Review of methods of calculating the field in optical cavities and choice of calculation method for free-flow chemical lasers

Abstract

Solution of the general problem of producing high-efficiency laser systems calls for research into the choice of the optimal cavity design, aimed also at studying the influence of the spatial inhomogeneity of the active medium on the character of the laser emission [9-18]. Understandably, in the general case it is more effective to carry out the investigation by numerical means. This reduces in essence to the self-consistent problem of the mutual influence of the field in the cavity and the physicochemical processes that form the active medium. In the case of a cw chemical HF laser, the pertinent calculations are made complicated by the multilevel character of the excitation of the working molecules, and also of the substantial role of the effects of mixing of a large number of chemically active jets. Undoubtedly the most general and rigorous mathematical model for the calculation of the magnetic field in a cavity should be taken to be the system of Maxwell equations and, for an active medium, the Navier-Stokes system of equations with allowance for physicochemical kinetics. This method, however, is too complicated and laborious in practice. We must therefore choose simplified approaches by starting from the specific features of the problem. The scientific literature has described by now a wide spectrum of computation methods that have become classical and are contained in textbooks and monographs [i, 2, 19-27]. The choice of the method must therefore be justified on the basis of the aims pursued and the problems to be solved. An advantage of analytic methods [i, 2, 28-43] is that they make it possible to obtain the solution in the most general form and to understand its physical meaning. Final analytic solutions, however, are frequently attainable only in some particular or limiting cases. Therefore, if specific numerical results are needed, computer methods turn out to be more useful. The known published numerous geometric-optics [i, 2, 20, 25, 44-56], ray-matrix [i, 23, 57-64], opticogeometric [65-70], etc., methods, including the eikonal method [21, 24, 71-72], while relatively simple and sufficiently effective for the solution of a definite group of problems, can nevertheless not represent in the general case completely the phase structure of the radiation field in a cavity with a free-flowing optically inhomogeneous medium. Principal attention is therefore paid in this review to the description and mutual comparison of various methods of wave optics [73-163] and also to methods of calculating active-medium characteristics [164-198], aimed at choosing a working method of solving complex comprehensive problems of the ratio of the parameters of a cw HF chemical laser with an unstable telescopic cavity, so as to improve the directivity pattern of its radiation.