Studies On The Kinetics Of Optically Pumped Metastable Argon Laser

Optically pumped metastable rare gas laser（OPRGL）has been an important concern by many research institutes worldwide for its potential to become a high energy laser with high beam quality.Experimental studies of OPRGL are currently in the low-pump-power and low-volume stage.The high level up to now is the diode pumped metastable argon（Ar^*）laser using parallel plates high repetition rate pulsed DC discharge.Its development is limited by the difficulty of producing a large gain region of high Ar^*number densities,and that the laser kinetics have not yet been understood accurately.In this dissertation,temporally and spatially resolved models were developed for the kinetic analysis of OPRGL,a series of experiments were conducted to test the reliability of the theory and accuracy of the models,and estimations were made for high power operations.Studies include the discharge process,laser process（optical pumping and laser radiation）and their interactions,effects of dual-pump schemes,gain characteristics of the laser system,and operation of MW-level output power.For Ar^*OPRGL using high repetition rate pulsed DC discharge between parallel plates,time-resolved spatially-uniform/zero-dimensional model was developed.Processes of electrons collisions were described by reduced electric field/.Time-dependent simultaneous computations of the discharge processes and the lasing processes in the positive column were realized.For calculations of the lasing processes,‘longitudinally double-pass averaged’method was applied to the longitudinal distributions of the pump intensity and the oscillating laser intensity.Rate coefficients and transmission coefficients of electron collisional reactions were calculated through BOLSIG+solver（a free computer program）based on time-dependent reduced electric field/.The model contained 194reactions,and can describe time-dependent evolutions of particle densities of 17 species,the voltage between electrodes,current,electron energy,output laser power and efficiencies.The results of the model were in great agreement with the experimental results.Periodic mean value error is about 5%compared to the reported experimental result of 4.1 W output laser.In addition,a low-power-pump closed-chamber experiment was carried out,and the experimental result of‘double peaks’was analyzed through the model.Applicabilities of the‘longitudinally double-pass averaged’method and the‘local field approximation’in BOLSIG+solver were confirmed through building a longitudinally resolved model and?zero-dimensional model（?:mean electron energy）respectively.In the?zero-dimensional model,electron-collision coefficients were calculated through BOLSIG+solver based on the time-dependent?.Through the?zero-dimensional model,time-dependent evolutions and their mechanisms of the number densities of populations at1s5,1s4,2p10,2p9 and 2p8 levels were analyzed.Descriptions of the time-dependent characteristics and the interrelations between discharge and lasing processes reflected the advantages of the models.E/N zero-dimensional model was used to predict the laser performance when dual-pump schemes were applied to break through the bottleneck caused by the population accumulation at 1s4 level.The 1s4→2p10 scheme was generalized to 1s4→2p8 and1s4→2p7 secondary pump lines.Results of the model demonstrated that,through these three schemes,the output power could be enhanced respectively by 3.1,4.1 and 4.1 times the single-pump scheme with a 10%（secondary pump power/main pump power）secondary pump at ambient temperature.In addition,experiments with continuous main pump and pulsed secondary pump were conducted,which verified the schemes,and showed consistent with the simulation results.The 1s4→2p7 method is expected to be more conveniently conducted since its line is closer to the main pump line compared to the other two.A one-dimensional model with temporal and spatial（in the discharge direction）resolutions was developed based on computational fluid dynamics.Analysis on the evolution behaviors and schemes in the positive column and the cathode fall of the plasma were specially focused on.Results of an ICCD recording experiment of the discharge glow demonstrated that the model could accurately present the dominance of‘secondary emission’in producing electrons.Through the results of the one-dimensional model,the rationality of the assumptions of cathode fall in the zero-dimensional models was discussed.A series of parameters were analyzed with temporal and spatial resolutions,including:the voltage between electrodes,reduced electric field,electron temperature,electron number density,and number densities of Ar（1s5）,et al.At last,optical pumping was added.The output laser power was a little lower than the power calculated through the zero-dimensional model,mainly because the cathode fall of one-dimensional model was a temporal result,calculated based on discharge conditions,rather than an assumed constant.The gain characteristics were analyzed and the power scaling was made for transversely pumped slab OPRGL.The relations between discharge deposition energy density and Ar（1s5）number density,absorbed pump power per unit volume,small-signal gain coefficient,saturation intensity,output laser power per unit volume and energy conversion efficiencies were analyzed for systems of various discharge repetitions,pulse durations and electrode areas.The effects of gas temperature,gas pressure and composition were also studied.At gas temperature 500 K and pressure 1 atm,for a system that can produce a density of metastable particles at the level of 1.0×10¹³ cm^-3,the discharge volume for producing mega Watt output laser power using the single-pump scheme is about 2.4 L.