Single Pass Free Electron Lasers as Sources of Short Wavelength Coherent Radiation

Abstract

Free electron lasers utilize a relativistic electron beam as the laser medium, which results in unique properties beneficial to the generation and amplification of short wavelength coherent radiation. Since the resonance condition can be adjusted by varying externally controlled parameters such as the electron energy or the wiggler magnetic field strength, these devices are well suited to produce sources with tunable wavelength. Also, there is no problem with the medium having undesirable absorption at particular wavelengths. Development of free electron laser oscillators utilizing optical cavities has proved very successful in the infrared. As one considers moving into the vacuum ultraviolet and x-ray regions, mirrors become more difficult, and it is worthwhile to consider devices based upon single pass amplification. One approach is to utilize an external laser to seed the FEL amplifier. At short wavelengths a seed at the operating wavelength may be unavailable, but one can use a subharmonic seed at longer wavelength and carry out harmonic generation in the FEL. This scheme has many merits especially in the ultraviolet region from 75nm-200nm. The temporal coherence of the output is inherited from the seed laser. Another approach is to start the FEL from the spontaneous radiation emitted at the beginning of the wiggler. This approach is viable at very short wavelengths where even subharmonic seeds are unavailable. Devices based on self-amplified spontaneous radiation are capable of delivering high intensity and good transverse coherence, but the frequency bandwidth cannot be as narrow as achievable in an oscillator or seeded amplifier.