Two-photon transitions in optically pumped submillimeter lasers

The emission frequency of an amplified spontaneous emission (ASE), optically pumped submillimeter (SMM) laser has been studied as a function of laser gas pressure for several different gases and SMM laser lines. For off resonant pumping, Fabry-Perot interfetometer scans indicate a shift in emission frequency between high- and low-pressure laser operation. In high-pressure operation, emission occurs via a Raman type, two-photon process with some additional frequency offset due to the ac Stark effect. The Raman process is favored by its higher quantum efficiency over a two-step process involving single-photon transitions at the pump and SMM wavelengths. In low-pressure operation, the SMM laser emission is found to be delayed in time with respect to the pump pulse. The Raman process thus cannot occur and the two-step process is favored. This results in a shift in the SMM laser emission frequency from the Raman line to the line center frequency of the SMM laser transition. The Raman emission gain bandwidth appears to be broadened by the CO2pump laser bandwidth.