Operational experience of contamination and damage of the larger aperture optics in the HELEN laser facility vacuum system at 1053 and 527 nanometres

Abstract

In this paper we review contamination and damage that has occurred since the HELEN laser was converted for use as a chirped pulse amplification (CPA) system. We concentrate on the largest components in the facility that operated in a vacuum environment in the short pulse (500fs) parts of the system. This experience will be valuable for planning commissioning and operations on the successor facility, ORION that is currently being constructed. The optical components located in the vacuum sections will be described. We have an interest in laser-induced damage and contamination from the long pulse only (~1ns), short pulse only (500fs) and combined regimes. For most of the operations the CPA system has operated at a wavelength of 1053nm (1ω). Some experiments have also been conducted with 527nm (2ω) CPA light derived by the use of a KDP doubling crystal. Damage to the infra red pulse compression gratings has occurred infrequently with a minority of high-energy shots. Contamination of multilayer dielectric plane turning mirrors has arisen from target disassembly. Focussing systems using off axis parabolic mirrors have sustained contamination from debris and a dielectric protected silver reflector used for green light suffered laser induced damage. Debris shields placed between the target and the parabolas have been used on selected experiments. The shields were anti-reflection coated with single layer, sol gel silica. These shields became contaminated on the target facing sides and in the case of 2ω operation also from blow off of the damaged mirror coating that caused a two-pass transmission loss in the system. A number of characterisation methods were used to evaluate and quantify the damage and contamination. These include macroscopic photography, microscopy, reflectometry and transmission spectroscopy.