Characterization of a High-Gain Harmonic-Generation Free-Electron Laser at Saturation

We report on an experimental investigation characterizing the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO₂ laser at a wavelength of 10.6 μm was used to generate amplified FEL output at 5.3 μm. Measurement of the frequency spectrum, pulse duration, and correlation length of the 5.3 μm output verified that the light is longitudinally coherent. Investigation of the electron energy distribution and output harmonic energies provides evidence for saturated HGHG FEL operation.