Capillary compressor of femtosecond laser pulses with nonlinear rotation of polarisation ellipse

The process of nonlinear rotation of the polarisation ellipse of laser radiation, occurring simultaneously with the broadening of the pulse spectrum due to nonlinear self-phase modulation in a gas-filled capillary, is studied. It is shown that the maximal rotation of the polarisation ellipse is experienced by the spectral components, shifted towards the short-wavelength side with respect to the central wavelength of the initial laser pulse. Using the effect of polarisation ellipse rotation, an eightfold increase in the energy contrast ratio of a 28-fs light pulse, obtained by compression of the radiation pulse from an ytterbium laser with the duration 290 fs, is implemented.