BIDIRECTIONAL 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 las ers operating either on a pressure broadened far-infrared transition o r on a Doppler broadened one, using a three-level density matrix theor y. 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 l ight shifts of the upper level, shared by pump and lasing transitions. We compute transferred Lamb-dip (TLD) line shapes in different condit ions of laser gain saturation and discuss conditions for the appearanc e of the TLD.