Multimode coupling and nonlinear-gain effects in distributed and helical feedback gas lasers

We present a detailed experimental and theoretical study on the waveguide modes of distributed feedback (DFB) and helical feedback (HFB) gas lasers including for the first time the experimental verification of multimode-coupling and nonlinear-gain phenomena. For this purpose we used oversized hollow metal waveguides with periodic or helical corrugations. The latter exhibit the symmetry of either the single helix or the double helix. For the interpretation of our observations we developed a coupled-wave theory extended to multi-mode coupling and adapted the nonlinear-gain approach for strong coupling by Haus. The experiments with DFB and HFB gas lasers give new relevant information on these phenomena.On leave from Academia Sinica, Beijing, People's Republic of China