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.
Surface incandescence properties of proton implanted fused silica have been researched with a focused CO2
laser. We have discovered that in the initial stage of incandescence a thermoluminescent peak appears. We call it
blackbody thermoluminescence. In our silica samples, with a 100 micron spatial resolution, the blackbody
thermoluminescence mapping reveals surface and sub surfaces defects made by the polishing process. We show how
laser damage and laser conditioning are the same two facets of this blackbody thermoluminescence occurrence.
Under vacuum conditions, the accumulation of low fluence laser pulses generally leads to an organic
contamination of the surface irradiated. This phenomenon reduces the optical component lifetime. Experimental
conditions such as laser characteristics, environment composition and structure of the coating strongly influence the
contamination mechanisms. Silica being the most employed material for optical coatings, this study aims at describing
the laser-induced contamination influence of silica coatings deposition techniques. E-Beam evaporated and Ion Beam
Sputtered silica thin films have been exposed to several billions 600 mJ/cm2 - 532 nm laser pulses under vacuum. This
paper presents the observations made on laser-induced contamination and discusses the physical mechanisms involved.
Laser damage at 3ω, 351 nm, of fused silica optical components is a major concern for LMJ maintenance.
Indeed, even a low density of damage sites is unacceptable due to the exponential growth of surface damage with a series
of laser shots. A technique is now used to prevent the growth of initiated damage sites : this mitigation technique consists
in a local melting and evaporation of silica by CO2 laser irradiation on the damage site. Even if the growth is stopped in
most cases, we showed previously that some of the mitigated sites re-initiate on their peripheral area, where most of redeposited
debris are located. To further increase the efficiency of mitigation technique, the treatment was improved by
varying the spatial profile of the CO2 laser beam. We present here the new set-up and the results obtained in terms of
laser damage resistance: about 98% of the mitigated sites sustained 200 shots of a 10 J/cm2 3ω YAG laser without
damage.
It is still assumed that optical components submitted to laser fluences orders of magnitude below their laser induced damage threshold (LIDT) will last for ever. However, depending upon environmental conditions, the accumulation of low fluence laser pulses leads to a progressive contamination and eventually to a damage of the optical components. In order to study the physics of the laser induceded contamination, a laser test bench has been developed. The experimental cell is dry-pumped and a mass spectrometer controls the environment around the optical component. An infrared camera diagnosis follows the sample surface temperature. This paper contains preliminary results obtained on anti-reflective coatings on fused silica tested at 532 nm with a pulse repetition rate of 10 kHz and a pulse width of 100 ns.
In the midst of the Mega Joule Laser project, a study of the impact of organic contamination on optical surfaces has been launched. Last year, we presented results on intentionally contaminated optics by outgassing products of a typical material of the LIL (Ligne d'Integration Laser, the prototype laser line of the future LMJ). A small quantity of organic contamination deposited on high reflective mirrors decreased their R/1 laser induced damage threshold. As the LIDT R/1 test procedure may "condition" the optical component, further raster scan tests have been implemented on new intentionally contaminated samples to assess the test procedure impact on the LIDT results for different contaminations. The aim of this work is double: -First, the impact of organic contamination deposited on optical surfaces by outgassing will be evaluated by laser induced damage threshold measurement, after a laser shot at nominal fluence ; -The second objective is to evaluate the real effects of "conditioning", notably towards organic contamination deposited on optics.
The pulsed Laser Induced Damage Threshold (LIDT) of optical components usually reaches several hundreds of MW/cm2. When exposed to laser power several order of magnitude below their LIDT, the optical component lifetime is, by default, considered infinite. Under specific conditions, the accumulation of laser pulses may lead to a contamination of the surface and a degradation of its optical properties and LIDT. In the first order, these phenomena depend on the experimental conditions such as the irradiation time, the laser power, and the environment. In order to better understand the physics emphasizing this degradation, we developed an experimental cell with an in-situ spectroscopic ellipsometry diagnostic. The dry-pumped cell sheltering the sample is associated with a mass spectrometer that enables us to follow the environmental conditions in which we experiment the ageing. Anti-reflection coatings on fused silica were tested under 10 kHz-532 nm laser ageing. We present first results of degradation obtained in these conditions.
A new facility has been designed to enhance laser damage resistance at 351 nm of large scale 3w KDP and silica optics by laser treatment. This facility is a prototype, and the process will be industrialized as a means of fabrication of the LMJ optics. The first step of the process is a conditioning/initiation step, which consists of a UV laser raster scan of the whole optics; the second step is a step of detection and analysis of damage possibly initiated during the previous step; the third step is a mitigation step, which consists of a local melting of the detected damage on silica surface, in order to stop their growth. The facility is equipped with a 3w Nd:YAG laser allowing the process of both KDP and silica. A CO2 laser is used for damage mitigation. Both the lifetime increase and the reduction of the process duration of large scale optics have been taken into account with a view to industrialize the process.
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.
The Ligne d’Integration Laser (LIL) is a prototype installation at scale 1 of one of the 30 lasers of future Laser Mega Joule. It is intended to validate technological choices made for LMJ and to prepare its exploitation. The facility will contain nearly 10.000 optics and over 4000 m2 of mirrors. Cleanliness will be an essential matter in the facility since contamination of optics can reduce their laser damage threshold. Hence, airborne molecular contamination (AMC) has been sampled near optics in strategic places of the LIL. These samplings have shown high levels of organic compounds, notably in the amplifying section, which is expected to be the most sensitive part in the LIL. Suspecting a local source of contamination, outgassing tests of typical materials constituting the amplifying section had been carried out. Among them, one sealing material has been identified as a source of organic contamination near the optics. Effects of this pollution have been investigated by a measurement of laser damage threshold after intentional contamination of optics. This work shows the complexity of the outgassing contamination issue, since several steps are necessary to evaluate the effects of this contamination on optical surfaces: air samplings, identification of sources, outgassing tests, intentional contamination of optics and finally measurement of laser damage threshold.
For high power laser applications like the "Laser Megajoule" facility under construction in France, laser-induced damage threshold (LIDT) in fused silica is a limitation. CEA has made efforts to improve LIDT at the wavelength of 351 nm. Polishing and post polishing processes have been optimized. Laser damage sites density was decreased by several orders of magnitude by combining different fabrication steps. In order to further enhance optical laser resistance and to remove damaged sites on full-size optics, several small-beam raster scanning techniques have been studied and developed to condition fused silica optics. To stop the growth of damage sites, a continuous CO2 laser was used to re-melt them. Laser induced damage tests, performed on instrumented and automated facilities, are reported in order to check and illustrate the effectiveness of these treatments. Damage initiation studies as well as damage growth measurements are presented.
The purpose of this paper is to gather experimental elements allowing for the prediction of laser damage on full size components installed on high power Nd-glass laser lines. Damage can initiated on material defects, which aren’t known in their nature, but the density of which can be measured. On transmissive optics, depending on the component thickness, and on the intensity distribution at the front surface, rear surface damage can also appear due to self-focusing of hot spots. These two contributions produce damage sites that are prone to grow. The growth rate has been shown to be proportional to the damaged area. The resulting exponential growth is the major limitation to the lifetime of optics. A representation of these phenomena in the plane Intensity/Fluence gives a practical description of the impact of laser damage on the lifetime of optical components. It also enlightens the comparison between different operating conditions.
A continuous CO2 laser is used to locally re-fuse silica and avoid growth of 3ω laser damaged sites. Temperature evolution on each spot is monitored by a radiometry diagnostic. Important temperature variations are observed from site to site at a mm scale. Such variations can only be induced by a non homogeneous, high temperature, thermal conductivity. Real time retroaction, on silica exposure to laser radiation, enables us to control surface silica evaporation and etching depth. The 3ω laser induced damage threshold test of the re-fused sites shows that the limit for the mitigation rate lies in the surrounding silica surface.
On the 3ω part of the LIL laser many optical components will have to sustain fluences above 10J/cm2. Even if current progress in silica substrate technology decreases the number of defects/cm2 which can induce a damage under such a laser flux, tens of damaged sites will appear on large surface optics. Knowing that these surface damaged sites grow exponentially with the number of laser shots, it is necessary to stop the growth of these defects before the use of the optical component is impaired. In this paper we use localized re-fusion of silica, induced by a continuous CO2 laser, as a means to reshape the damaged site and circumvent the growth of laser-induced surface damages. In a first part we compare the 1 and 2D model of the interaction of a gaussian laser beam with an homogeneous material and deduce that the 1D model is convenient down to a laser beam radius waist of 100 μm in silica. We show that at atmospheric pressure total mitigation might not be achieved due to silica evaporation and peripheral redeposit in air. This risk cannot be managed with predetermined laser power and interaction time, because thermal conductivity of silica is not homogeneous. In order to keep the process “vacuum free”, a radiometry diagnostic has been mounted to monitor the surface temperature of silica. Real time retroaction on silica exposure to laser radiation enables us to control surface silica evaporation.
Unlike other drive fusion class laser, Megajoule laser (LMJ) and its first prototype, the Laser Integration Line (LIL) are equiped with specific diffractive optical components. All these optics are situated in the final optic assembly.
An high efficiency diffraction focusing grating called 3w grating is used to focus the beam into the center of the target chamber instead of a classical focusing lens. Another large grating called 1w grating is used for optical path compensation purposes. Both gratings have a dimension of 420x470mm2 and are working at an incidence of 25°. Gratings are plano transmission holographic gratings directly engraved into fused silica substrates. The 1w grating is working at the wavelength of 1.053μm, its grooves are straight and equispaced. The 3w grating, is a focusing grating working at the wavelength of 0.351μm. Its grooves are curved and non equispaced.
Jobin Yvon was selected by CEA to manufacture these two types of diffraction graintgs. After processes developpements and facilitization, a complete batch of twelve 1w gratings and sixteen 3w gratings were delivered to CEA for integration.
After a brief presentation of CEA's specification for this diffractive components, we give some details on the manufacturing processes. We also demonstrate good agreement between specified and manufactured component. We give an overview of the global production performances
In the field of the development of LIL and LMJ fusion class high power lasers, CEA has made important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of 351 nm. Since several years, we have focused on optimizing the grinding, polishing and post polishing processes to overcome the existing performances with various industrials and academics partners. In this paper, we describe our understanding of the nature of the polished silica interface and our approach to rich our damage threshold goal. Our efforts were mainly put on reducing the cracks region extension and removing or optimizing the polishing top layer. We give also some details on the influences of each of the polishing process from rough material grinding to post processing. We demonstrate that some order of magnitude in laser damage initiation density can be gained by combining appropriate fabrication steps.
In the context of high power laser applications like the French "Laser Megajoule" or the "National Ignition Facility" in the United States, laser-induced-damage in fused silica appears to be a limitation. Although it has been established that nano-sized defects are suspected to be responsible of the damage initiation in optical
components, the induced-breakdown process is still unknown. The specific apparatus developed at the Institut Fresnel permits to study in real time the laser interaction on an isolated absorbing defect (around 100 nm): Indeed the coupling of a nanosecond focused laser (beam diameter about 10μm) and a high resolution photothermal
microscope (beam diameter 1μm) in a unique facility permits to highlight the evolution of the local absorption of the defect versus irradiation energy density. Furthermore, the morphology of this modification is followed thanks to a "refraction mapping" (photothermal deflection measured at frequency 0). An in-situ
dark field microscope and an ex-situ AFM complete the observations. In order to simulate the presence of the nano-defects, engineered sub-micronic gold particles (100-600 nm) were embedded in silica samples. The whole observations contribute in the understanding of the different stages of the damage initiation and more particularly, a determination of a "pre-damage" threshold can be performed.
In order to exhibit the role of laser damage precursor centers in silica, gold particles ranging from 0.2mm to 0.8mm have been included between two coatings of SiO2 deposited on silica substrates. UV and IR nano-second pulsed irradiations on samples with different SiO2 over-layer thicknesses (2, 5, 10 mm) has been performed. The damage morphologies observed with Nomarski and atomic force microscopes have shown to be dependent on fluence, wavelength and SiO2 over-layer thickness. In addition a localized irradiation study using 6mm spot size allows to aim accurately on an isolated particle. The measurement is compared to the experiments presented for a few hundred micrometers spot size. Indeed in this case we have to consider that several particles are simultaneously irradiated. A comparison between the respective morphologies could inform us on potential collective effects of the particles. The choice of experimental test conditions, and the whole associated results will be presented and discussed.
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.
A photothermal microscope has been combined with an experimental
set-up allowing damage threshold measurements at the same
wavelength. The microscope is based on photothermal deflection of
the transmitted probe beam: the CW pump beam (1.06 μm
wavelength) and the probe beam are collinear and focused through
the same objective. The diameter of the pump beam on the sample
surface is 1 μm. Laser damage thresholds are measured thanks
to a pulsed beam (1.06 μm wavelength and 6 nanosecond pulse)
and the spatial position of the pulsed beam is controlled by a CCD
camera. This experimental setup has been used to study the
behavior of metallic inclusions in dielectric materials in laser
damage processes. Results are presented with gold inclusions of
about 600 nm in diameter in silica.
The validation of numerical simulations of laser induced damage of fused silica requires detailed knowledge of the different parameters involved in the interaction. To approach the problem, we have performed simulations of laser energy deposition in spherical metallic defects and the surrounding fused silica. Our code DELPOR takes into account various laser/defect induced absorption mechanisms of SiO2, such as radiative ionization, avalanche and multiphotonic ionization. We have studied crater formation produced by the absorber explosion with a 2-D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. To validate our simulations, we have made and tested samples of ultra-pure silica thin film, containing gold nanoparticles of diameter 0.6 μm. The fused silica coating could have three different thickness. We compare experiment and simulations for two laser irradiations at wavelengths 0.351 and 1.053 μm.
On the 3ω part of the LIL laser many optical components will have to sustain fluences above 10J/cm2. Even if progress in silica substrate technology decreases the number of defects/cm2 which can induce a damage under such a laser flux, tens of damaged sites will appear on large surface optics. Knowing that these damaged sites grow exponentially with the number of laser shots, it is a necessity to stop the growth of these defects before the use of the optical component is impaired. In this paper we have used wet chemical etching as a way to circumvent the growth of laser-induced surface damages. SEM characterization of damages at all stages of the process has been carried out. We show that at a reduced damage creation fluence, the use of a highly concentrated HF acid leads to a 93% mitigation rate for those damaged sites that need mitigation. Due to the etching anisotropy, the HF acid concentration is more important than etch depth’s for the mitigation rate of laser induced surface damage in silica.
Simulations of laser-fused silica interactions at 0.351 μm are a key issue in predicting and quantifying laser damage in large laser systems such as LIL and LMJ. Validation of numerical simulations requires detailed knowledge of the different parameters involved in the interaction. To concentrate on a simple situation, we have made and tested a thin film system based on calibrated gold nanoparticles (0.2-0.8 μm diameter) inserted between two silica layers. The fused silica overcoat was either 2 or 10 microns thick. We have performed simulations of laser energy deposition in the engineered defect (i.e. nanoparticle) and the surrounding fused silica taking into account various laser/defect induced absorption mechanisms of SiO2 (radiative ionization, avalanche and multiphotonic ionization). We have studied crater formation produced by the absorber explosion with a 2D Lagrange-Euler code taking into account crack formation and propagation in the brittle material. We discuss the influence of the defect depth (with respect to the surface) on the damage morphology. The simulations are compared with our experimental results.
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.
Technological processing of bulk silica is needed at many points of the fabrication of optical components. In the Laser Integration Line (LIL) and Megajoule Laser (LMJ), with the usual but challenging optical constraints of keeping the wavefront quality on large optics the constraint of high flux laser resistance is added. This has led to many technological improvements of silica processing which have been transferred into the industrial tissue. Improved polishing and cleaning processes have been developed which avoid the contamination of surface with polishing agents and are now used for lenses, windows, and substrates preparation of mirrors and polarizers. But some components like the gratings which are to be used on LIL and LMJ need new processing steps which are typical of the semiconductor industry and whose effects are unknown in terms of laser induced damage threshold in silica surface and subsurface. After a summary of the specifications and the performances of these gratings at 1 and 3(omega) wavelength we will focus on the laser induced damage (LID) tests that were performed at different stages of the grating process and see how they impact on the LID threshold of the gratings.
To deviate and focus of the beams of the future Laser Integration Line (LIL) and Megajoule laser (LMJ), CEA has chosen an original setup using two large 420 x 470 mm2 transmission gratings. The first grating is an holographic plano transmission master grating with straight and equispaced ruling, 25 degree(s) incidence angle and working at 1.053 micrometers . The second one is an holographic plano transmission master grating, with curved and non equispaced ruling, 25 degree(s) incidence angle which combines both focusing and deviation properties. Groove profile of both gratings is deep laminar. High damage threshold, improved wavefront quality and high efficiencies are the main issues for those two gratings. Jobin Yvon's was selected by CEA in 1999 to develop, industrialize and manufacture gratings reaching LIL/LMJ specifications. A dedicated plant and facilities were built to manufacture the gratings directly engraved into the fused silica substrates provided by CEA. After process developments, Jobin Yvon manufactured the two first 1(omega) and 3(omega) gratings in mid 2001. After a short summary of the specification of these gratings, we present in this paper the production process and the performances of the 1(omega) and 3(omega) gratings manufactured. Wavefront data, efficiency measurements and damage threshold performances are detailed.
We present here recent developments obtained at LETI infrared laboratory in the field of infrared detectors made in HgCdTe material and using the molecular beam epitaxial growth technique (MBE). We discuss the metallurgical points (growth temperature and flux control) that lead to achieve excellent quality epitaxial layers grown by MBE. We show a run-to-run reproducibility measured on growth run of more than 15 layers. The crystalline quality, surface morphology, and composition uniformity are excellent. The etch pits density (EPD) are in the low 105.cm-2 when HgCdTe grows on a CdZnTe substrate. Transport properties reveal a low n-type carrier concentration in the 1014 to 1015.cm-3 range with a carrier mobility in excess of 105 cm2/V/sec at 77K for epilayers grown with 10 micrometers cutoff wavelength. We describe the performances of several kinds of our HgCdTe- MBE devices: single color MWIR and LWIR detectors on HgCdTe/CdZnTe operating at 77K in respectively (3-5 micrometers ) and (8-12 micrometers ) wavelength range; single color MWIR detectors on HgCdTe grown on germanium heterosubstrate operating at 77K in the (3-5 micrometers ) wavelength range; two color HgCdTe detectors operating within the MWIR (3-5 micrometers ) band.
The CIRS instrument is the Infrared spectrometer of the CASSINI orbiter. The flight and spare models performances of the IR Photovoltaic detector arrays are presented here. We discuss the efforts made to avoid 1/f noise dependent diodes to meet the severe requirements of this mission to Saturn.
Middle wavelength IR photovoltaic detectors were fabricated with HgCdTe epilayers grown by molecular beam epitaxy. The epilayers were grown on a slightly misoriented (111) B CdZnTe substrate and were twin free. Epitaxy was carried out with substrate rotation and the wavelength uniformity achieved on a 20 X 20 mm2 wafer was 0.80%. A planar photovoltaic technology was used to produce the IR photodiodes and the junction formation was obtained by ion implantation. Linear array photodiodes and 128 X 128 2D-arrays interconnected with a CCD readout circuit were achieved. RoA product values of 4.7 105 ohm cm2 were measured on diodes with a 4.7 micron cutoff wavelength at 77 K. The good homogeneity of the layer is reflected by the low value, 4.4%, of the standard deviation on the short circuit current histogram of the 2D-array.
For the Cassini probe, focal plane number four of the Composite IR Spectrometer is equipped with a HgCdTe photovoltaic linear array. This 10 X 1 array detects in the 1100 to 1400 cm-1 wavenumber range with 200 X 200 micrometers 2 diodes at a pitch of 215 micrometers . Although background flux is near zero, peak detectivity, which is set by intrinsic diode noise, reaches 7.2 1011 cmHz.5W-1 at -30 mV bias whereas the theoretical limit is 7.8 1011 cmHz.5W-1. These performances are obtained by a simple ion implanted n/p planar technology on a liquid phase HgCdTe epilayer grown on lattice matched CdZnTe substrate.
This paper presents the first results obtained at LETI/LIR on 12.5 micrometers HgCdTe (MCT) infrared photodetectors. The objective of this program is to develop long buttable linear arrays of a few thousand photosites for earth observation from satellites. In a preliminary phase, a prototype with three photovoltaic sub-modules has been achieved and presents promising performances which are described in this paper.
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