The optically pumped far-infrared laser: Rate equations and diagnostic experiments

A four-level rate-equation model for an optically pumped far-infrared (FIR) laser is presented, and an expression is derived for the output power as a function of pump power and gas pressure. The theory is compared with measurements on the 570.5-μm line of CH
3
OH, pumped by a CO
2
laser. The width of the gain curve is measured and is shown to go below the Doppler width of the laser transition at low pressure. This indicates that the use of a monochromatic pump source leads to a velocity selective pumping process.