Both the UCSB FIR-FEL and MM-FEL exhibit strong, stable, sideband lasing. This phenomenon occurs when gain is generated by resonant coupling between the FEL's radiation field and the coherent synchrotron motion of electrons trapped within the ponderomotive potential as the FEL reaches saturation.
A high order Fabry-Perot scan separates the sideband and fundamental components. Sideband power appears lower because the pulse is
integrated in this measurement and the fundamental has higher spectral density (narrower linewidth). The FIR FEL was operating at
140 µm wavelength for these measurements.
A full time resolved spectrum (time progresses left to right) shows buildup and saturation of the fundamental followed by
buildup and saturation of the sideband. A harmonics and intermodulation components are also present .