Design of freeform diffraction gratings: performance, limitations and potential applications

Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs

The reformatting advantage: photonics vs conventional optics!

In recent decades, spectroscopic capabilities have been significantly enhanced by new technological developments, in particular spatial reformatting. Spatial reformatting allows multiple functionalities: the observation of a larger area of sky, obtaining the spectra of all spatial elements under the same atmospheric conditions; modification of the shape and size of the field of view; focal-ratio conversion for the optimized coupling between the telescope and the spectrograph; increase in the spatial and spectral resolving power; the observation of multiple objects; homogeneity in the illumination; scrambling of spatial and/or phase induced structure with the instrument, thus improving the system stability; relocation of the exit pupil, especially important for telecentric systems. The impact of reformatting and the breadth of science cases is so great that many alternative methods and technologies have been proposed: image slicers using refractive or reflective solutions; optical fibers with different core sizes and geometries; microlenses used in isolation or combined with fibers and more recently, photonic devices such as Photonic lanterns to produce modal decomposition. In this paper, a comparison between all currently available options is presented, with a detailed analysis of their advantages and limitations and a proposal for a new reformatter combining slicers and photonic devices. This proposal presents the advantages of the other alternatives and additionally offers: minimization of focal-ratio degradation; produces image and modal decomposition; improves the throughput along the spectral range, increases the spectral resolving power and adds the functionality of scrambling. All of these advantages are combined in a system where photonic and astronomical instrumentation capabilities are joined in an innovative solution with many applications, like for example, the Extremely Large Telescope

Improved calibration of vertical scanning optical profilometers for spherical profiles measurements

A new method for calibrating optical scanning profilometers is presented. Especially adapted to spherical and aspherical profile measurements, it shows an increase of accuracy bigger than one order of magnitude for radius of curvature measurements. Calibration of vertical scaling is obtained with a reduction of its uncertainty by a factor larger than 2, which also demonstrates the advantage of this method for any surface measurements. Using commercially available reference balls, this method is easily implementabl

CUBES: application of image slicers to reformat the field for two spectral resolving powers

The Cassegrain U-Band Efficient Spectrograph (CUBES) is a high-efficiency spectrograph designed for observations from 305 to 400nm. It will be integrated at a Cassegrain focus of the Very Large Telescope (VLT). The image slicer technology is applied to reformat the field of view reducing the spectrograph entrance slit etendue and minimising the spectrograph volume and weight without slit losses. Two image slicers will provide CUBES with two spectral resolving powers: R≥20,000 for high resolution (HR) and R≥5,000 for low resolution (LR). Both image slicers are composed of two arrays of six spherical mirrors. For the HR mode, a rectangular field of view of 1.5arcsec by 10arcsec is reorganised into a slit of 0.19mm × 88mm; for the LR mode, a field of view of 6arcsec by 10arcsec is reformatted into a slit of 0.77mm × 88mm, with slicer mirrors of width 0.5mm and 2mm, respectively. CUBES is currently in the Preliminary Design Phase (Phase B). This communication presents the Conceptual (Phase A) design and the main performance for the HR and LR image slicers addressing the following technological challenges: compact layout with the minimum number of optical components to optimise throughput, near diffraction limited optical quality, telecentric design with overlapped exit pupils for all slices of the field of view, distribution of the slicer mirrors to reduce shadows and selection of the best substrate for the very short wavelengths at which CUBES will operate

Low loss coatings for the VIRGO large mirrors

présentée par L. PinardThe goal of the VIRGO program is to build a giant Michelson type interferometer (3 kilometer long arms) to detect gravitational waves. Large optical components (350 mm in diameter), having extremely low loss at 1064 nm, are needed. Today, the Ion beam Sputtering is the only deposition technique able to produce optical components with such performances. Consequently, a large ion beam sputtering deposition system was built to coat large optics up to 700 mm in diameter. The performances of this coater are described in term of layer uniformity on large scale and optical losses (absorption and scattering characterization). The VIRGO interferometer needs six main mirrors. The first set was ready in June 2002 and its installation is in progress on the VIRGO site (Italy). The optical performances of this first set are discussed. The requirements at 1064 nm are all satisfied. Indeed, the absorption level is close to 1 ppm (part per million), the scattering is lower than 5 ppm and the R.M.S. wavefront of these optics is lower than 8 nm on 150 mm in diameter. Finally, some solutions are proposed to further improve these performances, especially the absorption level (lower than 0.1 ppm) and the mechanical quality factor Q of the mirrors (thermal noise reduction)

Design of freeform diffraction gratings: performance, limitations and potential applications

Spectroscopy is a key technique in astronomy and nowadays most major telescopes include at least one spectrograph in their instrument suite. The dispersive element is one of the most important components and it defines the pupil size, spectral resolution and efficiency. Different types of dispersive elements have been developed including prisms, grisms, VPH and echelle gratings. In this paper, we investigate the design and optimization possibilities offered by metallic freeform gratings using diamond machining techniques. The incorporation of power in a diffraction grating enables several functionalities within the same optical component, such as the combination of dispersion, focusing and field reformat. The resulting benefit is a reduction in the number of surfaces and therefore, an improvement in the throughput. Freeform surfaces are also interesting for their enhanced optical performance by allowing extra degree of freedom in the optimization. These degrees of freedom include the shape of the substrate but also additional parameters such as the pitch or the number of blaze angle. Freeform gratings used as single optical component systems also present some limitations such as the trade-off between optical quality versus field of view or the spectral range versus spectral resolution. This paper discusses the possibility offered by the design of freeform gratings for low to medium spectral resolution, in the visible and near-infrared, for potential applications in ultra-compact integral field spectrographs

New opportunities of freeform gratings using diamond machining

With the recent development of new ultra fine aluminium alloys and progress in the field of directly machined freeform surfaces, diamond machined freeform gratings could play an important part in future spectrographs or integral field units, particularly at SWIR and LWIR wavelengths where the improved thermal performance of metal optics at cryogenic temperatures is well established. Freeform diamond machined gratings can offer a cost-effective, compact, and flexible alternative to gratings fabricated by other methods such as ion beam etching or complement these technologies. In this paper, both the advantages and limitations of 5 axis diamond machined freeform gratings are presented and potential applications are discussed

Optiques freeforms : défis et perspectives

International audienceLes optiques freeforms sont les optiques de demain. Elles inondent déjà notre quotidien dans de nombreux domaines et répondent à une attente profonde du marché. La révolution des optiques freeforms est en marche, une opportunité pour qui saura concevoir ces objets, les fabriquer, les mesurer et les intégrer

Low coherence interferometry to characterize the induced vibrations and topology change of the cryogenic mirror of the Einstein Telescope prototype

We describe the state of development of a white light interferometer to characterize the cryogenic mirrors for GW detector on operation. We include the first experimental results from the proof of concept of the metrology instrument. The instrument will characterize the topology as well as the vibration of the mirrors. This development takes place in the frame of the E-TEST project. E-TEST is one of the technology demonstrators for the future Einstein Telescope (ET). ET is dedicated to the measure and characterization of gravitational waves. The prototype built by E-TEST includes a large silicon mirror of 40 cm diameter suspended by innovative vibration isolation hanging modules. To reach the detection specification, the mirror is cooled down at cryogenic temperatures around 20 K. Nevertheless, even after the isolation, the mirror may not reach perfect stability once at cryogenic temperatures. Furthermore, the mirror may experience surface topology changes and wavefront deformation due to the extreme variations in temperature and gradient. With our metrology instrument, we can obtain on a single camera frame a set of interferogram maps of the area observed on the mirror at different optical path differences. To do this, we design an innovative phase mask for a white light low-coherence interferometer. In addition, we implement new algorithms for the white light interferogram analysis, avoiding the limitations of the conventional Phase Shifting Interferometry algorithms.</p

Polishing and coating of a F1250mm 90° off-axis parabola for PW lasers

International audienc