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 aP₂(3)vibrational-rotational transition and 7 percent/cm on aP₁(6)transition for flow rates corresponding to 1.8 kW of closed-cavity power. The variation of zero-power gain with rotational quantum numberJfits a simple rotational equilibrium model indicating increasing rotational temperatures with increasing distancexfrom the nozzle exit plane. The model also indicates a total inversion forx ≤ 0.2in for theP₂(J)transitions and a partial inversion further downstream.