Abstract

With respect to the propagation of a laser beam a slab-shaped laser medium may be considered as a waveguide in the transverse direction (narrow spacing) and as free space in the lateral direction (broad area). Using a common procedure of separation of variables, the mode formation in the transverse and lateral direction may be considered independently. The excess energy from the excited region should be removed by heat conduction to the walls resulting in the formation of refraction index gradients. The index gradients may strongly influence the waveguide mode shape and loss. On the other hand, the minors of the resonator are placed at some distance from the waveguide ends. Therefore, a cavity mode generally is formed as a result of the combination of free-space diffraction (FSD), the reflection from the resonator mirrors and waveguide propagation (WP). Losses are controlled by absorption in the waveguide walls, coupling of radiation reflected from the mirror to the waveguide and output coupling. In this paper numerical simulations of a cavity consisting of an index-graded waveguide and two plane mirrors were performed. Calculations were made for typical experimental conditions: a RF discharge pumped laser on atomic Xe lines (2.03, 2.65 and 3.51 µm); gas mixture He-Ar-Xe at pressure 0.15 bar, waveguide transverse width 2 mm, active medium length 37 cm. Beam profiles were measured in the far and near field zones with a one-dimensional photodetector array (128 elements 200 pm each)¹. Typical experimental results for the 3.51 µm line for two array positions are presented in Fig. 1, along with the results of numeric calculations. In the computations the index profile, gas composition and distances between waveguide and mirrors were varied. The effect of a possible non-uniform distribution of the gain will be discussed.

© 1998 IEEE