  Experimental and numerical studies of a temporal evolution of a light bullet
formed in isotropic LiF by Mid IR femtosecond pulse (2500 to 3250 nm) of power,
slightly exceeding the critical power for self-focusing, are presented. For the
first time regular oscillations of the light bullet intensity during its
propagation in a filament were registered by investigation of induced color
centers in LiF. It was revealed that color centers in a single laser pulse
filament have a strictly periodic structure with a length of separate sections
about 30 mcm, which increases with a laser pulse wavelength decreasing. It was
shown that the origin of light bullet modulation is a periodical change of the
light field amplitude of an extremely compressed single cycle wave packet in a
filament, due to the difference of the wave packet group velocity and the
carrier wave phase velocity.
