth tuning that is only possible with volume holograms. Unlike earlier holographic photopolymers, these new materials offer the advantages of no chemical or thermal processing combined with low shrinkage and detuning. Furthermore, these materials exhibit high transparency, a high resolution of more than 5000 lines/mm and are environmentally robust. Bayer MaterialScience plans to commercialize these materials, which combine excellent holographic characteristics with compatibility to mass-production processes. In this paper, we will briefly discuss the potential markets and applications for a new photopolymer, describe the attributes of this new class of photopolymers, relate their ease of use in holographic recording, and discuss potential applications of such materials..

Holographic recordings with high beam ratios on improved Bayfol® HX photopolymer

Bayfol® HX film is a new class of recording materials for volume holography. It was commercialized in 2010 and is offering the advantages for full-color recording and moisture resistance without any chemical or thermal processing, combined with low shrinkage and detuning. These photopolymers are based on the two-chemistry concept in which the writing chemistry is dissolved in a preformed polymeric network. This network provides the necessary mechanical stability to the material prior to recording. In addition to the well-known security and imaging applications, Bayfol® HX film also offers a new opportunity for the manufacturing of volume Holographic Optical Elements (vHOEs) in new optical and optoelectronic applications. For the implementation of holographic recording layouts and associated exposure schedules for these HOEs detailed understanding of the photopolymer material properties and the knowledge how to achieve the optical requirements of dedicated holographic applications are necessary. In this paper we extend the application of our simulation method for the writing mechanism for the Bayfol® HX photopolymer film. Different photopolymer product variations, including development of photopolymer grades with improved bleaching properties and increased dynamical range, which enable simultaneous multi-color recording, while maintaining a high diffraction efficiency of the recorded holograms are covered. The model is investigated experimentally by recording and evaluation of specifically designed directional diffuser vHOEs as they would be used e.g. for light shaping or light management purposes. One important observation is the capability of Bayfol® HX film type photopolymers to form highly efficient diffraction gratings even at very high intensity ratios of the reference beam versus the object beam.

Bayfol ® HX photopolymer for full-color transmission volume Bragg gratings

Manipulating light with optical gratings based on volume Holographic Optical Elements (vHOEs), also known as volume Bragg gratings, has the advantage to reconstruct only the first diffraction order and hence provide high diffraction efficiencies and angular selectivity. In addition, they offer the further benefit to be fully transparent in the off- Bragg condition like it is required in optical combiners. We present the latest status of our instant-developing photopolymer film technology (Bayfol® HX) and show beneficial recording parameters - specifically we discuss the challenges to record transmission vHOEs and how to overcome them. Experimental results on color transmission recordings are shown and it is demonstrated that those match perfectly to Kogelniks coupled wave theory. It is recommended to adopt the dynamic range of the recording media by proper choice of recording dosage, recording power, beam ratio and photopolymer film type to the desired transmission vHOE design.

Diffractive optics in large sizes: computer-generated holograms (CGH) based on Bayfol ® HX photopolymer

Volume Holographic Optical Elements (vHOE) offer angular and spectral Bragg selectivity that can be tuned by film thickness and holographic recording conditions. With the option to integrate complex optical function in a very thin plastic layer formerly heavy refractive optics can be made thin and lightweight especially for large area applications like liquid crystal displays, projection screens or photovoltaic. Additionally their Bragg selectivity enables the integration of several completely separated optical functions in the same film. The new instant developing photopolymer film (Bayfol® HX) paves the way towards new cost effective diffractive large optics, due to its easy holographic recording and environmental stability. A major bottleneck for large area applications has been the master hologram recording which traditionally needs expensive, large high precision optical equipment and high power laser with long coherence length. Further the recording setup needs to be rearranged for a change in optical design. In this paper we describe an alternative method for large area holographic master recording, using standard optics and low power lasers in combination with an x, y-translation stage. In this setup small sub-holograms generated by a phase only spatial light modulator (SLM) are recorded next to each other to generate a large size vHOE. The setup is flexible to generate various types of HOEs without the need of a change in the mechanical and optical construction by convenient SLM programming. One Application example and parameter studies for printed vHOEs based on Bayfol® HX Photopolymer will be given.

Diffractive optics with high Bragg selectivity: volume holographic optical elements in Bayfol® HX photopolymer film

For a long time volume Holographic Optical Elements (vHOE) have been discussed as an alternative, but were hampered by a lack of suitable materials. They provide several benefits over surface corrugated diffractive optical element like high diffraction efficiency due to their ability to reconstruct a single diffraction order, freedom of optical design by freely setting the replay angles and adjusting their bandwidth by a selection of the vHOE’s thickness. Additional interesting features are related to their high Bragg selectivity providing transparent films for off-Bragg illumination. In this paper we report on our newly developed photopolymer film technology (Bayfol® HX) that uniquely requires no post processing after holographic exposure. We explain the governing non-local polymerization driven diffusion process leading to an active mass transport triggered by constructive interference. Key aspects of the recording process and their impact on index modulation formation is discussed. The influence on photopolymer film thickness on the bandwidth is shown. A comparison between coupled wave theory (CWT) simulation and experimental results is given. There are two basic recording geometries: reflection and transmission vHOEs. We explain consequences of how to record them properly and discuss in more detail the special challenges in transmission hologram recording. Here beam ratio and customization of photopolymer film properties can be applied most beneficially to achieve highest diffraction efficiency.

Performance optimization in mass production of volume holographic optical elements (vHOEs) using Bayfol HX photopolymer film

Volume Holographic Optical Elements (vHOEs) gained wide attention as optical combiners for the use in smart glasses and augmented reality (SG and AR, respectively) consumer electronics and automotive head-up display applications. The unique characteristics of these diffractive grating structures – being lightweight, thin and flat – make them perfectly suitable for use in integrated optical components like spectacle lenses and car windshields. While being transparent in Off-Bragg condition, they provide full color capability and adjustable diffraction efficiency. The instant developing photopolymer Bayfol® HX film provides an ideal technology platform to optimize the performance of vHOEs in a wide range of applications. Important for any commercialization are simple and robust mass production schemes. In this paper, we present an efficient and easy to control one-beam recording scheme to copy a so-called master vHOE in a step-and-repeat process. In this contact-copy scheme, Bayfol® HX film is laminated to a master stack before being exposed by a scanning laser line. Subsequently, the film is delaminated in a controlled fashion and bleached. We explain working principles of the one-beam copy concept, discuss the opto-mechanical construction and outline the downstream process of the installed vHOE replication line. Moreover, we focus on aspects like performance optimization of the copy vHOE, the bleaching process and the suitable choice of protective cover film in the re-lamination step, preparing the integration of the vHOE into the final device.

Mass production of volume holographic optical elements (vHOEs) using Bayfol® HX photopolymer film in a roll-to-roll copy process

Volume Holographic Optical Elements (vHOEs) gained wide attention as optical combiners for the use in augmented and virtual reality (AR and VR, respectively) consumer electronics and automotive head-up display applications. The unique characteristics of these diffractive grating structures – being lightweight, thin and flat – make them perfectly suitable for use in integrated optical components like spectacle lenses and car windshields. While being transparent in Off-Bragg condition, they provide full color capability and adjustable diffraction efficiency. The instant developing photopolymer Bayfol® HX film provides an ideal technology platform to optimize the performance of vHOEs in a wide range of applications. Important for any commercialization are simple and robust mass production schemes. In this paper, we present an efficient and easy to control one-beam recording scheme to copy a so-called master vHOE in a step-and-repeat process. In this contact-copy scheme, Bayfol® HX film is laminated to a master stack before being exposed by a scanning laser line. Subsequently, the film is delaminated in a controlled fashion and bleached. We explain working principles of the one-beam copy concept and discuss the mechanical construction of the installed vHOE replication line. Moreover, we treat aspects like master design, effects of vibration and suppression of noise gratings. Furthermore, digital vHOEs are introduced as master holograms. They enable new ways of optical design and paths to large scale vHOEs.

Precision holographic optical elements in Bayfol HX photopolymer

The versatility of Volume Holographic Optical Elements (vHOE) is high, especially because of their tunable angular and spectral Bragg selectivity. Those unique lightweight, thin and flat optical elements are enabled by the new instant developing photopolymer film Bayfol® HX technology, which allows to mass produce cost effective diffractive optics due to its simplified and robust holographic recording process. From a pure scientific point of view volume holography is well established. In practice though, commercially available optical design software is not adapted to handle the specific characteristics of photopolymer diffractive optical elements and their recording. To achieve high quality vHOE precision optics, the recording setup needs to accommodate several aspects that will be covered in this paper. We report on means how to deal with photopolymer shrinkage and average refractive index changes of the recording media. An important part in diffractive optics design is the compensation of different conditions between the holographic recording setup and in a final product containing the vHOE. Usually substrates might need to be changed (in material, in refractive index) as well the illumination sources are using incoherent light having angular and spectral emission profiles with finite bandwidth. Recently special in- and out-coupling vHOEs are becoming attractive e.g. in near eye displays and in compact lighting devices. We will report on design considerations and adjustments to the recording condition for a specific in-coupling vHOE and demonstrate the effects of pre-compensation on this example.