Abstract
We analyze an optical phenomenon taking place in waveguide free-electron lasers, which disturbs, or forbids, operation in far infrared range. Waveguides in the optical cavity are used in far-infrared and THz ranges in order to avoid diffraction optical losses, and a hole coupling on output mirror is used for laser extraction. We show that, when the length of the waveguide exceeds a given limit, a phenomenon of ``mode disorder'' appears in the cavity, which makes the laser difficult, or impossible, to work properly. This phenomenon is even more important when the waveguide covers the whole length of the cavity. A numerical simulation describes this effect, which creates discontinuities of the laser power in the spectral domain. We show an example with an existing infrared Free-Electron Laser, which exhibits such discontinuities of the power, and where no convincing explanation was proposed until now.
Highlights
Several infrared free-electron lasers (FEL) are presently used, around the world, as light sources by a large scientific community [1,2]
The laser power extraction is generally performed by a hole coupling on the output mirror of the cavity, since no wideband mirror exists in infrared and most parts of substrates contain absorption bands
Eigenmode distribution in the waveguide section. This discussion does not involve the notion of eigenmodes in the waveguide section, but it is interesting to see the distribution of eigenmode of CLIO, and compare to the pattern displayed in Fig. 8 corresponding to the configuration of FLARE
Summary
Several infrared free-electron lasers (FEL) are presently used, around the world, as light sources by a large scientific community [1,2]. Optical cavity length: 7.5 m Mirrors radius of curvature: 4.81 m in horizontal Extraction hole: 2.5 × 10 mm (vertical slit) Waveguide transverse size: 200 × 10 mm Undulator period: 110 mm Number of undulator periods: 40 Undulator parameter Krms: 0.7–3.4 Electron beam energy: 10–15 MeV Energy spread: 0.8% (nominal 1⁄4 0.3%) Electron beam transverse size: σXY 1⁄4 5 × 4 mm RMS Bunch charge: 150 pC Micro-pulse duration: 10 ps (nominal 1⁄4 3 ps) Micro-pulse frequency: 3 GHz Macro-pulse duration: 10 μs Macro-pulse repetition rate: 10 Hz applied independently to each eigenmode. Several causes may be involved in such spiky behavior: the gain, the cavity losses and the extraction rate These parameters are strongly dependent on the transverse laser profile inside the optical cavity. These two effects, laser extraction and required gain variations, are responsible of the spiky behavior of the laser power observed in simulations on Fig. 4, and in measurements on Fig. 1
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
