NONLINEAR TRANSFORMATION OF A SPECTRUM OF FEMTOSECOND LASER PULSES IN A GAS-FILLED MICROCAPILLARY

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium- sapphire laser pulses (tau(0.5) = 27 fs, lambda(0) = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-mu m diameter core) filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of similar to 1.5.10(14) W/cm(2) in the fundamental EH(11) mode of the gas- filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather high relative (>= 95%) and absolute (similar to 60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive index n(2) = 4.5.10(-23) cm(2)/(W.Torr) (N(2)) and n(2) approximate to 2.8.10(-23) cm(2)/(W.Torr) (He) have been found. A short- wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed by the generated electron plasma at high pulse intensities (>= 10(14) W/cm(2)).