Coherent harmonic production using a two-section undulator FEL

We present measurements of a new method of generating harmonic radiation in a free-electron laser oscillator using a two-section undulator. To produce coherent harmonic radiation the undulator is arranged so that the resonance frequency of the downstream undulator section matches an harmonic of the upstream undulator. Both the fundamental and the harmonic optical fields evolve in the same optical cavity and are coupled out with different extraction efficiency, through a hole in one of the cavity mirrors. We present measurements that show that the optical power at the second and third harmonic can be enhanced by more than an order of magnitude. We examine the dependence of the harmonic power on the optical cavity desynchronism and the cavity roundtrip number to evaluate the influence of the fundamental power on the evolution of the harmonic. We compare our measurements with predictions of a multi-electron numerical model to evaluate the bunching at the harmonic frequency. This fundamental/harmonic mode of operation of the FEL may have useful applications in the production of coherent X-ray and VUV radiation, a spectral range where high reflectivity optical cavity mirrors are difficult or impossible to manufacture.