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The refocusing of femtosecond laser pulses forming the filament in air is studied both numerically and experimentally. The evolution of fundamental as well as third harmonic pulses and spectra during propagation in the filament is discussed. It is shown that the refocusing of the pulse gives rise to enhanced spectral shifts of fundamental and third harmonic waves.
The large spectral shifts of the fundamental and third harmonic waves (up to 20nm and 50nm, respectively) during femtosecond laser pulse filamentation in air have been demonstrated by using focusing lenses of long focal length. It was shown that in a wide range of laser intensities this process does not saturate and linearly depend on the pump pulse energy.
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