  Cylindrical density depressions generated by femtosecond laser pulses
filamenting in air for different energy depositions is investigated
numerically, by using a set of hydrodynamic equations. The evolution of density
profile is calculated for different temperature elevations, the results
indicate that the gas density hole is getting shallower and wider with the
increasing temperature elevations. A simulation of the propagation inside
low-density channel implies a new way to generate a type of bottle beam.
