We have clarified the influences of water-vapor absorption lines in air on femtosecond pulse propagations from experimental and theoretical points of view. Precise measurements for the femtosecond pulse shapes after propagation of as much as 300 m through air have been made in a semiunderground optical testing tunnel. We observed the pulse splitting and the enhancement of the pulse broadening due to the 100-m propagation in air. The experimental results are in good agreement with the theoretical analysis by use of the HITRAN database at the edge and the center of the water-vapor absorption regions in air. Measured autocorrelation traces are mostly reproduced by those calculated with only the real part (dispersion) of the refractive index modulated by water-vapor absorption lines in air. This fact enables us to conclude that the pulse deformations due to water absorption lines in air are caused mainly by the real part (dispersion), not by the imaginary part (absorption), of the refractive index of air.
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