Mode calculations in unstable resonators with flowing saturable gain 2: Fast Fourier transform method

We present further calculations of the three-dimensional mode patterns and power outputs from a high-power gas-dynamic laser, including a nonuniform flowing, saturable gain medium plus index inhomogeneities (shocks) inside the laser. The calculations are carried out using a plane-wave or k-space expansion together with the fast Fourier transform. A new expanding-beam coordinate transform converts all diverging or converging sections of the resonator mode into equivalent collimated beam sections. The resulting FFT propagation code is significantly faster than earlier propagation codes using other eigenmode expansions.