Flow Laser Diagnostic Techniques

Abstract

The great diversity of thermo-and gasdynamic states and substantial departures from thermodynamic equilibrium typical of molecular gas flow lasers specify the methods of the optical diagnostics of such systems. The most important characteristic of the inverse gas laser medium is the unsaturated gain coefficient, a, of unit length of the active medium, which is connected with the integral gain coefficient of the entire length of the active medium, k = P_out/P_in, by the relation k = = exp[(α−δ)L_o] where δ is the coefficient of distributed losses (scattering) and L_o is the active medium length. Except for the so-called method of calibrated losses, the gain coefficient is usually measured by passing through the active medium a monochromatic radiation with the same quantum transitions as those in the induced radiation of the medium. The remainder parameters of the diagnostic laser medium and of that tested (i.e. density, temperature, composition) may in this case differ substantially. The analysis of the conditions typical of gasdynamic and electric-discharge CO₂ lasers shows that the effective widths of the tested and diagnostic line contours may differ several-fold. This leads to a partial overlapping of lines and, consequently, to certain errors in the measurements of gain. Also, an important characteristic of the inverse molecular laser medium is the temperature of a particular vibrational state of the molecules. That the vibrational and rotational temperatures could be determined, the distributions of the gain (absorption) coefficient over a wide range of separate IR-spectrum lines are measured. The paper presents an analysis of some reasons for the errors occurring in determination of the gain coefficient for molecular gas laser media and describes the methods allowing simultaneous measurements of the vibrational and rotational temperatures in a CO₂ laser medium by means of a tuned diagnostic laser with high time resolutions.