We have measured the reduction in optical-mode size produced by optical guiding in a free-electron laser. The intracavity transverse mode is smaller than the diffraction-limited mode in the small-signal as a result of strong guiding regime, but it grows larger in saturation as a result of reduced gain. Comparison with theory shows agreement except for a discrepancy in the shape of the mode as saturation begins.

• Received 27 July 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.2047