Dispersion of femtosecond pulses in ambient and low pressure air

To measure the pressure dependence of dispersion, the Mach-Zender interferometer illuminated by 800nm, 16 fs pulses from a Ti:S oscillator working at 71 MHz repetition rate. The outcoming pulses from both arms were unified and sent to the entrance slit of an imaging spectrograph. The spectrally resolved interference fringes were then captured and evaluated, which provides directly the values of GDD, TOD and even higher order dispersion terms. When the tube was in air, the evaluation of the fringes provided the base dispersion values. Starting evacuating the tube, the spectrally resolved fringes were captured again at certain pressures. The dispersion falls exponentially with an exponent of 0.00149 mbar^-1 but still exhibits a surprisingly high value of 4.7 fs²/m at more than four order of magnitude lower level than the standard pressure. The air conditioning system maintained constant the room temperature at 23 °C and the relative humidity at 60 %.