This paper, “Faisabilitité d'un separateur spectral integre: étude de filtres optiques localisés et d'un diaphragme absorbant," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
Improving materials and devices reliability is a major concern to the spatial industry. Results are reported for satellite mirrors-like specimens consisting in oxide-protected metal systems. Optical coatings were deposited by electron beam evaporation.
Mechanical stress fields in multi-layered materials play an important role. The stress state can have far-reaching implications both in kinetics and thermodynamics. Therefore an integrated apparatus with four-point bending equipment was designed. The technique allowed us to exert stress into a film or a system of films on a substrate concurrently with thermal treatment. In order to achieve the first tests performed with the help of the apparatus, various preliminary characterizations were required. The article reports the preliminary micro-mechanical testing of the materials (ultra micro-indentation to evaluate the elastic modulus of the samples materials and wafer curvature technique to determine the specimen residual stress) and the first ageing experiment. Experimental evidence of accelerated ageing under stress is successfully reported.
The lifetime of optical components submitted to high laser fluences is degraded under organic contaminant environment.
The molecular background of the Ligne d'Integration Laser (LIL), prototype of the future Laser Megajoule, might reduce
the laser damage threshold of exposed fused silica surfaces. This paper reports the interaction effects between pure
model contaminant deposits and a pulsed 1064 nm laser radiation on the coming out of mirror damage. The experimental
setup allowed us to condense nanolayers of model contaminants on optics, the deposit impacts were then investigated by
Laser Induced Damage Threshold (LIDT) tests in Rasterscan mode. In order to highlight physical processes emphasizing
the emergence of optics damage, we characterized the irradiated deposit using interferometric microscopy analysis and
spectrophotometric analysis. The challenge was to determine physical and phenomenological processes occurring during
the irradiation of a pure contaminant deposit with a 1064 nm pulsed laser and to study the impact of this model
contaminant on the LIDT of dielectric SiO2/HfO2 mirrors.
Fluoride materials like Magnesium fluoride and Lanthanum fluoride exhibit unique properties for applications in mirror and anti reflecting coatings in the VUV spectral range (120- 230 nm). These large band gap materials provide low absorption and a usable refractive index contrast. Common deposition methods are thermal evaporation and electron beam evaporation. A columnar microcrystalline structure with a significant porosity is observed for such coatings. Furthermore, a high sensitivity for contamination processes resulting in an increasing absorption is often perceived. Investigations of mirror systems exposed to a harsh environment like the storage ring free electron laser at ELETTRA show a rapid degradation in respect to a reduced reflectivity, an increased hydrocarbon contamination, and a formation of colour centres.
An improved performance of the fluoride coatings could possibly be expected for films, which do not have the polycrystalline columnar structure. Ion beam sputtering deposition of fluoride materials demonstrated its applicability to deposit dense amorphous fluoride coatings down to 193 nm. An IBS deposition plant with a Kaufman ion source, using a reactive fluorine environment, is used to grow fluoride layers with comparably low absorption values. Single and multilayer coatings were optically characterised after deposition, exposed to synchrotron radiation at ELETTRA, and characterized again after irradiation. However, the first set of irradiated multilayer mirrors showed a strong degradation of reflectivity and a strong hydrocarbon contamination. Colour centres were not observed.
In the field of laser-induced surface damage, it has been shown that localized re-fusion of silica can be used as a mean to mitigate the damage and therefore stop its growth before the use of the optical component is impaired. In this paper, this localized re-fusion was produced using a continuous CO2 laser. As the damage is reshaped, we observed that a ring of evaporated silica is systematically deposited around the mitigated damage. This evaporated silica is likely to be non-stoechiometric and therefore to present absorption and luminescence properties.
Thus we decided to perform photoluminescence measurements in order to analyse the mitigated damages. We performed fluorescence imaging and spectroscopy using 351nm continuous laser excitation. Different experimental conditions were used for the re-fusion process and the consequences on the photoluminescence properties were studied. We also compared these properties to the properties of non-mitigated damages.
Mirrors with a graded reflectance profile have been used for many years in unstable cavities for improving the optical quality of the laser output beams. All the variable reflectivity mirrors are realized with multilayer-coatings containing one or more profiled layers inside the stack. They generally exhibit high reflectance in the central area and very low reflectance in the external area. In particular, phase-unifying (PU) mirrors are graded mirrors properly designed in order to obtain a low wave-front distortion in the transmitted laser beam. In this paper, the laser damage resistance properties of a PU mirror designed for XeF excimer lasers (351 nm) have been studied. The laser-induced damage threshold has been measured by a XeF laser on the high and low reflectivity areas. A correlation between the damage threshold values and the standing wave electric field profile, which settles inside the two coating structures during laser irradiation, has been found.
Improving materials and devices reliability is a major concern to the spatial industry.
Results are reported for satellite mirrors-like specimens consisting in oxide-protected metal systems. Optical coatings were deposited by electron beam evaporation.
Mechanical stress fields in multi-layered materials play an important role. The stress state can have far-reaching implications both in kinetics and thermodynamics. Therefore an integrated apparatus with four-point bending equipment was designed. The technique allowed us to exert stress into a film or a system of films on a substrate concurrently with thermal treatment.
In order to achieve the first tests performed with the help of the apparatus, various preliminary characterizations were required. The article reports the preliminary micro-mechanical testing of the materials (ultra micro-indentation to evaluate the elastic modulus of the samples materials and wafer curvature technique to determine the specimen residual stress) and the first ageing experiment. Experimental evidence of accelerated ageing under stress is successfully reported.
Phase-unifying mirrors used as output couplers of laser resonators allow to improve the output laser beam divergence. In this paper, a phase-unifying unstable cavity has for the first time been applied to a high-power, commercially available XeF laser (Lamda Physik 210 i), and a laser output beam of 280 mJ, of 30 ns duration (FWHM), and with a divergence close to the diffraction limited value has been obtained at λ = 351 nm.
Achieving ultra-high laser resistance for high reflectivity coatings has become possible due to technological progress during the last years; keeping these performances up to their initial level over the storage or operation period seems a new challenge, as preliminary results reported last year in this conference showed a degradation of LIDT @ 1.06μm as a function of elapsed time and storage conditions. This work is clearly the continuation of last year's effort, as we report more observations on the same objects, with an emphasis on the use of surface-analysis techniques (TD-GCMS e.g.) to check potential correlation between degraded LIDT behavior and organic contamination.
Contamination of surfaces prior to or during deposition of dielectric multilayers has been a classical subject of technological studies, but post deposition contamination has not been reported as often. Previous results typically dealt with sealed optics and exposition to laser or UV conditions, or on-orbit optical systems: we report here observations on HfO2/SiO2 mirrors designed for the Megajoules project, with very high laser resistance. Performances in terms of laser damage thresholds at 1.06 micrometers have been measured periodically over periods of several months, on different sets of samples. Comparison of R/1 LIDT distributions are presented versus time and storage conditions. Degradation of LIDT is clearly observed. Recovery methods are investigated.
Very high damage threshold Hafnia Silica mirror coating have been developed successfully thanks to a clear understanding of the main limitations of the available mirrors. Coating with a 3ns pulse width threshold above 100J/cm2 are reported. These coatings proved to be much more resistant than very high quality bare silica substrate tested the same way. The limitation of these coatings is still extrinsic. The potential of this technology for large size high damage mirrors for fusion laser application is discussed.
The laser damage threshold determination is linked to many factors as the damage criteria, the test conditions, etc. Among them, the spatial beam characterization as well as the power or the energy in the beam plays a key role. Indeed, an appropriate value has to be assigned to each tested site. This number is important to certify a component or to compare the components. This paper is focused on determining the beam parameters for 2 kinds of laser damage. The first part addresses the peak fluence measurement on YAG laser damage test facilities. The proposed method is applied to stable laser. Stable lasers mean the spatial profile is similar during the experiments. Before the laser damage tests, the spatial beam is visualized with a CCD camera. Them, the spatial beam profile is cautiously processed to extract the relation between the peak fluence and the energy, which is easy to measure in real time. This method is illustrated at 1064 nm. Finally, this procedure is validated on silicon wafer. Indeed, at 1064 nm, the theoretical laser damage threshold of silicon wafer is well known. Furthermore, it is independent of the pulse length in the nanosecond regime. The second part will be shortly devoted to a more specific problem: the CW damage threshold of CO2 mirrors. It has been previously shown that the 'short' term threshold of these cooled components is well described by a critical temperature. Due to the test geometry, the temperature rise is proportional to the beam power divided by a value homogeneous to the beam radius. In this article, it is sown the knife-edged method allows to literally extract the beam size parameter which appears in the theoretical formula of the critical temperature. The beam just needs to have a symmetry of revolution.
We report surface damage measurements on single crystal silicon under single and multiple 3 ns pulse laser irradiation at 1.06 micrometers wavelength. First, single shot damage threshold is measured. Then the numbers of pulses required to damage with a probability of 0.5 at various fluences below this 1/1 threshold are obtained at room temperature. These results are fitted with various life models. We look at temperature as a means to accelerate life test. A new feature of the laser test facility in our laboratory is described, allowing surface damage measurements under controlled thermal environment, with testing temperatures ranging from room temperature to 180 Celsius degrees. Experimental demonstration of accelerated failure time for silicon at 1.06 micrometers is provided.
Since the value and the shape of the pulse widths and also the spatial profile vary from one test bench to another, it appears interesting to see how the damage threshold evolves with these two parameters. The scaling laws have 2 main impacts: the possibility to compare easily the results between the laboratories and to help the understanding of the damage phenomena. But it can be also a great help on the laser damage threshold specifications of the optical components. Moreover, the spatial law versus the component area could become a request in the future specifications. The first part of this paper is devoted to experimental damage result performed with YAG laser at 1.064 micrometers in R- on-1 mode and in raster scan. 2 pulse widths close to a Gaussian profile is used: around 3 ns and 12.5 ns. Furthermore, 3 laser spot sizes with a shape close to a Gaussian beam are used. Their impact on the laser damage threshold is studied. The results are analyzed with the purpose of finding a tool to extrapolate the result at larger components. To test full large optics is time consuming. A primary automatic test is suggested too quickly eliminate the components below the specifications.
The automatic damage test benches allow to obtain more data in less time but also to imagine new laser damage tests. Thereby, new questions can be asked and more answers can be provided. In this paper, the 3 YAG automatic damage test facilities build up at the laboratory will be shown. There are based on well-known principles : an half wave plate to adjust the wished peak fluence and a scattered light measurement in real time to detect the damage appearances. Many tests are developed around these benches as the usual hon-i, N-on-i, R-on-1, S-on-i tests and raster scans previously proposed by other laboratories. On all our test benches, these standard tests are driven by the same software designed at the laboratory : "Wintfl". But new tests and characterizations are also added. Indeed, on these thcilities the aging ofthe optics at various repetition rate under constant fluence (like S-on-i with or without a limit number of shots) are carried out. An automatic defect count has been also implemented. This tool is able to evaluate the number of defects after cleaning or after coating but also between laser irradiation. New tests are also imagined. The so-called HR-on-i test (High Ramp), which is a R-on4 (Ramp) test without beginning at zero has been achieved. It allows to compare 1-on-I and R-on-1 distributions performed with exactly the same damage criterion. Other parameters can be investigated as the speed of the motor ramp during the R-on-1 test, the pulse repetition frequency during the S-on-i test,... In this article, after a short presentation of our test bench facilities, the convenient possibilities of automatic YAG test benches are illustrated with experimental results.
Other defects than nodules are responsible for laser damage of High Reflection coatings at 1.06 micrometers . Among them, flat bottom pits (FBP) are particularly interesting because they are surface induced damages not related with preexisting cosmetic defects. The damage morphology observed with a Scanning Electron Microscope suggest that damages are related to absorbing nanometric centers. Following this idea, a model of FBP formation is developed in this paper. This model predicts a size dependence of the FBP versus the thickness of the buckled layers; this fact is experimentally confirmed. Two different kinds of FBP have been observed; the second one might be related with the seed of nodular defects.