Two-color operation in high-gain free-electron lasers

Two-color operation in free-electron laser (FEL) amplifiers is studied using a 3D polychromatic simulation. We study the growth of two seed signals at closely spaced wavelengths within the gain band as well as a discrete spectrum of sidebands that are outside the gain band. The sidebands grow parasitically with growth rates higher than the seeded waves due to bunching in the seeded waves. An example of an X-ray FEL is discussed; however, the physics is applicable to all spectral ranges.