Zero-power gain measurements in a CW HF laser using a pulsed-probe laser

Zero-power gain measurements in a supersonic diffusion-type CW HF laser have been made using a pulsed HF probe laser. A peak zero-power gain of 10 percent/cm was measured on a
P_{2}(3)
vibrational-rotational transition and 7 percent/cm on a
P_{1}(6)
transition for flow rates corresponding to 1.8 kW of closed-cavity power. The variation of zero-power gain with rotational quantum number
J
fits a simple rotational equilibrium model indicating increasing rotational temperatures with increasing distance
x
from the nozzle exit plane. The model also indicates a total inversion for
x \leq 0.2
in for the
P_{2}(J)
transitions and a partial inversion further downstream.