Optical diagnostics for a ring resonator free-electron laser

Abstract

The optical cavity of the Boeing free-electron laser (FEL) was reconfigured as a semiconfocal ring resonator with two glancing incidence hyperboloid-paraboloid telescopes. The challenge for this experiment was the complexity of the ring resonator compared to the simplicity of a concentric cavity. The ring resonator's nonspherical mirror surfaces, its multiple elements, and the size of the components contributed to the problems of keeping the optical mode of the resonator matched to the electron beam in the wiggler. Several new optical diagnostics were developed to determine when the optical mode in the FEL was spatially and temporally matched to the electron beam through the wiggler. These included measurements of the focus position and Rayleigh range of the ring resonator optics to determine the spatial match of the optical mode through the wiggler, and a measurement of the position of the optical axis for multiple passes around the ring resonatorto determine the stability of the resonator alignment. This paper also describes the optical measurements that were necessary to achieve reliable lasing. The techniques for measuring ring resonator Rayleigh range and focus position, multiple pass alignment, cavity length, optical energy per micropulse, peak power, optical extraction, small signal gain, ringdown loss, lasing wavelength, electron bunch pulse width, and energy slew are discussed.