With the aim to expand its capability to offer state-of-the-art space qualified multispectral optical filters assemblies, Sodern continues its effort to evaluate and incorporate new technologies in its designs. For many years, Sodern has been developing complex optical devices incorporating spectral filters.
We present hereafter the results on the fabrication of complex optical filters within the Institut Fresnel in close collaboration with CILAS. Bandpass optical filters dedicated to astronomy and space applications, with central wavelengths ranging from ultraviolet to near infrared, were deposited on both sides of glass substrates with performances in very good congruence with theoretical designs. For these applications, the required functions are particularly complex as they must present a very narrow bandwidth as well as a high level of rejection over a broad spectral range. In addition to those severe optical performances, insensitivity to environmental conditions is necessary. For this purpose, robust solutions with particularly stable performances have to be proposed.
We report on the sub-picosecond laser-induced damage of optical thin films of different thickness made by Magnetron sputtering, Ion assisted deposition and Ion plating, and submitted to single irradiation of the first and the third harmonics of an Ytterbium laser (1030 and 343nm). Using a single rate equation approach for free electron excitation coupled with calculation of the spatial and temporal distribution of the electric field, we investigate numerically the spatial density distribution of the absorbed energy and evaluate its capacity to describe damage phenomena especially damage threshold and morphologies (damage diameter, ablation deepness). Laser-induced damage thresholds are compared for different film thicknesses and different irradiation conditions.
We report on the laser-induced damage threshold at 500fs of optical films made by Magnetron Sputtering and
submitted to single and multiple irradiations at different harmonics of an Ytterbium laser (1030nm, 515nm
and 343nm). Single layers of SiO2, HfO2, and Nb2O5 as bare fused silica samples are under investigation.
We investigate quantitative phase imaging as a measurement method for laser damage detection and analysis of laser induced modification of optical materials. Experiments have been conducted with a wavefront sensor based on lateral shearing interferometry technique associated to a high magnification optical microscope. The system has been used for in situ observation of optical thin films and bulk samples irradiated by 500fs pulses. It is shown that the technique realizes high sensitivity, convenient use and can provide quantitative information on the refractive index or surface modification of the samples under test.
Various scratches on fused silica optics after polishing have been characterized with confocal microscopy and then tested
with nanosecond UV laser. Scratches are identified as a major contributor to laser damage even if they are only a few
micrometers wide. We propose a process in order to remove these scratches whose depth ranges from 2 to 16 μm. We
use a CO2 laser to heat the scratched areas at high temperature which will heal fractures due to viscous flow. The
characterizations were completed by laser damage tests that finally prove the effectiveness of the repair. We conclude
also that this repair process proves to be fast, localized to the scratch and clean because no debris are generated.
Irradiation experiments were conducted at Prague Asterix Laser System (PALS) with the Ne-like zinc soft x-ray laser
(SXRL) at 21.2 nm (58.5 eV) delivering up to 4 mJ (~4 x 1014 photons), 100-ps pulses in a narrowly collimated beam.
The SXRL beam was focused using a 1 inch diameter off-axis parabolic mirror (f = 253 mm at 14 degrees) with a Mo:Si
multilayer coating (R = 30% at 21 nm) placed 2825 mm from the SXRL. The diameter of the SXRL beam incident on
the mirror was about 11 mm. Ablation experiments with a gradually attenuated beam were performed to determine the
single-shot damage threshold of various materials. In this case, the sample was positioned at the tightest focus of the
SXRL whose pulse energy was attenuated by aluminum filters of various thickness to adjust the fluence. Both the focal
spot area and single-shot damage threshold were determined from the plot of damaged surface areas as a function of a
pulse energy logarithm to dete. For PMMA, the focal spot area and the ablation threshold inferred from the data are
Sfoc = (1172±230) μm2 and Fth = (1.25±0.4) J/cm2, respectively. Inorganic materials have thresholds significantly higher
than organic polymers, e.g., amorphous and monocrystalline silicon gave values 2.5 J/cm2 and 4.2 J/cm2, respectively.
For prospective SASE FEL optical elements, the SiC coating is of great interest. Its damage threshold is of 20 J/cm2, i.e.,
slightly lower than that of monocrystalline silicon. The thresholds determined with the 100-ps pulses from plasma-based,
quasi-steady state SXRL are significantly higher than the thresholds obtained for 20-fs pulses provided by the SXR freeelectron
laser in Hamburg. There is a difference in PMMA thresholds of two orders of magnitude for these two sources.
We present the characterization of Al/SiC periodic multilayers designed for optical applications. In some samples, a thin
layer of W or Mo is added at the SiC-on-Al interfaces. We use x-ray reflectivity (XRR) in order to determine the
parameters of the stacks, i.e. thickness and roughness of all the layers. We have performed x-ray emission spectroscopy
(XES) to identify the chemical state of the Al and Si atoms present within the structure from an analysis of the shape of
the Al Kβ and Si Kβ emission bands. Finally, time of flight secondary ion mass spectrometry (ToF-SIMS) is used to
obtain the depth profile of the different elements present within the studied stacks. A fit of the XRR curves shows that
the Al/SiC multilayer present large interfacial roughness (up to 2.8 nm), which is decreased considerably (down to 1 nm
or less) when the refractory metal layers are introduced in the periodic structure. The combination of XES and ToFSIMS
allows us to conclude that in these systems the roughness is a purely geometrical parameter and not related to
chemical interfacial reactions.
Through its participation to European programs, SAGEM has worked on the design and manufacturing of normal
incidence collectors for EUV sources. By opposition to grazing incidence, normal incidence collectors are expected to
collect more light with a simpler and cheaper design. Designs are presented for the two current types of existing sources:
Discharge Produced Plasma (DPP) and Laser Produced Plasma (LPP). Collection efficiency is calculated in both cases. It
is shown that these collectors can achieve about 10 % efficiency for DPP sources and 40 % for LPP sources. SAGEM
works on the collectors manufacturability are also presented, including polishing, coating and cooling. The feasibility of
polishing has been demonstrated with a roughness better than 2 angstroms obtained on several materials (glass, silicon,
Silicon Carbide, metals...). SAGEM is currently working with the Institut d'Optique and the Laboratoire des Materiaux
Avancés on the design and the process of EUV coatings for large mirrors. Lastly, SAGEM has studied the design and
feasibility of an efficient thermal control, based on a liquid cooling through slim channels machined close to the optical
HECOR (HElium CORonagraph) is a coronagraph designed to observe the solar corona at 30.4 nm between 1.2 and 4
solar radii. The instrument is part of the Herschel sounding rocket payload to be flown from White Sands Missile Range
in December 2007. Much like for neutral hydrogen, the residual singly ionized helium present in the corona can be
detected because it resonantly scatters the intense underlying chromospheric radiation. Combined with the simultaneous
measurements of the neutral hydrogen corona made by SCORE, the other coronagraph of the Herschel payload, the
HECOR observations will provide novel diagnostics of the solar wind outflow. HECOR is an externally occulted
coronagraph of very simple design. It uses a triple-disc external occulting system, a single off axis multilayer coated
mirror and a CCD camera. We present measurements of the EUV mirror roughness and reflectivity, tests of the image
quality, and measurements of the stray light rejection performance. The mirror uses a novel multilayer design with three
components that give HECOR a high throughput.
The development of new high power EUV sources and EUV space imaging requires optics having specific
properties which depend on applications and operating conditions. These both applications are very different in the
working multilayers environment. For the high power sources, multilayers are submitted to short pulses with high
energy peak whereas, for the space imaging, multilayers are submitted to continuous flux with low level. Moreover
photon energy and environment for both applications may be different. The environment may affect structure and top
layer contamination when optics are stored, handled, mounted on the final device and finally operating. Main
environmental parameters investigated are temperature and humidity variation.
One objective is the optimisation of multilayer coatings to offer the highest resistance under photonic, ionic
fluxes and temperature cycle. This means that interfacial diffusion between thin layers and degradation of the capping
layers have to be avoided or reduced. The present study relies with designing, depositing and testing different structures
of multilayer coatings in order to minimise the influence of the environment.
Multilayer coatings based on molybdenum, silicon and silicon carbide materials have been deposited by magnetron
sputtering on silicon and zerodur substrates. Samples were submitted to radiations emitted by an EUV source at
wavelength closed to 13.5 nm. Furthermore they were also submitted to thermal cycles and annealing under warm
humidity in the aim to simulate extremes storage or handling conditions as space mission's conditions.
The damages and the performance of the multilayers were evaluated by using grazing incidence reflectometry
at 0.154 nm and EUV reflectometry at the operating wavelength.
After a presentation of the multilayer design, deposition and metrology tools, we will describe the different
environmental effects on the coatings to take in care during EUV source exposure, handling and storage conditions.
First results on multilayers performances to EUV source exposure and space specification tests are presented. Main
damages studies were on annealing, thermal cycling and warm humidity.