| Optically pumped gas terahertz laser is one powerful way to generate high-power and high-energy coherent terahertz radiation. Unfortunately, for gas terahertz laser pumped by TEA CO2laser,some problems such aslow efficiency, large volumeand complex structure are difficult to overcome.High efficient, high-power,high beam quality and compact optically pumped gas THz laser is of special interest in areas such as terahertz imaging, terahertz differential absorption lidar, terahertz heterodyne remote sensing and terahertz nondestructive evaluation. In view of this condition, the technologies to obtain high efficient optically pumped D2O gas385m THz laser have been investigated theoretically and experimentally in this dissertation.Theoretically,based on density matrix rate equation and time evolution equation of laser cavity-field intensity, the laser kinetics model of pulsed optically pumped gas THz laser has been given. Considering the transient coherent effects and multiphoton effects, such as stimulated Raman emission, density matrix rate equation was deduced based on semi-classical density matrix theory. Time evolution equation of laser cavity-field intensity was established by taking into account the gain and optical loss in the THz cavity. Actually, both the pump intensity and THz intensity in THz laser cavity are changed with time and space, and the general solutions to laser kinetics equations are difficult and complex.In order to simplify the laser kinetics model, we measured D2O energy level structure, absorption coefficient and pump pulse shape by using the experimental method, and the measuring results provided reliable basis for the approximate treatment of the laser kinetics model. Considering the optical loss in cavity, a three-energy level geometry was used for385m THz laser theoretical calculation. Take into account the influence of the absorption of pump radiation, the pump intensity was approximated by the equivalent pump intensity in cavity. Pump pulse shape was approximately described by a superposition equation which composed by four gaussian functions.According to the above approximations, we numerically calculated and analyzed the high efficient and compact optically pumped D2O gas THz laser. When the cavity length is140cm, the output characteristics of THz laser such as output power, pulse shape and pulse width were theoretically investigated with the variation of pump energy, pump pulse shape and gas pressure. Then the influence of pump energy, pump detuning, gas pressure and output coupler transmission on the output energy of THz laser have been theoretical researched, respectively. In experiment research, a simple and compact optically pumped D2O gas super radiation THz laser has been designed and some performance optimization experiment has been carried out. A scanning Fabry-Perot(F-P) interferometer which composed by two Ge plates was designed and the super radiation spectrum of D2O gas THz laser was measured. For fundamental mode TEACO2laser as pump source, with the cavity length of120cm, THz pulse energy of1.3mJ and peak power of10kW were achieved at a wavelength of385μm, and the Photon conversion efficiency (PCE) is about29%. When pumped by multi-mode TEACO2laser, THz energy of6.2mJ and peak power of51.7kW, PCE of about36.5%was obtained. Experiment measured pulse shape of residuary pump pulse which contained vibrational relaxation time of the D2O gas molecules, and vibrational relaxation time of about5.5μs Torr~5.7μs Torr has been measured.We demonstrate a highly efficient and compact terahertz cavity oscillator that is based on z-cut crystal quartz used as the dichroic beam splitter, for the first time to the best of our knowledge. With D2O gas as the active medium, pumped with a TEACO2laser, performance optimization experiment has been carried out to obtain high efficient, high beam quality and compact optically pumped THz laser. With the cavity length of120cm, pulse energy of7.4mJ, peak power of82.2kW, and beam quality M2factor of1.77were achieved at a wavelength of385μm. The corresponding PCE is about44%, to our knowledge, which is the highest efficiency ever reported in D2O gas385μm terahertz cavity laser systems. The influences of output coupling ratio and the length of THz cavity on the output THz energy pumped by multimode TEACO2laser have been experimental research, respectively. The maximum385μm terahertz output energy of10.1mJ and peak power of112.2kW were obtained with cavity length of140cm and gas pressure of500Pa.PCE of54.2%was obtained at the maximum output level.In the aspect of THz laser pumped by fundamental mode TEACO2laser, the output characteristics of THz laser(length of140cm) such as output energy, pulse shape and pulse width were experimental investigated with the variation of pump energy, pump pulse shape and gas pressure.The maximum PCE of56.1%at wavelength of385m was obtained with output energy of2.31mJ. The corresponding numerical simulation results of the optically pumped D2O gas THz laser are in good agreement with experimental measurements, which can demonstrate that the THz laser kinetics model established by us is correct. |
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