Bayfol HX Research Articles

Comparison of holographic recording options for reflection-format and transmission-format coupler-type diffractive optical elements: theoretical exploration and experimental validation

The holographic recording of gratings that have very large diffraction angles and/or slants is challenging because recording requires beam angles that are not possible without the use of prisms. However, by using a recording wavelength that is different from (usually shorter than) the intended operating wavelength, it is possible to record with less challenging beam angles. In this paper, a recently developed model that allows systematic investigation of the potential and limits of this wavelength-shift recording technique is extended to include a reflection format. Transmission and reflection recording options are compared systematically for the first time, and it is shown that in reflection recording, some couplers can be recorded more easily when the recording wavelength is longer than the operating wavelength. This opens up new design options for previously challenging regions of the spectrum. Experimental validation of the reflection version of the model is carried out by holographic recording in reflection mode at 532 nm in Bayfol HX 200 photopolymer, demonstrating the coupling of blue light.

Characterization of the holographic photopolymer Bayfol HX in the IR region

The possibility of creating holographic optical elements operating in the near infrared spectral range based on the Bayfol HX holographic photopolymer has been considered. The dynamic range of the refractive index of the photopolymer and the amplitude-phase nature of the holograms in the infrared range have been studied. The influence of recording parameters (power density of recording radiation, recording time) on the distribution of the dynamic range of the refractive index between grating harmonics has been studied. The analysis of the amplitude-phase nature of holograms was carried out by measuring the transmission spectra of the studied photopolymer after the photopolymerization reaction. The dynamic range of the refractive index of a photopolymer evaluated in the spectral range from 405 nm to 2099 nm. For this purpose, the angular selectivity contours of holograms with periods from 414 nm to 2100 nm, optimized for different parts of the specified spectral range, were measured and analyzed. The influence of recording parameters on the distribution of the dynamic range of the refractive index between the grating harmonics was analyzed by calculating the amplitudes of the first and second harmonics of the refractive index modulation from the experimentally measured angular selectivity contours of holograms recorded with different recording time at a constant irradiation dose. It was shown that the dynamic range of the refractive index of the photopolymer in the near infrared spectral range, as compared with the long-wavelength part of the visible region of the spectrum, differs by a value that does not exceed the measurement accuracy. A pronounced violation of the reciprocity was demonstrated with scaling of the interference pattern or with changing of the power density of the recording radiation. The optimal recording conditions for holograms calculated for the infrared spectral range for the studied photopolymer were found. The possibility of using of the studied holographic material in telecommunication optics has been demonstrated.

Modelling and Design of Holographic Optical Elements for Beam-Coupling Applications for a Range of Incident Beam Angles

Theoretical modelling has been used to calculate the holographic recording beam angles required in air (at any recording wavelength) to produce a Volume Holographic Optical Element (VHOE) for any defined input and output beam angles. The approach is used to facilitate the design and fabrication of diffractive coupling elements through a holographic process that avoids the use of coupling prisms during recording and will help in the design of recording arrangements that better suit the mass production of low-cost elements, especially those designed for non-normal incidence. In this study, the recording angles needed for a range of recording wavelengths were explored for VHOE couplers designed for input angles (in air) ranging from 0° to −55°. Then, in order to validate the model, holographic recording in Bayfol HX 200 photopolymer at 532 nm was used to fabricate photopolymer VHOE couplers for 633 nm light (−45° input angle in air). Bragg curves obtained experimentally for different probe wavelengths (403 nm, 532 nm and 633 nm) confirm the recording of the desired grating structures to a precision of ±1°, and coupling is demonstrated at 633 nm with a diffraction efficiency of up to 72%. Furthermore, the model is used to identify the origins of some weaker spurious gratings observed alongside the expected ones.

Study of Full-Color Multiplexed Transmission Holograms of Diffusing Objects Recorded in Photopolymer Bayfol HX

A wavelength multiplexing procedure for color transmission volume holograms of diffusing objects recorded in Bayfol® HX200 photopolymer is proposed. For the recording, three lasers of 442 nm, 532 nm and 633 nm, and a Spectralon® diffusing object were used for monochromatic and polychromatic calibration. Monochromatic calibration shows that the maximum index modulation obtained for each wavelength was not enough to give 100% efficiency, although the efficiency values achieved in the case of monochromatic recordings with diffusing objects were high, at around 90% for 633 nm and 532 nm recordings, and 60% for 442 nm. The efficiency values obtained for multiplexed holograms were 19.1% for the 442 nm hologram, 25.9% for the 532 nm hologram and 15.2% for the 633 nm.

Dynamics of Diffraction Efficiency of Superimposed Volume Reflection Holograms at Their Simultaneous Recording in Photopolymer Material

The new experimental data on the dynamics of diffraction efficiency (DE) of superimposed volume reflection holograms with a grating period of $${\sim}$$ 250 nm is obtained at their simultaneous recording in the photopolymer material BAYFOL HX TP. The character and parameters of their interaction are established when exposure of one hologram is delayed relative to the other one. It is revealed that the DE dynamics is practically not affected by variations in diffusion time of monomer in the range $$0.12<t_{D}<0.17$$ s, but the transient behavior of the photopolymerization parameter $$\tau_{p}$$ , which increased in our experiments from 5 to 15 s, becomes considerably visible. The well-known equations of the photoinduced variation in the photopolymer refraction index are improved with account for the character of variation in the parameter $$\tau_{p}$$ . The calculated dependences of DE dynamics obtained with refined formulas are in good agreement with experimental data (mean square deviation is $${\sim}2\%$$ ).

Experimental method for determination of shrinkage direction during holographic recording in Bayfol HX photopolymer

Subject of Research. The paper presents experimental determination of the main shrinkage direction caused by photopolymerization during holographic recording in Bayfol HX material. Method. The determination method for the main shrinkage direction is based on registration of the change in volume holograms characteristics (period, the orientation of isophase planes relative to material surfaces and hologram thickness) caused by the shrinkage. For the implementation of this method a 2D-model is presented which describes shrinkage as a vector. The effect of the shrinkage on characteristics of holograms is evaluated based on geometrical laws. We performed modeling of the shrinkage with different directions and determined its effect on a variety of arbitrarily oriented holograms-gratings. Namely, the cases of isotropic shrinkage, as well as shrinkage in the direction of the material surfaces, the lattice vector and the isophase planes were considered. The experiment is carried out involving a simultaneous recording of two total internal reflection holograms in a photopolymerizable Bayfol HX composite. The changes of their characteristics make it possible to determine clearly the main shrinkage direction. Main Results. The main shrinkage direction has been determined along the less side of the material (considering thickness). However, the experiment has demonstrated a deviation of this direction along the lattice vector as well. Practical Relevance. The developed model and the experimental method allow for taking into account the shrinkage in Bayfol HX while modeling of photopolymerization and multicomponent diffusion in this material. The studied technique also facilitates predictions of hologram characteristics after the shrinkage aimed at its compensation.

Application of Photopolymer Materials in Holographic Technologies.

The possibility of the application of acrylate compositions and Bayfol HX photopolymers in holographic technologies is considered. The holographic characteristics of materials, their advantages, and limitations in relation to the tasks of obtaining holographic elements based on periodic structures are given. The conditions for obtaining controlled two and multichannel diffraction beam splitters are determined with advantages in terms of the simplicity of the fabrication process. The diffraction and selective properties of volume and hybrid periodic structures by radiation incidence in a wide range of angles in three-dimensional space are investigated, and new properties are identified that are of interest for the development of elements of holographic solar concentrators with advantages in the material used and the range of incidence angles. A new application of polymer materials in a new method of holographic 3D printing for polymer objects with arbitrary shape fabrication based on the projection of a holographic image of the object into the volume of photopolymerizable material is proposed, the advantage of which, relative to additive 3D printing technologies, is the elimination of the sequential synthesis of a three-dimensional object. The factors determining the requirements for the material, fabrication conditions, and properties of three-dimensional objects are identified and investigated.

Full modeling and experimental validation of cylindrical holographic lenses recorded in Bayfol HX photopolymer and partly operating in the transition regime for solar concentration.

Concentrating photovoltaics for building integration can be successfully carried out with Holographic Optical Elements (HOEs) because of their behavior analogous to refractive optical elements and their tuning ability to the spectral range that the photovoltaic (PV) cell is sensitive to. That way, concentration of spectral ranges that would cause overheating of the cell is avoided. Volume HOEs are usually chosen because they provide high efficiencies. However, their chromatic selectivity is also very high, and only a small part of the desired spectral range reaches the PV cell. A novel approach is theoretically and experimentally explored to overcome this problem: the use of HOEs operating in the transition regime, which yield lower chromatic selectivity while keeping rather high efficiencies. A model that considers the recording material's response, by determining the index modulation reached for each spatial frequency and exposure dosage, has been developed. It has been validated with experimental measurements of three cylindrical holographic lenses with different spatial frequency ranges recorded in Bayfol HX photopolymer. Simulations of systems comprising two lenses and a mono-c Si PV cell are carried out with the standard AM 1.5D solar spectrum. Promising results are obtained when using the system with lower spatial frequencies lenses: a total current intensity equal to 3.72 times the one that would be reached without the concentrator.

High efficiency, low cost holographic optical elements for ultracold atom trapping.

We demonstrate a method of creating high efficiency, high fidelity, holographic optical elements for the generation of complex optical fields, in a low cost photopolymer, Bayfol HX. The desired optical field profile is generated by a spatial light modulator and written into an optically addressable photopolymer as a volume hologram. We demonstrate the utility of this approach by trapping a Bose-Einstein condensate of rubidium-87 atoms in the nodal plane of an HG0,1 mode generated by one of these holographic optical elements. We also extend this method to the generation holograms with twice the angular momentum per photon than can be generated with a given spatial light modulator.

Characterization of volume holographic optical elements recorded in Bayfol HX photopolymer for solar photovoltaic applications.

Volume Holographic Optical Elements (HOEs) present interesting characteristics for photovoltaic applications as they can select spectrum for concentrating the target bandwidth and avoiding non-desired wavelengths, which can cause the decrease of the performance on the cell, for instance by overheating it. Volume HOEs have been recorded on Bayfol HX photopolymer to test the suitability of this material for solar concentrating photovoltaic systems. The HOEs were recorded at 532 nm and provided a dynamic range, reaching close to 100% efficiency at 800 nm. The diffracted spectrum had a FWHM of 230 nm when illuminating at Bragg angle. These characteristics prove HOEs recorded on Bayfol HX photopolymer are suitable for concentrating solar light onto photovoltaic cells sensitive to that wavelength range.

Photopolymeric films with highly tunable refractive index modulation for high precision diffractive optics

The design and manufacturing of high efficiency and reliable volume phase holographic optical elements require photosensitive material where it is possible to finely control the refractive index modulation. Bayfol HX photopolymers show this feature together with other interesting advantages, in particular the self-developing and the large refractive index modulation. In this paper, the design of Volume Phase Holographic Gratings (VPHGs) is reported underlying the relationship of gratings’ performances with the refractive index modulation. The trend of this property with the change of the laser power density and the ratio of the two writing beams is shown. Based on these results, VPHGs for astronomical instrumentation have been designed and manufactured.