Аннотация:We use time-resolved spectral probing to study the dynamics of transformation of the opticalproperties of a nonlinear medium during formation and propagation of an extremelycompressed mid-IR femtosecond wave packet in calcium fluoride. Broadening, attenuation anddistortion of the probe pulse spectrum have been experimentally observed varying the delaytime of the probe pulse relative to the pump. By solving the unidirectional pulse propagationequation that describes propagation and filamentation of a pump pulse in a calcium fluoridecrystal we determine temporal changes of the refractive index and absorption coefficientinduced by an extremely compressed wave packet. A mechanism of the observed broadeningand breakup of the probe pulse spectrum is proposed.