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.

pulse with sideband lasing
Radiation pulse from the FIR-FEL during sideband lasing. The fundamental starts up and saturates at ~2400 W. Gain is then present for the sideband which then follows a similar startup and saturation at ~5000w. Finally another sideband component adds another KW to the pulse.

fabry-perot scan of lasing 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.

time resolved spectrum 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 .