  Studies of a broad bandwidth, two-colour FEL amplifier using one
monoenergetic electron beam are presented. The two-colour FEL interaction is
achieved using a series of undulator modules alternately tuned to two
well-separated resonant frequencies. Using the broad bandwidth FEL simulation
code Puffin, the electron beam is shown to bunch strongly and simultaneously at
the two resonant frequencies. Electron bunching components are also generated
at the sum and difference of the resonant frequencies.
