The design and realization of a plane-grating monochromator mainly intended for high-energy resolution in the extreme-ultraviolet and soft-X-ray spectral regions is presented. The principal application is the spectral selection of high-order laser harmonics generated in gas combined with the possibility to achieve sub-20-meV bandwidth, approaching synchrotron-like beamlines resolution performances. This geometry has three optical elements: a uniform-line-spaced plane grating illuminated by the converging light coming from a focusing cylindrical mirror and an additional plane mirror that is needed to change the grating subtended angle to keep the system in focus on a fixed slit. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. A monochromator for the 12-50 eV region has been realized to show the feasibility and the performances of the configuration.
We discuss the design of double plane grating compressors to be used for chirped pulse amplification on free-electron laser (FEL) ultrashort pulses at energies higher than 100 eV. In particular, the design of a grating compressor for wavelength emission centered at 10 nm is demonstrated. The XUV efficiency measurements in the 8-12 nm range of the gratings to be used in a double plane grating compressor realized for an upcoming experiment planned at FERMI is presented and discussed. The overall instrument efficiency is furthermore analyzed.
We present the optical layout of soft X-rays compressors using reflective grating specifically designed to give both
positive or negative group-delay dispersion (GDD). They are tailored for chirped-pulse-amplification experiments with
FEL sources. The optical design originates from an existing compressor with plane gratings already realized and tested at
FERMI, that has been demonstrated capable to introduce tunable negative GDD. Here, we discuss two novel designs for
compressors using deformable gratings capable to give both negative and positive GDD. Two novel designs are
discussed: 1) a design with two deformable gratings and an intermediate focus between the twos, that is demonstrated
capable to introduce positive GDD; 2) a design with one deformable grating giving an intermediate focus, followed by a
concave mirror and a plane grating, that is capable to give both positive and negative GDD depending on the distance
between the second mirror and the second grating. Both the designs are tunable in wavelength and GDD, by acting on
the deformable gratings, that are rotated to tune the wavelength and the GDD and deformed to introduce the radius
required to keep the spectral focus. The deformable gratings have a laminar profile and are ruled on a thin silicon plane
substrate. A piezoelectric actuator is glued on the back of the substrate and is actuated to give a radius of curvature that is
varying from infinite (plane) to few meters. The ruling procedure, the piezoelectric actuator and the efficiency
measurements in the soft X-rays will be presented. Some test cases are discussed for wavelengths shorter than 12 nm.
The main topics related to the optical design of table-top ultrafast beamlines with femtosecond or sub-femtosecond resolution for the generation and use of high-order laser harmonics are here discussed. After the generation through laser-gas interaction, the extreme-ultraviolet (XUV) photon beam has to be conditioned and handled. The paper is focused on two main issues: 1) beam monochromatization and 2) beam focusing. The available techniques to realize XUV ultrafast tunable monochromators using gratings are discussed. The main issue to be faced when designing a monochromator is the preservation of the ultrashort duration of the pulse after the monochromatization. The different available grating geometries and some practical realizations are presented. Furthermore, the problems related to the design of the focusing section of XUV ultrafast beamlines are discussed. The effects of the focusing properties on the ultrashort duration of the pulse are considered, namely the focal aberrations due to the optical design. Some optical solutions for XUV ultrafast micro-focusing and results measured on existing beamlines are discussed.
Polarimetry is a powerful tool to interpret how the coronal plasma is involved in the energy transfer processes from the Sun’s inner parts to the outer space. Space polarimetry in the far ultraviolet (FUV) provides essential information of processes governed by the Doppler and Hanle resonant electron scattering effects. Among the key FUV spectral lines to observe these processes, H I Lyman α (121.6 nm) is the most intense. Some developing or proposed solar physics missions, such as CLASP, SolmeX, and COMPASS, plan to perform polarimetry at 121.6 nm. Classical solutions, such as a parallel plate of a transparent material, either MgF2 or LiF, result in a modest efficiency of the passing polarization component. The development of more efficient linear polarizers at this wavelength will benefit future space instruments. A research has been conducted to develop polarizers based on (Al/MgF2)n multilayer coatings in a band containing 121.6 nm, to obtain a significant efficiency increase over plates. Coatings operating by reflectance resulted in a high efficiency after approximately one year of storage under nitrogen. In parallel, coating polarizers operating by transmittance have been prepared for the first time. Transmissive polarizers have the advantage that they involve no deviation of the beam. As a further benefit, the developed transmittance polarizers additionally incorporate filtering properties to help reject wavelengths both shortwards and longwards of a band containing 121.6 nm. Hence a polarizer combined with a filter is obtained with a single device. The combined polarizer-filter could enable a higher performance polarimeter for solar physics if the use of a separate filter to isolate Lyman α turns unnecessary.
We present the optical layout of a reflective grating compressor specifically designed for extreme-ultraviolet FEL sources. The working principle is based on the use of a couple of constant-line-spaced gratings used at grazing incidence and illuminated in divergent light. The two possible grating configurations, namely the on-plane and off-plane, are analyzed and compared. The Group Delay Dispersion (GDD) introduced by the compressor is analytically analyzed and quantified. The spatial chirp also is considered, and its effect analyzed. The deviation from the ideal case in which the instrument is feed with a collimated beam is considered. The effect of the beam divergence on the compressor response is quantified and the attenuation of this effect by a “de-tuning” of the compressor is proposed. This solution avoids the use of a pre-collimating optics, therefore incrementing the total instrumental throughput. Finally, it is shown the optical design of an actual compressor for the FERMI FEL, that can be inserted in the optical path without any deviation or translation of the photon beam with respect to the nominal path.
We present the design and characterization of a compact and portable spectrometer realized for photon in-photon out
experiments (in particular X-Ray Emission Spectroscopy, XES), in particular tailored to be used at the FERMI freeelectron-
laser (FEL) at ELETTRA (Italy). The spectrometer can be installed on different end stations at variable
distances from the target area both at synchrotron and FEL beamlines. Different input sections can be accommodated in
order to fit the experimental requests, with/without an entrance slit and with/without an additional relay mirror. The
design is compact in order to realize a portable instrument within a total footprint of less than one square meter. The
instrument is based on the use of two flat-field grazing-incidence gratings and an EUV-enhanced CCD detector to cover
the 25-800 eV spectral range, with spectral resolution better than 0.2%. The absolute response of the spectrometer, has
been measured in the whole spectral region of operation, allowing calibrated measurements of the photon flux. The
characterization on the Gas Phase beamline at ELETTRA Synchrotron as instrument for XES and some experimental
data of the FEL emission taken at EIS-TIMEX beamline at FERMI, where the instrument has been used for photon beam
diagnostics, are presented.
METIS, the Multi Element Telescope for Imaging and Spectroscopy, is a coronagraph selected by the European Space
Agency to be part of the payload of the Solar Orbiter mission. The original METIS proposal included four optical paths,
for observations in: 1) linearly polarized visible-light (590-650 nm), 2) narrow-band ultraviolet HI Lyman-alpha (121.6
nm), 3) narrow-band extreme-ultraviolet HeII Lyman-alpha (30.4 nm), 4) spectrographic mode for the HI Lyman- alpha
and He II Lyman-alpha in corona. The design, coating performances, and test activities of the grating for the
spectroscopic path are here described. The grating is optimized to work at near normal incidence and to diffract the 121.6
nm radiation at the first order and the 30.4 nm at its 4th order, consequently the two spectroscopic channels are
overlapped on the focal plane. The grating is spherical with variable-line-spaced rulings, 1800 gr/mm central density.
The selection of the spectroscopic channel to be acquired, either the 121.6 nm or the 30.4 nm, is made by a suitable filter
wheel. The grating is multilayer-coated, to have high efficiency in both the spectral channels. In this paper we describe
the tests made on a prototype with flat surface and constant groove spacing. The measures have been carried out at the
BEAR beamline at the ELETTRA Synchrotron in Trieste (Italy). The grating was initially coated by gold and
successively by a Mo-Si multilayer optimized at 30.4 nm. The efficiencies at the first and fourth order (121.6 and 30.4
nm) have been measured before and after the multilayer deposition. The quality of the multilayer deposition has been
tested by atomic force microscope measurements on the grating surface and by reflectivity measurements performed on a
test reference mirror. The experimental data are compared with numerical simulations accounting for the coating
roughness and the smoothening effect on the blaze profile after the multilayer deposition. To our knowledge, this is the
first time that such a grating configuration is proposed.
VUV polarimetry has been recognized as one of the most powerful diagnostic tools for remote sensing of the solar
corona, with the potential of providing accurate space resolved information on magnetic activity through observation of
resonance lines of the most abundant species. In an on-going collaboration between our groups from Spain and Italy, a
program to design, build and characterize optical components for the VUV region has been activated. In particular, using
the beamline BEAR at the synchrotron facility Elettra in Trieste (Italy) we have characterized some thin film reflecting
linear polarizers, designed and optimized for the study of polarimetric properties of the HI Ly-alpha at 121.6 nm. The
characterizations are performed from 100 to 150 nm at different angles of incidence (40 – 80 deg). Some polarizers have
shown excellent performances with an average reflectivity R ≈ 34% and a modulation factor exceeding 95%. The
calibration of several samples is reported and aging effects on some old samples is discussed. One of the calibrated
sample will be used for the evaluation of the performances of a new fast calibration set-up facility for VUV.
Polarimetry in the far ultraviolet (FUV) is a powerful tool for the interpretation of the role of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. FUV polarimetry from space provides more accurate observations on the kinetics of the features and on local magnetic fields through the Doppler and Hanle resonant electron scattering effects. Particularly interesting lines for FUV polarimetry are H Lyman α (121.6 nm) and β (102.6 nm), along with OVI lines at 103.2 and 103.8 nm. One key element to perform polarimetry measurements at these wavelengths is the need of efficient polarizers. A limitation of the available polarizers, such as crystal plates of MgF2 and LiF working at Brewster angle, is their moderate reflectance at the non-extinguished component of the electric field, which results in a modest polarizer efficiency.
We present the design and the characteristics of a portable and compact photon spectrometer to be installed in freeelectron- laser (FEL) beamlines for photon in – photon out experiments, in particular single-shot X-ray emission spectroscopy. The instrument is operated in the 30 – 800 eV energy range with two channels and is designed to be initially used in the LDM (Low-Density Matter) and TIMEX (TIme-resolved studies of Matter under EXtreme conditions) beamlines of the FERMI@ELETTRA FEL, covering the whole spectral range of FERMI-1 and FERMI-2 emissions. The design of the instrument is tailored to achieve high spectral resolution in the whole interval of operation, high acceptance angle and high dynamic range. These characteristics are achieved in a compact environment to give a portable instrument that may be easily installed in different experimental chambers. The design consists of an entrance slit, a grazing-incidence diffraction grating and a detector. The number of elements within the optical path is kept to a single component, to minimize the losses due to reflectivity. The grating is spherical with variable line spacing along its surface, to provide an almost flat spectral focal plane that is perpendicular to the direction of the diffracted light. The detector is a back-illuminated CCD. The spectral resolution is better than 0.2% in the 30 – 800 eV region and the acceptance angles are 10 × 17 mrad in the 30-250 eV and 5 × 17 mrad in the at 250-800 eV.
The design of an imaging spectrograph operating at grazing incidence and stigmatic in a large field-of-view is
presented. It is realized coupling a double telescope and a spectrometer. The instrument may be used for XUV space
observations of extended sources, and is particularized to the Sun observation from the Earth. The performances of a
laboratory prototype are presented. This instrument covers the 4-20 nm (310-62 eV) spectral region, with a spectral
resolution of 0.1% at 10 nm and a spatial resolution of 3.5 arcsec over a field-of-view of 0.5 deg, within a total
envelope of 1.2 m. The design and the characterization of the instrument in the whole spectral region of operation