Temporal structure of the CLIO laser micropulses in the two-color regime

By using sum-frequency generation in a Michelson arrangement, we have directly measured the temporal overlap between single-frequency micropulses in a two-color free-electron laser. As expected, the average separation appears to be related to the slippage length between the electrons and light when passing through the undulator. The two colors are then partly simultaneous, although there is not necessarily two-frequency bunching in the electron beam.