Jean-Luc Rullier

Initiation of laser-induced damage sites in fused silica optical components

Significant improvement in the polishing process of fused silica optical components has increased their lifetimes at 351 nm. Nevertheless, for large laser facilities like the LaserMegaJoule (LMJ), zero defect optical components are not yet available. Therefore, a damage mitigation technique has been developed to prevent the growth of the laser-initiated damage sites. Because of the difficulty to produce mitigated sites with sufficiently large depth, the initial morphology of damage to mitigate is a critical issue. The aim of this work is to determine laser parameters (pulse duration, fluence) which permit us to initiate damage sites in accordance with our mitigation process. Confocal microscopy is used to observe damage sites that have sub-surface cracks and consequently to measure precisely the diameter and the depth of the area to mitigate.

Localized pulsed laser interaction with sub-micronic gold particles embedded in silica: a method for investigating laser damage initiation

Laser damage phenomena in fused silica are currently under study because of numerous related high power laser applications. Nanosized defects are believed to be responsible for some laser damage initiation. In order to predict and to quantify this initiation process, engineered submicronic gold defects were embedded in silica. The study of these samples by localized pulsed irradiation of isolated gold particles coupled with Nomarski, atomic force and photothermal microscope observations permits us to discriminate between two distinct stages of material modification: one detectable at the surface and the second in the neighbourhood of the embedded particle. Comparison between the observations and simulations results in good agreement if we assume that inclusion melting initiates the damage.

Influence of longitudinal mode beating on laser-induced damage in fused silica

In our study, the laser-induced damage densities on a fused silica surface produced by multiple longitudinal mode (MLM) pulses are found to be higher than those produced by single longitudinal mode pulses at 1064 nm. This behavior is explained by the enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating. At 355 nm, the absorption is linear and an opposite behavior occurs. It can be explained with the help of a process involving thermomechanics coupled with the fine time structure of MLM pulses, leading to the possible annealing of part of the absorbent defects.

Origin of the damage ring pattern in fused silica induced by multiple longitudinal modes laser pulses

Ring patterns surrounding laser damage sites at the exit surface of fused silica are systematically observed when initiated by multiple longitudinal modes nanosecond laser pulses at 1064 nm. The appearance chronology of rings is found to be closely related to the temporal shape of the laser pulses. This supports that the damage morphology originates from the coupling of a laser-supported detonation wave propagating in air with an ablation mechanism in silica. In our experiments, the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube root of the laser intensity, in good agreement with theory.

Evaluation of the fused silica thermal conductivity by comparing infrared thermometry measurements with two-dimensional simulations

A self-consistent approach is proposed to determine the temperature dependent thermal conductivity k(T) of fused silica, for a range of temperatures up to material evaporation using a CO2 laser irradiation. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was linked step by step as the laser power was increased with experimental measurements using infrared thermography. We show that previously reported k(T) does not reproduce the temporal profile as well as our adaptive fit which shows that k(T) evolves with slope discontinuities at the annealing temperature and the softening temperature.

Impact of two CO(2) laser heatings for damage repairing on fused silica surface.

CO(2) laser is an interesting tool to repair defects on silica optics. We studied UV nanosecond laser-induced damage in fused silica after CO(2) laser heating. The localization of damage sites and the laser damage threshold are closely related to stress area in silica induced by heating. By applying a suitable second laser heating, we managed to eliminate the debris issued from redeposited silica and to modify the stress area. As a consequence, a significant increase of laser resistance has been observed. This process offers the possibility to improve damage repairing sufficiently to extend the lifetime of the silica components.

Observation by photothermal microscopy of increased silica absorption in laser damage induced by gold nanoparticles

In order to understand laser-induced damage in glass, we subjected engineered SiO2 thin films containing sub-micron gold inclusions to high fluences, and observed the results using several means of analysis. We found decoupling in time between the emission of gold and that of silicon with samples containing gold spheres of diameter 3 nm. We have analyzed the changes in the silica optical absorption at 1064 nm, using photothermal deflection microscopy. We find, upon exceeding a sharp fluence threshold, a thousand-fold increase in absorption of the silica matrix around the inclusion. We conclude that ions from the inclusion permeate the surrounding silica, and form a highly absorbent mixture.

Removal of scratches on fused silica optics by using a CO 2 laser

We investigate the efficiency of local CO₂laser processing of scratches on silica optics in order to enhance the nanosecond UV-laser damage resistance. The surface deformations induced by the process have been measured for different CO₂laser parameters and then the pulse duration and the beam diameter have been chosen accordingly to limit those deformations below 1 µm. From the study of the laser damage resistance as a function of different material modifications we identify a range of optimal radiation parameters allowing a complete elimination of scratches associated with a high threshold of laser damage. Calculation of the temperature of silica using a two-dimensional axi-symmetric code was compared with experiment, supporting an optimization of the laser parameter as a function of the maximal dimensions of scratches that could be removed by this process.

Integrated photothermal microscope and laser damage test facility for in-situ investigation of nanodefect induced damage.

An integrated setup allowing high resolution photothermal microscopy and laser damage measurements at the same wavelength has been implemented. The microscope is based on photothermal deflection of a transmitted probe beam : the probe beam (633 nm wavelength) and the CW pump beam (1.06 microm wavelength) are collinear and focused through the same objective. In-situ laser irradiation tests are performed thanks to a pulsed beam (1.06 microm wavelength and 6 nanosecond pulse). We describe this new facility and show that it is well adapted to the detection of sub-micronic absorbing defects, that, once located, can be precisely aimed and irradiated. Photothermal mappings are performed before and after shot, on metallic inclusions in dielectric. Results obtained on gold inclusions of about 600 nm in diameter embedded in silica are presented.

Infrared thermometry and interferential microscopy for analysis of crater formation at the surface of fused silica under CO2 laser irradiation

In situ spatial and temporal temperature measurements of a fused silica surface heated by a 10.6 μm CO2 laser were performed using an infrared camera. These measurements were derived from heat flux emission of the fused silica. High temperature measurements—in the range 400–2500 K—were performed at the surface of a semi-transparent media with a high spatial resolution. Particular attention was given to the experimental conception and to the calibration of the infrared device. Moreover, both conventional and interferential microscopes were used to characterize the silica surfaces after CO2 laser irradiation. By associating these results with thermal camera measurements we identified the major surface temperature levels of silica transformation when heated during 250 ms. Surface deformation of silica is observed for temperatures <2000 K. This is consistent with other recent work using CO2 laser heating. At higher temperatures, matter ejection, as deduced from microscope observations, occurs at temperatures ...