So far, we have discussed single filamentation with re-focusing if the peak power is high. That is to say, the beam quality of the laser pulse is assumed to be very smooth. The transverse fluence distribution of the pulse is similar to, if not equal to a Gaussian distribution in the experiment and is assumed Gaussian in the simulation. Only one filament occurs if the peak power is not more than 2–3 critical power in an experiment whereas in the simulation, a single filament always occurs. However, once the peak power is higher than about 3 critical power in an experiment, unavoidable spatial irregularity across the wave front would be induced due to the imperfection of the beam quality or due to propagation through a non-homogeneous medium in the real environment. This would result in self-focusing of the local higher intensity zone resulting in the formation of more than one filament; i.e., multiple filamentation takes place.