Study of optimal gas pressure in optically pumped D 2 O gas terahertz laser

Abstract

Heavy water vapor (D₂O gas) which owns special structure properties, can generate terahertz (THz) radiation by optically pumped technology, and its 385 &mu;m 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 (&lambda;=9.26 &mu;m) acted as pumping source, D₂O gas molecules were operating medium, the expressions of pumping absorption coefficient <i>G</i><i>_p</i> and THz signal gain coefficient <i>G</i><i>_s</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 D₂O 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-CO₂ 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.