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We are currently focusing on the improvement of contrast pedestal (CP) in the compressed laser pulse of PW Ti:Sapphire lasers. In our previous studies, we have identified the stretcher in our laser system as the source of CP. In order to underpin the true origins of CP, we have quantitatively characterised the surface quality of large optics used in the Gemini laser stretcher, where the laser beam is spatially dispersed and the spectral phase noise is induced by the optical surface roughness. We have measured the surface profiles of 2 different gold gratings, the new and old grating, and back mirror to a very high precision (~ a fraction of nm) by using ZYGO Dynafiz, with a spatial resolution of ~50µm over a width up to ~320mm, an unprecedented combination of very high spatial resolution with a very wide field of view. The surface roughness of the large curved mirror was determined experimentally. We have developed a simple physical model to deal with the influence of the surface roughness on the contrast pedestal. Based on the measured surface profiles and by taking the actual laser beam size into account, we are able to determine the spectral phase noise induced by the optical surface roughness in the stretcher. Consequently, we are able to accurately evaluate the impact of individual large optics in the stretcher and an overall impact of the stretcher on the contrast pedestal. The calculated contrast induced by both stretches with the new and old gratings are in an excellent agreement with the experimental results measured by the Sequoia scan. For the stretcher with the old grating, the grating is the dominant impact factor on the contrast. However, for the stretcher with the new gold grating of higher quality, the impact of the curved mirror on the contrast is comparable to that of grating. This implies that the influence of curved mirror on the contrast pedestal becomes more significant when the surface quality of grating is further improved. It is clearly observed that the impact of back mirror on the contrast is more than one order of magnitude lower than that of gratings and also much lower than that of curved mirror.
In conclusion, we have demonstrated a novel method to evaluate the impact of large optics in the stretcher on the contrast pedestal by precisely quantitative characterization of optical surface quality. It is possible to accurately predict the contrast pedestal based on the stretcher configuration and precise characterisation of the optical surface in the stretcher prior to the construction of actual CPA high power laser system.
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