Abnormal wavelength dependence of the self-cleaning phenomenon during femtosecond-laser-pulse filamentation

Through investigating the laser beam patterns at various wavelengths of the filamentation supercontinuum, abnormal wavelength dependence of the beam profile self-cleaning phenomenon is reported. It is founded that only the Stokes light has a nice spatial profile, while the patterns at the anti-Stokes side and the fundamental wavelength are not smooth and regular. Furthermore, by numerical simulations, we propose that the intensity perturbation of the initial beam can be considered as high order spatial modes superpositioning on a fundamental mode. The fundamental mode self-focuses, first producing a single mode profile on the axis. The higher order modes undergo strong diffraction constituting the energy reservoir. That gives rise to the overall 'bad' spatial properties at the fundamental wavelength. On the other hand, the distortion of the beam patterns at the anti-Stokes side is due mainly to the interaction of the back part of the pulse with the plasma, which gives rise to anti-Stokes shifts. Finally, a possible application of our results to high beam quality tunable Ti: sapphire laser amplification is suggested.