Study of Optimal Cavity Parameter in Optically Pumped D2O Gas Terahertz Laser

Abstract

Heavy water gas (D2O gas) which owns special structure property, can generate terahertz radiation by optically pumping technology, and its 385 μm wavelength radiation can be widely used. In this research, on the base of semi-classical density matrix theory, we set up a three-level energy system as its theoretical model, a TEA-CO2 laser 9R (22) output line (λ = 9.26 μm) acted as pumping source, D2O gas molecules were operating medium, the expressions of pumping absorption coefficient G p and Terahertz signal gain coefficient G s were deduced. It was shown that the gain of Terahertz signal was related with the energy-level parameters of operating molecules and some operating parameters of the Terahertz laser cavity, mainly including cavity length. By means of iteration method, the output power density of Terahertz pulse signal was calculated numerically. Changing the parameter of cavity length and keeping others steady, the relationship curve between the output power intensity (Is) of Terahertz pulse laser and the operating cavity length (L) was obtained. The curve showed that the power intensity (Is) increased with cavity length (L) in a certain range, but decreased when the length (L) exceeded some value because of the absorption effect, and there was an optimal cavity length for the highest output power. We used a grating tuned TEA-CO2 laser as pumping power and a sample tube of variable length in 70–160 cm as terahertz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other, and it is helpful for miniaturizing terahertz laser volume to make it practical.