Sub-nanosecond laser induced air-breakdown with giant-pulse duration tuned Nd:YAG ceramic micro-laser by cavity-length control.

We first demonstrated a continuously and widely giant-pulse duration tunable laser based on a short monolithic Nd:YAG/Cr:YAG ceramic by cavity-length control in 100 Hz operation. The tuning range of pulse duration τ was from 0.5 to 9 ns as keeping peak powers of over 0.5 MW up to 6 MW. The characteristics of the ceramic laser was discussed in detail such as cavity-length dependent beam pattern and divergence, pulse shape, pulse energy due to the transverse and longitudinal modes, and an elliptical polarization status. Laser induced breakdown in laboratory air was investigated as a function of τ in sub-nanosecond region using the developed laser. Air-breakdown threshold intensity I_th was measured using three different focusing conditions. We confirmed that 1) the measured I_th was almost constant at the longer τ than τ_CI named as limit-pulse-duration of cascade ionization (CI), 2) I_th had ~τ^-2 scaling for τ < τ_CI, 3) the increase of I_th is not connected to a specific intensity level, and 4) τ_CI was not constant and depended on focusing conditions. These phenomena were discussed with considering temporal-spatial intensity of laser.