Effect of Splitting of the Femtosecond Pulse in the Linear Transfer Mode
One of the required problems of physical optics consists in obtaining optical images of objects hidden in the optically dense disperse medium [37—49]. This problem arises, first of all, in the development of efficient methods of transmission optical tomography and optical imaging [36,49], laser sensing of the atmosphere and ocean , confocal microscopy , etc. The recent progress in the technology of femtosecond lasers  has given a new impulse to these investigations, allowing measurements with the ultrahigh spatial resolution close to the diffraction limit. However, in this case, one faces some methodological difficulties connected with the information content of the transmitted and backscattered laser radiation, which, in contrast to X-ray and microwave radiation, interacts actively with the disperse medium, losing the coherence and forming the multiple-scattering noise. In addition to the diffusion broadening of the lidar pulse, multiple scattering can be a source of a false signal leading to uncontrolled error of tomographic or lidar measurements. For the first time, this was noticed in early experiments by R. R. Alfano, one of the founders of femtosecond optics, with colleagues [40—42]. The experiments dealt with the use of ultrashort radiation pulses for imaging of small objects in dense scattering media. In some situations, as the duration of the initial pulse decreased, the time splitting of the transmitted signal into two barely distinguishable components was observed. These components, in opinion of the authors of Reference 40, owe their origin to the coherent interaction of ballistic and diffusely scattered photons. The observation of this effect requires the precision recording of the temporal envelope of the signal. Therefore, the number of works, in which this effect was noted, is limited. Only in the recent time, some experiments were reported [43,44], in which the splitting effect was confirmed with high accuracy. At the same time, these experiments have shown that the short duration of a pulse is necessary, but not sufficient condition for appearance of the bimodal configuration of the envelope of transmitted signal. In References 45—47, it is shown that the shape and position of the secondary peak caused by the diffuse component of the signal depends on the optical parameters of the disperse medium (first of all, the scattering coefficient) and its optical thickness. In Section 5.3, based on the numerical solution of the radiation transfer equation, we make an attempt to specify the parametric range, in which the effect of splitting of an ultrashort optical pulse takes place.