We demonstrate complete cancellation of the nonlinear astigmatism in a mode locked Ti:Sapphire laser, which otherwise needs to be compensated in standard cavity designs.

Astigmatism is a well known aberration in folded optical cavities that include Brewster-cut crystals and/or off-axis focusing elements. For continuous-wave (CW) operation, astigmatism is linear and can be fully compensated by correctly choosing the folding angles of the focusing elements in accordance with the length of the Brewster-cut windows. This results in a circular non-astigmatic beam at the output of the laser. For mode-locked (ML) operation induced by the nonlinear Kerr effect, an additional nonlinear astigmatism from the Kerr lens is added that is power-dependent and needs to be taken into account. The standard technique to compensate for nonlinear astigmatism is to deliberately introduce linear astigmatism in the "opposite" direction, which results in compensation of the overall astigmatism for a specific intra-cavity intensity. Changing any of the cavity parameters that affect the intra-cavity intensity will require re-compensation. Here we demonstrate a novel type of cavity folding that eliminates from the source nonlinear astigmatism in Kerr-lens ML lasers. Our method uses a novel cavity folding in which the intra-cavity laser beam propagates in two perpendicular planes such that the astigmatism of one mirror is compensated by the other mirror, enabling the introduction of an astigmatic free, planar-cut gain medium. We demonstrate that this configuration is inherently free of nonlinear astigmatism, which in standard cavity folding needs a special, power specific compensation. In this work, the spatial mode of the beam in both sagital and tangential planes does not change as it enters the crystal, thus cancelling the nonlinear Kerr-lens astigmatism from the source.