Two-colour operation of a Free-Electron Laser and applications in the mid-infraredR. Prazeres, F. Glotin, C. Insa, D. A. Jaroszynski and J. M. OrtegaEur. Phys. J. D, 3 1 (1998) 87-93DOI: https://doi.org/10.1007/s100530050151