Two-colour operation of a Free-Electron Laser and applications in the mid-infrared R. Prazeres, F. Glotin, C. Insa, D. A. Jaroszynski and J. M. Ortega Eur. Phys. J. D, 3 1 (1998) 87-93 Published online: 15 July 1998 DOI: 10.1007/s100530050151