Competition and coupling between forward and backward waves in optically-pumped linear-cavity fir lasers

We study theoretically the gain for a linear-cavity (Fabry-Perot) FIR laser pumped by a travelling-wave infrared beam. Arbitrary field intensities and pressure and Doppler broadenings are considered. The simultaneous presence inside the laser cavity of the forward and backward generated waves produces strong competition and coupling effects between these waves which are interpreted in terms of Lamb-dip and multiphoton processes. These effects determine the main differences in the amplifying behaviour between the bi-directional (linear) and uni-directional (ring) laser configurations.