| To reach highest laser intensity possible, two approaches were addressed. First, amplifier performance limited by parasitic oscillations was improved by coating an index-matched, absorptive layer - diamond-like-carbon film. Secondly, the aberrations of a f/1 paraboloidal mirror used at the final focusing step were corrected for the first time by directly compensating measured wave-front distortion. In conjunction with the correction process, the importance of wave-front characterization in evaluating intensity at the focus is discussed comparing the problems of spot size measurement or ionization experiments. Instead of using these methods, the Fresnel-Kirchhoff diffraction integral with measured wave-front was used to calculate the amplitude and phase anywhere in the focal volume. The diffraction method indicates that the intensity of 1021 W/cm2 was generated by correcting f/1 off-axis paraboloidal aberrations in a 30 TW diffraction-limited laser system. While we measured and corrected wave-front after the paraboloid at a low energy level, the additional high energy aberration was measured before focusing and added to the corrected low energy wave-front to calculate the intensity at the focus by using a diffraction integral. This way, the available energy was used very efficiently to reach highest intensity with the full characterization of amplitude and phase anywhere in the entire focal volume. |
