Propagation Effects

Abstract

The laser theory discussed in Chaps. 7 through 9 is based on the uniform amplitude approximation, which assumes that the laser field can be written as a superposition of the passive resonator modes. In general this approximation is valid only for mirror reflectivities close to unity, which are typically not used in most commercial edge-emitting lasers or in laser amplifiers, where the facets are likely to be uncoated or antireflection coated. For these devices, propagation effects may be important, which necessitates a different method of analysis. A conceptually straightforward approach, that was originally developed by W. W. Rigrod (1963), involves writing the laser field as the sum of forward and backward travelling waves, whose amplitudes may vary considerably along the laser axis. This approach is widely used in gas and solid state lasers and has been proven to be accurate in many cases. When the homogeneously broadened gain formula g ₀ /(1 + I/I _s ) applies, analytical solutions may exist. For the more complicated semiconductor gain, the Rigrod equations have to be solved numerically.