Fig. 4.

Geometric optics model. (a) The beam was ray-traced through the focus of a 25 mm focal length, 90 degree off-axis parabola. The center wavelength and the FWHM edges of the spectrum were represented by green, blue and red rays, respectively. (b) The shape of the beam spot in x (chirped dimension) and y (unchirped dimension) was simulated at several axial positions through focus. At focus (z = 0) the beam spot was symmetric. (c) An image of the focal plasma in air. (d) The shape of the beam spot was asymmetric at focus with the addition of 6 mm of fused silica to simulate backside machining.