Effect of spherical aberration on the propagation of a tightly focused femtosecond laser pulse inside fused silica

We show the effect of a spherical aberration on the propagation of a single tightly focused femtosecond laser pulse inside fused silica. At high power, a long plasma channel across the geometrical focus can be observed under deep focusing. The total channel length decreases with the decrease of the focusing depth at fixed focal length. We found that the spherical aberration induced by the air–sample interface extends the focal region and plays an important role in the formation of the long plasma channel. In certain conditions this spherical aberration can be compensated by a spherical aberration induced by the objective used to focus the laser.