Observation of the diffraction induced violation of the Madey theorem

We have simultaneously measured [1] the spectra of the gain and the forward spontaneous radiation in the Orsay free electron laser so that we can extract the relative shift of these two curves. Our results are in agreement with the Madey theorem in the low divergence limit, and demonstrate the violation of the theorem produced as the Rayleigh range of the wave becomes comparable to or smaller than the length of the interaction region.