Measuring the focal spot of a high power laser is challenging, at best.
The main issue is that when a high power laser is focused down to a small point, the power density can be extremely high, typically high enough to damage any sort of measurement equipment you would use.
Measure the laser beam without letting the laser touch the sensor. How can this be done? John McCauley explains the new technology at ALAW (Advanced Laser Applications Workshop).
Hint: Why is the sky blue?