In this paper, we report, for the first time, a specially designed optical gating system of spatially angular multiplexing for ultra-short pulsed digital holography, which can be employed to record ultra-fast processes of the order of femto-second. The sampling resolution of the dynamic process ranges from 50 fs to 10 ps. By adjusting the time delays and the angles included between the sub-pulses, the system sampling interval ranging from 67 fs to 170 ps is obtained.
The laser induced ionization of ambient air is studied experimentally with laser pulses whose durations range from 50 fs up to 10 ps at 800 nm. It is found that the minimum pulse energy for detectable air ionization follows the scaling law of ε<i><sub>th</sub></i> varies direct as <i>t<sub>p</sub><sup>x</sup></i>, with 0.23 < x < 0.5, and x tends to rise for longer pulses within the range of 50 fs - 500 fs. For laser pulses from 0.7 ps to 10 ps, however, x is approximately equal to 0.8. The dependence of the critical intensity for air ionization on the beam spot size is also examined with a variety of focused laser beam spot sizes in the experiments.