Ionization spectrum transformation and compression of powerful femtosecond laser pulses in experiments on the propagation in gas-filled dielectric capillaries

The propagation of an intense ($I\leq10^6\,$ W/cm$^2$) femtosecond laser radiation with a duration of $\sim100$ fs through gas-filled dielectric capillaries was studied. The radiation with a power up to 0.2  TW propagates along the paths up to 20  cm with a transmission efficiency of $\sim45$%. The beam transverse structure at the output is close to the capillary fundamental mode under gas-ionization conditions. The transformation of pulse spectrum was studied as a function of input intensity. It is demonstrated experimentally that the pulse is compressed to a duration of $\sim30\,$ fs due to the compensation of ionization-induced self-phase modulation in a linear dispersive element at the capillary output.