Efficiency of resonance laser-collisional conversion of hydrogen fluoride laser radiation

Abstract

High-power hydrogen fluoride lasers with good energy parameters are now available.1 However, there are several factors why the radiation emitted from these lasers is not of satisfactory quality. In the case of cw hydrogen fluoride la- sers one of the main problems is the loss of optical homo- geneity by the active media as a result of mixing of the rea- gents. In the case of pulsed hydrogen fluoride lasers the optical homogeneity is disturbed by inhomogeneous initia- tion of the reaction. Suppression of these inhomogeneities is sometimes a very difficult task. Successful solution of many scientific and applied problems by hydrogen fluoride lasers requires that the divergence of the radiation from these la- sers should be close to the diffraction limit. A method for reducing the divergence of hydrogen flu- oride laser radiation based on resonance laser-collisional conversion (see Ref. 2-5 and the bibliography cited there) is currently being discussed. In this method the molecules of HF in an optically homogeneous gas mixture are pumped by HF laser radiation. This results in stimulated emission from HF molecules involving high rotational sublevels when the populations are inverted for transitions in the P branch. Wang et al? realized such resonance laser-collisional conversion emitted by a cw hydrogen fluoride laser. The maximum efficiency of the conversion process was 5.3% rel- ative to the pump power. In predicting the capabilities of the method it is important to know also the efficiency of conver- sion relative to the absorbed power. This efficiency was not determined in Ref. 3. The small-signal gain of a converter of radiation from a cw hydrogen fluoride laser was calculated in Ref. 4. The results obtained indicated that, in principle, the conversion process could have a high efficiency. We shall report the results of calculations of the effi- ciency of resonance laser-collisional conversion of radiation from a cw hydrogen fluoride laser. We shall assume that the laser radiation is absorbed and reemitted by a homogeneous gas stream of constant density and velocity. This allows us to ignore the equations of gasdynamics and to assume that χ = ut, where χ is the downstream coordinate, u is the stream velocity, and t is the time. Our calculations allowed for laser pumping, for VT, VV, and/?7relaxation processes, and for stimulated emission. The absorption cross sections were calculated using formulas from Ref. 6 on the assump- tion of an exact resonance. The FT relaxation rate of the first vibrational level of the HF molecule was taken from Ref. 7. It was assumed that the dependence of the FT relaxation rate on the serial number of the vibrational level was cubic: K υ. υ-ι — y3AT1>0.Theendothermal KFrelaxation rates were taken from Ref. 8 and their temperature dependences from Ref. 7. The R T relaxation processes were described by the model of strong collisions.910 The rotational relaxation times were taken from Ref. 11. The power of stimulated radi- ation was calculated in the constant-gain approximation. The following assumptions were made about the spectral composition of the pump radiation: 1) 45% of the total pow- er corresponded to the 0-1 band and 55% to the 1-2 band (such a distribution is typical of cw hydrogen lasers—see Ref. 12); 2) in each vibrational band the pump laser emits as a result of one transition in the P branch with / = 5 (this re- striction is unimportant for the model in question, but it makes it possible to reduce greatly the calculation time). We also assumed that reemission in the converter occurs due to one transition in the P branch for each vibrational band of the HF molecule. The rotational quantum number/, of this transition was treated as a parameter. Calculations showed that Jx = 9 was optimal from the point of view of the conver- sion efficiency in the case of long durations of interaction between the pump radiation and the gas stream.