Transformation of the spectrum of high-intensity femtosecond laser pulses in a gas-filled microcapillary

Peculiarities of propagation and nonlinear-optical transformation of femtosecond laser pulses (τ0.5 ≈ 27 fs, λ0 800 nm) in a cylindrical hollow microcapillary waveguide (280 μm in core diameter and 50 cm in length) filled with saturated water vapor have been experimentally studied. Precise focusing of a laser spot on the microcapillary entrance allowed us to achieve stable waveguide propagation of strong laser pulses in the fundamental modes (EH11 and EH12) without damage. It has been found that the influence of high intensity laser pulses (≅1014 W/cm2) on the spectra of femtosecond pulses is caused not only by Kerr nonlinearity, but also an electron component of generated plasma. The latter leads to 'a blue shift' of passed femtosecond laser pulses. (authors)