Instability processes and optimisation in multi-kilowatt coaxial CO2 lasers
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This thesis presents research carried out on a high power diffusion-cooled CO2 laser excited by a radiofrequency (RF) electric discharge in the annular configuration, and with a folded hybrid optical resonator. The main objective of the research was to investigate certain key aspects of the laser, so as to optimise overall performance while increasing power stability and reproducibility and ensuring safe and reliable operation at minimum cost. The work presented in the thesis contributed significantly to introduce the TruCoax Version 6 laser, which has 40% lower cost compared to the preceding version. The technological challenges encountered are divided into four main research areas as follows: The first area relates to RF discharge stability especially during the discharge ignition process and in the region of the RF feed-through, where an improved design eliminated damage due to electric breakdown and suppressed the occurrence of local γ-type RF discharge. Secondly, experimental acoustic measurements matched with a theoretical characterization of the acoustic properties of the annular CO2 laser vacuum vessel were used to generate information on the acoustic resonant frequencies and standing-wave patterns. Implementation of laser control algorithms based on this information prevented from laser damage due to acoustic resonances. Thirdly, the impact of thermal lensing in a gas laser gain medium located within a freespace stable resonator has been studied. The results show that the thermal lensing is an integral part of resonator configuration in the stable direction and can be applied towards a unified design for a range of RF input power levels. In the fourth key research area, temporal fluctuations in the laser output power (of order of 100 W) which were observed to occur on a timescale of minutes have been investigated for a laser with a power output of ~3.5 kW. Small power fluctuations in the range of ± 25 W could be attributed to increases in the coupling losses of rotational lines, and this was confirmed experimentally.