The temporal development of the intensity distribution in a focussed laser beam

This paper extends the steady-state calculation of the intensity distribution near the focus of a laser beam, to the case when the laser intensity is changing rapidly, over a time of order 1 picosecond. An efficient procedure for computation is outlined, allowing for the primary spherical aberration of the focussing lens and arbitrary dependence of the incident amplitude on position and time, and results are given for two trial calculations.