Wavelength dependence of the mechanisms governing the formation of nanosecond laser-induced damage in fused silica.

The influence of the wavelength on the morphology of nanosecond laser-induced damage on the exit surface of fused silica is investigated. A combination between the typical features of damage sites initiated at 1064 nm and 355 nm is observed at 532 nm, including ring patterns sporadically exhibited, in good agreement with calculations of the development of an electron avalanche at this wavelength. The associated ring appearance speed scales as the cube root of the laser intensity, and is ~10.5 km/s while it is ~20 km/s when initiated by infrared pulses. The whole set of results sheds light on the different wavelength-dependent mechanisms governing damage formation.

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.

Investigation of stress induced by CO2 laser processing of fused silica optics for laser damage growth mitigation.

Laser damage mitigation' is a process developed to prevent the growth of nanosecond laser-initiated damage sites under successive irradiation. It consists of re-fusing the damage area with a CO2 laser. In this paper we investigate the stress field created around mitigated sites which could have an influence on the efficiency of the process. A numerical model of CO2 laser interaction with fused silica is developed. It takes into account laser energy absorption, heat transfer, thermally induced stress and birefringence. Residual stress near mitigated sites in fused silica samples is characterized with specific photoelastic methods and theoretical data are compared to experiments. The stress distribution and quantitative values of stress levels are obtained for sites treated with the CO2 laser in various conditions of energy deposition (beam size, pulse duration, incident power). The results provided evidence that the presence of birefringence/residual stress around the mitigated sites has an effect on their laser damage resistance.

Effect of laser irradiation on silica substrate contaminated by aluminum particles.

A major issue in the use of high-power lasers, such as the Laser Megajoule (LMJ), is laser-induced damage of optical components. One potential damage initiator is particulate contamination, but its effect is hard to distinguish from that of other damage precursors. To do so, we introduced artificial contaminants typical of metallic pollution likely to be present on the optical components of the LMJ chains. More precisely, aluminum particles of two different sizes were placed on a silica sample. These dots were characterized by optical microscopy and profilometry. Then they were exposed to a laser beam with a pulse length of 6.5 ns at 1064 nm and fluences in the range from 1 to 40 J/cm(2). Each dot was characterized again with the same techniques and also by photothermal microscopy. To complete the experimental results, we performed numerical simulations with a one-dimensional Lagrangian hydrodynamics code. We show that the particle removal by laser irradiation produces a modification of the silica surface that does not evolve into catastrophic damage under subsequent irradiation. However, the effect does depend on the size of the dots. We demonstrate that a procedure exists that removes the dot and leaves the site capable of resisting high fluence.

Localized pulsed laser interaction with submicronic 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.

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.