In this paper, we present different procedures of laser conditioning realized on KDP doubler crystals. First, components
are treated either with an excimer laser (SOCRATE facility, 351 nm, 12 ns) or a Nd: YAG laser (MISTRAL facility,
355 nm, 7 ns). Then damage tests are performed at 2ω (532 nm - 5 ns BLANCO facility) and 3ω (355 nm - 2.5ns
LUTIN facility) in order to estimate the conditioning gain for these two wavelengths.
For the best procedures, results show that it is possible to increase laser damage threshold at 532 nm so that it becomes
compatible with the nominal specifications of the LMJ. Moreover, tests realized at 355 nm highlight also an
encouraging improvement for the laser conditioning of tripler crystals.
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.
The Megajoule laser, designed for the study of high energy density plasma, is currently being constructed at the CEA Cesta near Bordeaux in France. Constituted of 240 laser beams, this facility will by able to concentrate 1.8MJ of energy on a target placed in the centre of a vacuum chamber in order to obtain fusion. The 240 beams of the LMJ have a right section of 40 x 40 cm2 and are equipped with about 40 optical parts of various types: laser slabs, lenses, mirrors, diffractive optics. All of them have to sustain very high fluence induced by the laser beam. Manufacturing 9000 large laser optics of this type is a real technological and economical challenge. This presentation gives an overview of this activity and details the main recent development realized. In addition, we present results on the current development program made to improve lifetime of fused silica optics at the wavelength of 351 nm.
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.
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.
In order to understand the mechanisms of laser damage initiation, we study “model” samples constituted of pure silica seeded with 3 nm gold particles. Numerical simulations are performed with a 1-D hydrodynamic code to determine the laser light absorption by a spherical nanoparticle. This code also simulates the thermal conduction, radiative transfer and ionization by UV light emitted by the heated metallic particles. The setup used for experimental studies is a high resolution, high sensitivity photothermal microscope. This setup allows correlation between optical absorption and laser irradiation. We observe the silica transformation in terms of absorption modification as a function of the irradiation fluence. The morphology of irradiated samples surface is observed thanks to “Nomarski” and “atomic force” microscopy and compared to photothermal microscopy results. A correlation is observed between flaked silica and strongly absorbing areas.
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.
The development of applications of high power lasers require new characterization techniques for studying behavior of optical materials under intensive illumination, laser damage phenomena. Destructive investigations in silica have led to the conclusion that absorbing defects, typically a few nanometers in size, were responsible for laser damage initiation. The measured precursor densities are very low. The understanding of the true nature of these defects and damage phenomena requires the development of non destructive evaluation techniques with both high spatial resolution and high detectivity. The capability of collinear photothermal deflection to reach sub-micrometric resolution by reduction of the pump beam diameter has been theoretically and experimentally explored. Its ability to detect single absorbing particles has been studied. Currently 100-nm-diameter gold inclusions can be imaged with a signal-to-noise ratio of 8 at the wavelength 1064nm. Such a photothermal microscope has been coupled with an experimental set-up allowing damage threshold measurement at the same wavelength. Thus behavior of 100-nm-gold inclusions in silica can be studied under irradiation. Further improvements by decreasing wavelength, increasing modulation frequency and by using piezoelectric translation stages, will allow to study 10-nm-inclusions. We present an overview of last developments in the field of photothermal microscopy in connection with laser damage.
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.
Photothermal deflection is widely used to study defects in optical coatings and role of these defects in laser damage. Because defects responsible for laser damage are probably submicrometer sized, both high spatial resolution and high sensitivity are required to detect defects as small as possible. In this work we theoretically and experimentally explore the capability of collinear photothermal deflection to give submicrometric resolution by reduction of the pump beam diameter to one micrometer. We have developed a microscope based on the photothermal deflection of a transmitted probe beam and well-suited for multiscale studies of defects in thin films. The pump and probe beams are collinear and focused through the same optics, which can be chosen in order to change the diameter at 1/e2 of the pump beam on the sample surface from 100 micrometers to 1 micrometers . We present our first results obtained on specially prepared absorption targets and show that a lateral spatial resolution lower than 1 micrometers is reached.
Absorption and scattering present local defects which can be imaged simultaneously in exactly the same conditions, allowing a precise study of correlation between absorption and scattering spatial variations in thin film materials and surfaces. Absorption A is linearly related to the extinction coefficient whereas scattering S is mainly due to surface profile and refractive index variations. Furthermore spatial frequencies involved in these variations take a great part in scattering. However imaginary and real parts of the complex index are related by KK relations, and correlation A/S depending on the wavelength can be expected. Simulations in fused silica are presented. Studies of correlation give information about nature of defects.
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