Step-tapered operation of the FEL: Efficiency enhancement and two-colour operation

We present measurements of the temporal and spectral properties of radiation produced from the step-tapered undulator infrared free-electron lasers (FELs), CLIO in France and FELIX in the Netherlands. Using a two section undulator with independently adjustable deflection parameters, K, the FEL will operate either with an enhanced efficiency and improved spectral properties (with a small positive Delta K step) or simultaneously at two frequencies (for large Delta K). Using a dispersion-free hole output coupler the maximum wavelength difference delta lambda/lambda, has now been extended to more than delta lambda/lambda approximate to 0.6. We also present measurements that show that the FEL FELIX will produce significant power simultaneously at two wavelengths by coherent spontaneous emission when the wavelength is long and the electron bunch is short. The efficiency, spectral and temporal properties have been measured. We show that at the maximum efficiency, Delta K approximate to 0.02, the optical pulses generated are smooth and close to Fourier transform limited. By adjusting Delta K the optical pulse duration can be varied by a factor of 2 or more and sidebands due to synchrotron oscillations can be suppressed.