Demonstration of harmonic lasing in a free-electron laser

It has long been known that gain exists at the odd harmonics of a free-electron laser. Using a dispersive intracavity element to alter the cavity lengths for the fundamental and harmonic lasing, we have demonstrated harmonic lasing between 1.4 and 1.8 {μ}m using the Stanford Mark III infrared free-electron laser. Gain at the third harmonic was seen to agree with theory to within experimental limits. The wavelength of the third harmonic was not quite equal to one third of the fundamental due to saturation and bandwidth effects. Power output was somewhat smaller than expected. This may have been due to losses in the optical transport system.