Time-resolved study of sideband generation and transition to chaos on an infrared FEL

We report observations on sideband generation and transition to chaos in an infrared Free Electron Laser operating at a wavelength {lambda}{sub r} of 4 {mu}m with a low loss optical cavity. The temporal evolution of the radiation spectrum through the complete macropulse has been measured on a micropulse to micropulse basis. Stable sidebands are observed for small cavity detuning with frequency offsets in good agreement with calculations. For even smaller cavity detuning, self-oscillations occur with the main laser wavelength varying by up to 2%. Using a wavelength stabilization system based on RF feedback, we have suppressed these wavelength fluctuations without loss of peak power.