The spectral analysis of absorption of molecular gases belongs to very powerful methods for detection of their concentration in a tested sample. In case of the human breath it consists of hundreds of molecules of gasses and their concentrations are known as biomarkers. An increased concentration of more of them indicates certain diseases or symptoms. The presenting work covers a pilot experiment where femtosecond pulse probe laser and a detection chain based on cross-correlation makes an analysis of the specimen of the absorption gas. In the pilot experimental set-up the detection part utilizes a Michelson interferometer with movable measuring mirror. We developed a novel digital method for correction of the measuring mirror speed during the cross-correlation process of pulses. The caught optical signals are calculated by the fast Fourier transform from the time to wavelength domain after the speed correction The work proofed positive the possibility of using the femtosecond laser for measurement of the femtosecond laser spectrum in the infrared region when a continuously working laser was mixed with the femtosecond laser. The work presents the recorded spectra of this configuration of cw and pulse laser at the same time.