Research On Mechanism Of Non-chain Pulsed Electric-discharge DF Laser

Deuterium fluoride (DF) laser，based on chemical reactions, radiates in thespectral range3.5~4.2μm, which is the infrared atmospheric transparency windowcovering the absorption peaks of many atoms and molecules, so this laser hassignificant applications in many fields such as spectroscopy, laser radar transmitters,laser ranging, atmospheric monitoring, military, etc. DF laser can be divided into twotypes, one is the traditional chain reaction and the other is non-chain reaction, theformer’s output laser energy is not directly limited by the input energy, which meansit can reach high energy, high efficiency laser output. However due to the branchedchain reaction, chain DF laser has the risk of premature ignition and explosion. Onthe contrary non-chain pulsed DF laser has no risk of corrosion and explosion.Besides, it has other advantages, such as compact structure, easy operation, goodbeam quality, high peak power, high energy, etc. Non-chain DF laser becomes moreattractive due to all these advantages recently. The mechanism of non-chain pulsedelectric-discharge DF laser is studied theoretically and experimentally in this paper.Non-chain pulsed electric-discharge DF laser usually uses SF6gas as the donorof F atoms, so the analysis of the production process of F atoms is the premise of thebuilding of dynamical model. According to the impact cross sections, thresholdelectron energy and rate coefficients of different impact interactions between SF6 and electrons, the important processes which affect the generation of F atoms and thebreakdown of SF6are found, and the breakdown mechanism of pure SF6and SF6-D2mixture are obtained.With SF6-D2as gas medium, according to the reaction mechanism in non-chainpulsed DF laser, a dynamical model for this laser is presented by using rate equationstheory. By solving this model with Runge-Kutta method, the dependence of speciesconcentration in laser cavity on time and the photons radiation by differentvibrational level of excited state DF molecules are obtained. Then the effects of theworking gas ratio and the reflectance of output mirror on photon number density,single pulse energy, pulse width, and output power are studied with this model. Theoptimum parameters of the working gas ratio and the reflectance of output mirror areattained based on the theory calculation.The experiment platform of non-chain pulsed DF laser is built, on which theinfluences of the ratio and total pressure of working gas, the charging voltage and thereflectance of output mirror on the output energy of non-chain pulsed DF laser areinvestigated. The experimental research is based on UV-preionized transversedischarge method with a stable optical resonator, and the working mixtures used arenon-toxic and non-corrosive SF6and D2. The results show that the optimum ratio ofSF6-D2and the reflectance of output mirror, respectively, are10:1and30%, whichagree well with the theoretical calculation. The laser energy increases linearly withcharging voltage, but there will occur the phenomenon of arc discharge when thecharging voltage is too high, which will decrease the laser energy. There exists anoptimum gas pressure for each certain charging voltage, that is working gas has theoptimum E/P value. The maximum single pulse energy of4.95J, pulse duration of148.8ns, and peak power of33.27MW are achieved under the best workingconditions (SF6:D2=10:1, R=30%) when the charging voltage is39kV. Theinfluences of the mixture ratio, total pressure, charging voltage and the reflectance ofoutput mirror on impulse characteristics of non-chain DF laser are researched. Byusing a DF laser spectrum analyzer,20P-branch transition lines are recorded for the output spectrum of DF laser, which the laser energy concentrates on v=2vibrationlevel. Besides, by measuring laser spot with the laser ablation method, the laserbeam divergence angles of1mrad and6mrad in near field and far field respectivelyare attained. By replacing the resonator with a non-stable optical resonator, the farfield laser beam divergence angle can be compressed to1.2mrad in the sameexperimental conditions.