Michel A. Josse

An accurate, repeatable, and well characterized measurement of laser damage density of optical materials.

Known for more than 40 years, laser damage phenomena have not been measured reproducibly up to now. Laser resistance of optical components is decreased by the presence of material defects, the distribution of which can initiate a distribution of damage sites. A raster scan test procedure has been used for several years in order to determine laser damage density of large aperture UV fused silica optics. This procedure was improved in terms of accuracy and repeatability. We describe the equipment, test procedure, and data analysis to perform this damage test of large aperture optics with small beams. The originality of the refined procedure is that a shot to shot correlation is performed between the damage occurrence and the corresponding fluence by recording beam parameters of hundreds of thousands of shots during the test at 10 Hz. We characterize the distribution of damaging defects by the fluence at which they cause damage. Because tests are realized with small Gaussian beams (about 1 mm at 1e), beam overlap and beam shape are two key parameters which have to be taken into account in order to determine damage density. After complete data analysis and treatment, we reached a repeatable metrology of laser damage performance. The measurement is destructive for the sample. However, the consideration of error bars on defect distributions in a series of parts allows us to compare data with other installations. This will permit to look for reproducibility, a necessary condition in order to test theoretical predictions.

Parametric study of the growth of damage sites on the rear surface of fused silica windows

The growth of damage sites on the rear surface of fused silica plates was studied as a function of fluence and angle of incidence. At 1053 nm, a 70 J beam, 3 ns in pulselength, was directed to a 5 cm2 zone on a bare fused silica window. Initiation and growth was observed. The growth of previously initiated sites was also studied. Growth is exponential in nature. The experiments allow for the determination of the growth coefficient as a function of fluence. At 355 nm, damage sites were irradiated at various angles of incidence, with a tripled Nd:Yag laser, spatially Gaussian, 2.5 ns in pulselength. By fitting growth with an exponential law, it was determined that the relevant fluence for growth was that taken inside the material.

Study of the evolution of mechanical defects on silica samples under laser irradiation at 355 nm

During the life of a high-power laser chain, optical components may be damaged due to local high fluence levels in the inhomogeneous beam. The origin of the laser damage can be impurities, surface defects or flaws and cracks resulting from polishing, or it may be produced by self-focusing in the component. The aim of this study is to better understand the correlation between a surface crack on a silica optical component and laser damage. To accomplish this, calibrated indentations were made on silica samples. Observations of the sites were made with an optical microscope, and three different morphologies were recognized. Then the zones containing the indentations were irradiated (single shot mode) with a Nd Yag laser at 355 nm for various fluences. Subsequent observations of the sites were made with an optical microscope, with the aim of correlating site morphology and laser-induced damage. Some sites were believed to have undergone laser conditioning. They were further irradiated (raster scan mode) at high fluence, and some evidence for a laser conditioning effect was obtained.

Laser-induced damage growth with small and large beams: comparison between laboratory experiments and large-scale laser data

Experiments have been performed to measure the rate of laser-induced damage growth at the rear surface of fused silica windows at 1064, 1053 and 351 nm. One test bench delivered 9 ns monomode gaussian pulses at 10 Hz and 1064 nm. The size of the focused beam on the sample was a few mm2. Another test bench delivered 2.5 ns single or multimode pulses at 1053 and 351 nm. The focused spot on the sample was a few cm2. We compare and discuss our laboratory experimental results, the larger scale ALISE laser data and other results obtained at LULI.

Automatic damage test benches: from samples to large-aperture optical components

The functional lifetime of large aperture components used in high power lasers, like LIL and LMJ facilities, is mainly determined by laser damage measurements. Automatic damage test benches allow to obtain more data in less time than traditional tests. We present, first experimental procedures and statistical analysis made on small samples with mm-size beams, to determine damage densities and damage growth laws. The presented methods are the usual 1on1, Non1, Ron1 and Son1 tests and more specially the raster scan procedure. The tests and analysis are compared to other results obtained with larger beams (few cm2) on large optics. We show that the exact knowledge of each shot parameters (energy, surface and pulse duration) permits to determine the damage growth rate (and then to predict the lifetime of each optics), to precisely study self-focusing phenomenon and more to finely observe pre-damage-levels. In this way, the main parameters like fluence or intensity are associated to the observed phenomenon. Moreover laser beam diagnostics, many diagnostics used for the detection and the observation of damage occurrence are equally very important. It is also necessary to develop test procedures entirely computed which permit to scan all the surface of a component and to acquire in real time the beam parameters and the results of laser-matter interaction. Experimental results are reported to illustrate what could be achieved on an instrumented and automated facility.

Observation of giant enhanced backscattering of light from weakly rough dielectric films on reflecting metal substrates

The enhanced backscattering of light from a randomly rough surface, which is manifested as a narrow peak in the retroreflection di- rection in the angular distribution of the intensity of the light that is scat- tered diffusely, has been extensively studied. Both theoretical and ex- perimental investigations have shown that the height of the peak is never more than twice the height of the background at the position of the peak. We report the observation of a giant enhanced backscattering of light from a randomly weak rough dielectric film on a reflecting metal sub- strate, in which the ratio of the height of the peak to the height of the background at its position is greater than 10. It is found that this giant enhanced backscattering peak is accompanied by concentric circular in- terference fringes, whose axis is normal to the mean scattering surface, with both the specular and backscattering peak on the same ring. A possible mechanism for the giant backscattering is suggested.

Effects of scratch speed on laser-induced damage

A major issue in high power lasers for fusion is laser-induced damage on optical components. Since damage is often initiated by a surface crack resulting from polishing, it is important to understand the physics involved in this phenomenon. In this study, calibrated surface scratches have been realized on two silica samples using an indenter-scratcher apparatus. A variety of scratches have been tested by applying different speeds and forces on the scratcher needle. Optical microscope observations show that the scratches made at highest speed create irregular dashed lines. In addition, we have observed, at intermediate speed, an evolution in time of the scratches due to local stresses in silica. One of the samples was irradiated by a Nd:YAG laser beam at 355 nm with the scratches on the exit surface. Microscope observations were made before and after irradiation. Strong dependence on the scratch speed was observed on the local laser damage. Again, temporal evolution of the damage has been observed.

Description and calibration of a fully automated infrared scatterometer

A fully automated scatterometer, designed for BRDF measurements in the IR at about 10 micrometers , is described. Basically, it works around a reflecting parabola (464 mm diameter, F/0.25) and permits measurements in and out of the plane of incidence. Optical properties of the parabolic mirror are emphasized by a ray-tracing technique which permits determination of the correct illumination on the sample and detection conditions of scattered light. Advantages and drawbacks of such an instrument are discussed, as well as calibration procedures. As a conclusion, we present experimental results to illustrate the instrument capabilities.

Time-resolved measurements of reflectivity, plasma formation, and damage of hafnia/silica multilayer mirrors at 1064 nm

High damage threshold hafnia/silica HR mirrors were damage tested. The transient reflectivity of these mirrors was studied during laser irradiation and particularly during catastrophic damage by top layer chipping. The tests were performed in R/1 mode on 50 sites in order to correctly assess the statistical behavior of the damage threshold. During the ramp, we observed the formation of a first plasma coupled to a modification of the reflected pulse without formation of any pit or chipping damage. For further irradiations at higher fluences, the optical properties of the mirror were unchanged up to the creation of a catastrophic chipping damage coming with a more intense plasma. The statistical distributions of fluences obtained in the two cases were different. For the moment, there is no proven correlation between first plasma and chipping: plasma detection cannot be used as a nondestructive quality control.

The possibility of new high-transmission, high finesse, Perot-Fabry interferometers for far IR spectroscopy

Finesse as high as 260 and peak transmission close to 98% at frequencies around 30 cm−1 are predicted at 10 K by coating the Perot-Fabry (PF) mirrors with a suitable thickness of High Tc (HTC) superconductor YBaCuO. In fact, Finesse will be limited by the lack of parallelism and the lack of perfect flatness of the mirrors to values around 100. These predictions are based upon phenomenological models for both the YBaCuO coating and the MgO support, i.e. from direct transmission measurements of MgO and YBaCuO in the Far IR, which had to be fitted by the models.