High numerical aperture focal spot diagnostic and optimization

In this paper, we will show the principle of focus measurement and optimization with a quadriwave lateral shearing interferometer. We will present an application in the case of high power lasers. Once a laser calibration done at focus in the target chamber, a corresponding reference is acquired on the classical diagnostic beam using a leakage on the laser upstream from the chamber. The focal spot optimisation can be then controlled from this diagnostic beamline. An example of a focal spot optimized using a f/1.5 focusing optics is given on the figure 1. The focal spot energy distribution is measured coupling a CCD camera and a 150x microscope objective.