Bi-directional pumping of far-infrared lasers: Theoretical gain spectrum and transferred Lamb-dip line shape

We study the gain spectrum of bi-directionally pumped far-infrared lasers operating either on a pressure broadened far-infrared transition or on a Doppler broadened one, using a three-level density matrix theory. We show that in both cases resonant standing-wave pumping leads to gains larger than those corresponding to most commonly used travelling-wave pumping. This is due to the exact compensation of pump induced light shifts of the upper level, shared by pump and lasing transitions. We compute transferred Lamb-dip (TLD) line shapes in different conditions of laser gain saturation and discuss conditions for the appearance of the TLD. © 1995.