Alessio Zanutta

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.

Materials for VPHGs: practical considerations in the case of astronomical instrumentation

Volume Phase Holographic Gratings are interesting dispersing elements for astronomical instrumentation. An important point, in the realization of the grating, is the choice of the holographic material. Dichromated Gelatines (DCGs) are the best candidate, but they show some drawback especially regarding their water sensitivity and the complex developing process required to enhance their performances. New holographic materials are becoming interesting, such as photopolymers and photochromic materials. An exhaustive review of these classes of materials will be reported and their performances compared to those of DCGs, focusing mainly to the astronomical instrumentation field.

Photopolymer based VPHGs: from materials to sky results

Volume Phase Holographic Gratings cover a relevant position as transmission dispersing elements in astronomical spectrographs and each astronomical observation could take advantage of specific dispersive elements with features tailored for achieving the best performances. The design and manufacturing of high efficiency and reliable VPHGs require photosensitive materials where it is possible to control both the refractive index modulation and the film thickness. By means of Bayfol® HX photopolymers, we designed and manufactured six VPHGs for astronomical instrumentation in a GRISM configuration. We demonstrated how photopolymers are reliable holographic materials for making astronomical VPHGs with performances comparable to those provided by VPHGs based on Dichromated Gelatins (DCGs), but with a much simpler production process.

Compressive sampling for multispectral imaging in the vis-NIR-TIR: optical design of space telescopes

Micro-satellites equipped with multispectral payloads are now under development to acquire information on the radiation reflected and emitted from the earth in the vis-NIR-TIR bands. In this framework, we are studying different approaches based on the compressive sampling technique supported by innovative multispectral detectors, where the image sampling is performed on the telescope focal plane with a Digital Micromirror Device (DMD). We will describe in the paper the possibilities and the constraints given by the use of the DMD in the focal plane. The optical design of the telescope, relay system and detector in two different application cases will be provided.

Compact spectral multiplexing VPHGs using stacked photopolymeric layers

Within the astronomical field, many focal-reducer spectrographs that are currently available at state-of-the-art telescopes facilities, would benefit from a simple refurbishing that aims to increase both the resolution and spectral range. This kind of upgrade would cope with the progressively challenging scientific requirements, but, in order to make it appealing, it should minimize the changes in the existing structure of the instruments. In the past, many authors already tried to propose solutions based on stacking subsequently many dispersive elements and recording multiple spectra in one shot (multiplexing). Although this idea is very promising, it brings several drawbacks and complexities that prevent the straightforward integration of such a device in a spectrograph. Fortunately, today, the situation has changed dramatically, thanks to the availability of new materials such as the photopolymeric holographic films, that have proven their reliability in the fabrication of volume-phase holographic gratings (VPHGs) for astronomy. Thanks to the various advantages made available by these materials in this context, we propose an innovative solution for designing stacked multiplexed VPHG that is able to secure efficiently different spectra in a single shot. This will allow to increase resolution and spectral range enabling astronomers to greatly economize their awarded time at the telescope. In this proceeding, we demonstrate the applicability of our solution, both in terms of expected performance and feasibility, supposing the upgrade of the Gran Telescopio CANARIAS (GTC) Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS).

Holographic diffraction gratings based on photopolymers: achieved results and new opportunities in astronomical spectroscopy

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.

VPHGs for WEAVE: design, manufacturing and characterization

WEAVE is the next-generation optical spectroscopy facility for the William Herschel Telescope (WHT). It shows two channels (blue and red) and two working modes, a low-resolution (R=3,000-7,500) and a high-resolution (R=13,000- 25,000). The dispersing elements of the spectrograph are Volume Phase Holographic Gratings (VPHGs), two for the lower resolution mode and three for the higher resolution mode. Such gratings have a large size (clear aperture > 190 mm) and they are characterized by some key features, i.e. diffraction efficiency, wavefront error and dispersion that affect the final performances of the spectrograph. The VPHGs have been produced by KOSI based on the WEAVE design. After that, the VPHGs have been characterized, showing interesting results in terms of diffraction efficiency that reached peak values of 90%. As for the wavefront distortion, which is one of the critical aspect in VPHG technology, a different behavior between medium and high resolution elements was found. A larger wavefront distortion have been measured in the high resolution elements, because of the higher aspect ratio. A polishing process on the assembled VPHGs has been performed in order to reduce the wavefront distortion. Here, the results are presented and the specific issues discussed.

Photopolymer-based VPHGs for astronomy: update and new possibilities

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.

New over-octave VPHG architecture for DOLORES spectrograph at TNG

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.

Spectral multiplexed VPHG based on photopolymers: the first application on a spectrograph

Many of the current spectrographs available at state-of-the-art telescopes facilities, possess specifications that are strongly limited by the dispersing elements that are used. Therefore, a refurbishment of these devices would potentially increase the performances if innovative designs are considered. We propose a solution for designing stacked VPHG that is able to secure efficiently different spectra in a single shot. This could be possible considering parameters that are specific for a particular class of holographic material, the photopolymers, that are well known for bringing reliability and precise throughput. We demonstrate the applicability of our solution, through the example of the new spectrograph designed for the 1m telescope at SAO RAS. The spectrograph will cover a spectral range 444-706 nm with the spectral resolving power of R=4273-5176 and the throughput maximum of 64%. The working ranges of the gratings are selected to provide more diffraction efficiency around the main important lines used in astrophysics.