Gerard Raze

The impact of laser damage on the lifetime of optical components in fusion lasers

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.

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.

HIGH AVERAGE POWER DIODE ARRAY PUMPED FREQUENCY DOUBLED YAG LASER

We report the demonstration of a transversally diode array pumped Nd:Yag laser using thirty 20 watts CW linear diode arrays. At a 9 kHz repetition rate the laser produces 58 watts average power at 532 nm when intracavity doubled with a KTP crystal, leading to a 3.2 % optical/electrical efficiency.

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.

Self-focusing and surface damage in fused-silica windows of variable thickness with UV nanosecond pulses

Variable experimental conditions were used to measure the occurrence of front surface, rear surface and filamentation damage in synthetic fused silica windows. Experiments were performed at 355 nm with a table-top beam of mm-size, and at 351 nm with ALISE laser, a 100 J installation. The 351 nm beam was about 3 cm wide at the entrance surface; it was single-mode temporally, with or without a frequency modulation which has the function of widening the spectrum to decrease Stimulated Brillouin Scattering. The 355 nm was single-mode temporally. Thin windows showed very scarce front damage and no filament damage at intensities which cause a high density of rear surface damage. Without any spectral widening, the thicker windows (4.3 cm) showed appreciable amount of front surface damage; filaments were observed and but no filaments. When a spectral modulation was added, front surface damage vanished, filaments and rear surface damage were observed.

High average power diode array pumped frequency doubled YAG laser

In order to obtain high average power in the green, a diode pumped laser program is under development. High average power (tens of watts) and high repetition rate (tens of kilohertz) require: cw diode array pumping, high Q-switching and intra-cavity second harmonic array generation. We report the demonstration of a transversely diode array pumped frequency doubled Nd:YAG laser. The 35 linear diode arrays are arranged radially around a single Nd:YAG rod providing up to 700 watts of pumping power around 808 nm.

Growth of damage sites due to platinum inclusions in Nd-doped laser glass irradiated by the beam of a large-scale Nd:glass laser

Samples of Neodymium doped laser glass were irradiated by the 1ω beam of a Nd:glass laser that delivers up to 80 J during a 3-ns pulse duration. Prior to this experiment, platinum inclusions were revealed by a systematic scanning with a lab-scale Nd:Yag laser. The damage sites due to impurity inclusions were subjected to tens of shots of the centimeter-size beam. Several inclusions were irradiated by a series of shots, at a fixed fluence comprised between 10 and 20 J/cm2. The incidence on the optical component was taken at Brewster-angle. In each case, the damage zone began to grow, then the growth rate slowed down and finally stopped. Thus, a stabilization of the growth was obtained for this bulk damage as opposed to steady growth observed in the case of silica surfaces.

Laser damage measurements on optics: from small to large beams, from samples to large aperture components

The functional lifetime of large aperture optical components used in high power lasers, like LIL and LMJ facilities, is mainly determined by laser damage measurements. We present experimental procedures and statistical analysis, made on small samples with mm-size beams, to determine damage densities and damage growth laws. The tests and analysis are compared to other results obtained with larger beams (few cm2) on large aperture components.

Design and Performance of High Average Power Diode Array Pumped Frequency Doubled YAG Laser

In order to obtain high average power in the green, a diode pumped laser program is under development. High average power (tens of watts) and high repetition rate (tens of kilohertz) require: cw diode array pumping, high Q-switching and intra-cavity second harmonic array generation. We report the demonstration of a transversely diode array pumped frequency doubled Nd:YAG laser. The 35 linear diode arrays are arranged radially around a single Nd:YAG rod providing up to 700 watts of pumping power around 808 nm.