Direct measurement of the group refractive index of air with interferometry between adjacent femtosecond pulses

The group refractive index of air in laboratory conditions is measured directly between adjacent femtosecond laser pulses by a new interferometry technique. Measurement of the repetition rate of the mode-locked pulse train that gives the maximum amplitude of the interference-signal envelope enables us to determine the group refractive index of air within a standard deviation of 2 x 10(-7). This simple method without vacuum reference is attractive for measuring the group refractive index needed for precise distance measurements in open fields.