Heavy water vapor (D<sub>2</sub>O gas) which owns special structure properties, can generate terahertz (THz) radiation by
optically pumped 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-CO<sub>2</sub> laser 9R
(22) output line (λ=9.26 μm) acted as pumping source, D<sub>2</sub>O gas molecules were operating medium, the expressions of
pumping absorption coefficient <i>G</i><i><sub>p</sub></i> and THz signal gain coefficient <i>G</i><i><sub>s</sub></i> were deduced , It was shown that the gain of THz signal was related with the energy-level parameters of operating molecules and some operating parameters of the THz laser cavity, mainly including gas pressure, temperature etc.; By means of iteration method, the output power density of
THz pulse signal was calculated numerically as its initial power density was known; Changing the parameter of gas pressure and keeping others steady, the relationship curve between the output power intensity (Is) of Tera-Hz pulse laser and the operating D<sub>2</sub>O gas pressure (P) was obtained. The curve showed that the power intensity (Is) increased with gas pressure (P) in a certain range, but decreased when the pressure (P) exceeded some value because of the bottleneck effect, and there was an optimal gas pressure for the highest output power. We used a grating tuned TEA-CO<sub>2</sub> laser as pumping power and a sample tube of 97cm length as THz laser operating cavity to experiment. The results of theoretical calculation and experiment matched with each other.