SMALL-SIGNAL ANALYSIS OF COHERENT MULTIMODE COUPLING AND OPTICAL GUIDING IN A RAMAN FREE-ELECTRON LASER

A coherent multimode-coupling theory is developed to investigate optic al guiding in a Raman free-electron laser (FEL) amplifier. On the assu mption of using an electron beam with such a small radius that the tra nsverse gradient effect of the radiation field may be ignored, the mul timode dispersion relation of coherent coupling among TE(n0) modes ina rectangular waveguide is derived. Theoretical analysis and numerical results show, under proper conditions, that optical guiding in FEL's c an be realized by the coherent coupling of the TE(10) and TE(30) modes . However, the dependence of the field coupling coefficient on radiati on frequency causes a strong frequency-dispersion effect on the transv erse distribution of the power density of the radiation field. The ran ge of operating frequency is limited.