Two-colour operation of the free-electron laser using a step-tapered undulator

Two-colour operation of the CLIO free electron laser (FEL) has been demonstrated experimentally. Using a step-tapered undulator which produces two independently tunable resonances we have obtained two-colour FEL operation with a frequency difference over a range of ±20%. We show that the range is currently limited by the influence of optical dispersion in the dielectric output coupler. We present first order autocorrelation measurements that show 100% modulation due to beating of the two colours over the range of the spectrum. We present power measurements that show that “optical klystron” type of interference effects influence the small signal gain.