Second harmonic FEL oscillation - art. no. 084801

We have produced and measured for the first time second harmonic oscillatio n in the infrared region by a free electron laser. Although such lasing is ideally forbidden, since the gain of a plane wave is zero on axis for an el ectron beam perfectly aligned with a wiggler, a transverse mode antisymmetr y allows sufficient gain in this experiment for lasing to occur. We lased a t pulse rates up to 74.85 MHz and could produce over 4.5 W average and 40 k W peak of IR power in a 40 mn FWHM bandwidth at 2925 nm. In agreement with predictions, the source preferentially lased in a TEM01 mode.