OPTIMIZATION OF A COHERENT TE-CO2-LASER WITH VARIABLE REFLECTIVITY OUTCOUPLERS

To optimize the parameters (energy, pulse duration, beam quality, mode discrimination, frequency chirping) of a TE-CO2-laser for coherent LIDAR applications, we investigated the influence of different discharge conditions and resonator configurations. Space- and time-resolved measurements of the small-signal gain coefficient have been used to study the evolution of plasma inhomogeneities. Maximum energy extraction (up to 4 J from an active volume of 2 * 2 * 70 cm3) has been found for a mixture of CO2:N2:He = 1:3:2 at 4 * 10(4) Pa and 24 kV (foot-to-foot pulse duration 5-mu-s). Hybrid and injection regimes have been tested to generate single axial and transversal modes. An optimum filling of the medium by the laser field has been reached by using unstable resonators with variable reflectivity mirrors and gratings. Homogeneous, nearly diffraction-limited far-fields have been obtained. In the case of apodizing dielectric coatings, focusing properties have been found which are governing the radiation fields. The orientation of the coated mirror face (inside/outside the cavity) is strongly determining the field distribution. The frequency chirping has been detected for different working conditions by a heterodyne system (100-600 kHz/mu-s2).