Gain Measurements from Start-Up and Spectrum of a Raman Free-Electron-Laser Oscillator

Results from a Raman free-electron-laser oscillator producing \AA{}2 MW at 2.5-mm wavelength ($\frac{\ensuremath{\Delta}\ensuremath{\lambda}}{\ensuremath{\lambda}}\ensuremath{\simeq}1%$) are reported. A novel method of extracting regenerative-gain and power-coupling-loss measurements from signal delay times is described and the experimental analysis compared with 3D theory. The first observation of long-wavelength sideband power in the Raman regime is reported at high undulator strength. Temporal behavior of the output after saturation is reached is discussed.