Volume Phase Holographic Gratings (VPHGs) are optical element widely used in astronomical spectrographs as main disperser or cross-disperser in high resolution echelle instruments. In spite of the fact that other technologies are available on the market, the VPH technology remain a key one. In the EU funded OPTICON project, different activities were carried out in order to consolidate the design and production of VPHGs for astronomy. In particular: i) a production process based on innovative high performance holographic materials (by COVESTRO AG) has been defined; ii) high quality VPHGs ⪆ 170 mm in diameter were manufactured; iii) innovative configurations, such as multiorder and multiplexed gratings were proposed and the devices realized. Now, more than 10 devices based on this technology are mounted on observing facilities and several more are in development or planned. Here, we retrace the achievement of the VPH activities in the last years and we propose our roadmap for future improvements in the VPHG design and production for supporting the requests of the astronomical community starting from the ORP EU project.
Triazene derivatives are studied as potential doping systems in polymeric films for direct laser wiring (DLW) of volume phase hologram. In particular, aryltriazenes can undergo to the group cleavage with the formation of volatile nitrogen and two radicals when irradiated with UV/violet light. Three derivatives, bearing different substituents, are characterized in n-hexane solution with UV-vis spectroscopy and the different photo-reactivity is highlighted. The best candidate, with a nitro group on the phenyl ring, is then added to a polyurethane matrix at 10% wt and the photoreactivity is evaluated together with the phase modulation in thin films deposited on Si wafers. A 5% change in the product of thickness and refractive index is measured by means of spectral reflectance. Simple patterns are transferred to the films using a DLW facility equipped with a 405 nm laser and studied with the phase contrast microscope.
Compressive sensing (CS) is a new acquisition technique that can potentially open the way to multi- and hyper-spectral imaging in wide spectral regions with a simplified optical scheme. In this framework, we are studying different approaches applied to Earth observation in different contexts. We describe the design of a vis-NIR telescope, where the image sampling is performed in its focal plane with a Digital Micromirror Device (DMD), and the hyperspectral acquisition is made possible by a compact spectrometer with a multispectral detector. We show the approach for a pushbroom multispectral acquisition from space, working in both the visible and thermal bands, to monitor the status of the cryosphere. Finally, we describe a compact wideband system working in the 0.4- 2.5 micron spectral range, based on three parallel spectrometers, where the acquisition is performed with a pushbroom scanning and a rotating coding mask. Opportunities and constraints given by the CS approach in these different contexts are highlighted.
The thiol-ene polymerization between a tetrafunctional thiol and a cyclic allylic sulfide (CAS) monomer is used to generate a crosslinked structure with high concentration of sulfur atoms, which allows for the manufacturing of volume phase holographic gratings with high refractive index modulation (▵n). The writing chemistry is dispersed in a cellulose acetate butyrate (CAB) matrix together with Irgacure® 784 as free radical photoinitiator, making the system sensible to blue light. After the writing step, the performances of the grating can be further improved by means of a thermal treatment, which induces a strong enhancement of the refractive index modulation. This effect could be explained as a reorganization of the polymeric chains that leads to a marked separation between the writing chemistry and the polymeric binder and to local variations of the material density. This hypothesis is supported by different experimental evidences. Values of ▵n up to 0.0346 are reached, which are among the largest reported in the literature.
A new photopolymer system for holographic recording is presented. It exploits the free-radical ring opening polymerization of a cyclic allylic sulfide monomer, 7-methylene-1,5-dithiacyclooctan-3-yl acetate (MDTOA). Such a system allows for the transferring of the sinusoidal holographic pattern with great fidelity, independently on the rigidity of the host matrix. For this purpose, experiments were carried out in a cellulose acetate butyrate (CAB-531-1, Eastman) matrix, using Irgacure 784® as photoinitiating system. The films show excellent optical properties in terms of uniformity and transparency (complete bleaching). The measured diffraction efficiency curves indicate that the written pattern is nearly ideal and it well matches with the theoretical curves computed according to the Kogelnik model. The stability during time of the gratings is monitored and a decrease of efficiency is evident. This means that the formed polymer chains tend to countermigrate smoothing the refractive index modulation. The formation of a crosslinked network is therefore mandatory to obtain stable gratings.
Within the astronomical field, Volume Phase Holographic Gratings (VPHGs) cover nowadays a relevant position as dispersing elements (DE) because each observation could take advantage of specific devices with design and features tailored for achieving the best performances. The manufacturing of highly efficient and reliable VPHGs require holographic materials where it is possible to precisely control the parameters that define the throughput of the device (namely both the refractive index modulation and the film thickness), this is especially true for complex and novel optical designs, where the realization tolerances have to be strictly fulfilled to achieve the theoretical expectations. Moreover, in the design phase, it is crucial to take into account scattering effects and absorption losses to predict with accuracy the final behavior of the optical element. For this application, the most promising materials are the photopolymers because, beside the ability to provide the tuning feature, they bring also advantages such as self-developing, high refractive index modulation and ease of use thanks to their simple thin structure, which is insensitive from the external environment. Thanks to the advantages made available by photopolymeric materials, in this paper we propose innovative solutions for designing stacked spectral-multiplexed VPHGs and transmission dispersing elements (GRISMs) that can cover more than one octave in spectral range by using more than one diffraction order, providing huge advantages for the astronomers in terms of spectral resolution R or time allocation of the instrument for performing the observations. In this context, we also give hints for the process optimization that is crucial for achieving the results reported.
Volume Phase Holographic Gratings (VPHGs) are diffractive elements widely employed in the field of astronomical spectrographs. Photosensitive materials are used for the production of such elements and photopolymers represent a very interesting possibility. In particular, Bayfol® HX solid photopolymers are high performance holographic materials that have been already used for the realization of VPHGs working in the visible for small spectrographs. Recently, a new set of GRISMs have been commissioned at BFOSC spectrograph in order to replace worn or outperforming ones and improve the instrument throughput. The first dispersing element covers the Hα band, while the second one is designed to work in the UV down to 330 nm. Issues related to the material absorption and to the light scattering were faced at short wavelengths. A step forward in the implementation of this class of holographic materials is the design of VPHGs working in the infrared. Two gratings were designed, covering the ZJ band (0.8 – 1.35 μm) and the JH band (1.05 – 1.9 μm). RCWA simulations were performed to find the parameters (refractive index modulation and thickness) required to obtain high efficiency in the target spectral ranges. Material absorptions are not negligible in the NIR and have to be taken into account during the design phase. Preliminary writing tests were performed giving interesting results. In order to make the design phase more reliable, a study of the dependence of the refractive index modulation on wavelength was performed.
Specific astronomical science cases could take advantage of VPHG devices with design and features tailored for achieving the best performances. The manufacturing process require materials where it is possible to precisely control the efficiency response, specially in complex optical designs, where the realization tolerances have to be strictly fulfilled. In this paper, we present an innovative design for the DOLORES spectrograph @ TNG as an example of complex VPHG (in GRISM mode) based on photopolymers. This dispersing element and its prisms were designed to cover, with low R, more than one octave and to disentangle 1st and 2nd diffraction orders avoiding the typical contamination. The ok-sky results are finally presented.
In the astronomical field, the progressive increase in telescope size and in the complexity of the spectroscopic instrumentation has highlighted how the current technologies and traditional materials for dispersing elements do not completely meet the present and future requirements. Therefore, new materials and solutions have to be developed, not only to realize future astronomical facilities, but also to improve the performances of already available instruments and devices. In this context, the use of photopolymeric materials for the production of Volume Phase Holographic Gratings (VPHGs) is becoming an interesting approach thanks to their key properties, in particular the self-developing, high sensitivity and the simple manufacturing process. Here, the main design parameters and the strategy to address them will be presented considering the whole UV-NIR spectral range showing the actual capabilities together with the results obtained on real observing astronomical facilities.
Volume phase holographic elements are becoming attractive thanks to the large efficiency and good optical quality. They are based on photosensitive materials where a modulation of the refractive index is induced. In this paper, we highlight the strategies to obtain a change in the refractive index in a dielectric material, namely a change in the material density and/or in the molecular polarizability. Moreover, we show the results achieved for materials that undergo the photo-Fries reaction as function of the molecular structure and the illumination conditions. We also report the results on a system based on the diazo Meldrum’s acid where volatile molecules are produced upon light exposure.
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