Study On Pulsed Operation Of An Optically Pumped CH₃OH Molecular THz Laser

THz laser is a new source of radiation with many unique advantages, employing a new interdisciplinary technology which has great significance to national security and economic development. In this paper, an all-metal Terahertz laser is designed with simple structure and small volume,which can be applied to high-speed THz imaging, THz radar, medical diagnostics, etc.This paper analyzes the molecular dynamic processes of the optically pumped CH₃OH with the rate equation of the typical lasers. First of all, a model of three-energy-level system is established based on the analysis of the energy levels of CH₃OH. Secondly, the rate equation of the three energy levels of CH₃OH is also established according to the changes in population of photons on the upper and lower levels in the process of THz generating. Thirdly, the effective molecules in the laser process and average lifetime of the photons in laser resonant cavity are then calculated. In the end, through the simulative solution to the rate equation, the curves of the number of ground level particles and of the number of laser particles of the upper and lower energy level, together with the pulse wave of THz output, are obtained and analyzed.This self-developed all-metal Terahertz laser system consists of two main parts: the pump system and the THz laser generator. The pump source is grating tuning all-metal RF waveguide CO2 laser. THz laser generator is also designed to be all-metal, composed of five parts: the resonant cavity, the laser channel, the gas control system, the windows and the shell. The laser cavity adopts the traditional flat - concave resonator while the pump laser input and THz laser output adopt small aperture coupling. Owing to the all-metal design, it removes the water-cooling system, so that it achieves the advantages of simple structure and small volume.The waveforms of THz output are measured in the experiments. The relationship curve for the gas pressure of the resonant cavity's working gas CH₃OH and the intensity of THz output is obtained and compared. The waveforms of the THz output when the pump repetition rate ranges from 100Hz to 1000Hz are also obtained and compared. Due to the limit of the waveband of the detector, the highest repetitive frequency of the THz output observed is 1000Hz. In the end, transmittances of germanium film and silicon film are tested with the THz laser designed.